NISTIR 6964 

Results from Simulation and Laboratory 

Testing of Air Handling Unit and Variable 

Air Volume Box Diagnostic Tools 

Natascha S. Castro 

Jeffrey Schein 

Cheol Park 

Michael A. Galler 

Steven T. Bushby 

U.S. DEPARTMENT OF COMMERCE 

National Institute of Standards and Technology

Building Environment Division 

Building and Fire Research Laboratory 

Gaithersburg, MD 20899-8631 

John M. House 

Iowa Energy Center 

Ames, IA
NISTIR 6964 

Results from Simulation and Laboratory 

Testing of Air Handling Unit and Variable 

Air Volume Box Diagnostic Tools 

Natascha S. Castro 

Jeffrey Schein 

Cheol Park 

Michael A.Galler 

Steven T. Bushby 

U.S. DEPARTMENT OF COMMERCE 

National Institute of Standards and Technology 

Building Environment Division 

Building and Fire Research Laboratory 

Gaithersburg, MD 20899-8631 

John M. House 

Iowa Energy Center 

Ames, IA 

Prepared for: 

Architectural Energy Corporation 

Boulder, Colorado 

January 2003

 

U.S. Department of Commerce 

Donald L. Evans, Secretary

Technology Administration 

Phillip J. Bond, Under Secretary for Technology 

National Institute of Standards and Technology 

Arden L. Bement, Jr., Director
Abstract 

Control of building systems is becoming increasingly more intelligent and complex. This

development both necessitates the use of automated diagnostics to ensure fault-free operation 

and enables diagnostic capabilities for the various building systems by providing a distributed 

platform that is powerful and flexible enough to perform fault detection and diagnostic (FDD). 

Most of today’s emerging FDD tools are stand-alone software products that do not reside in a 

building control system. Thus, trend data files must be processed off-line, or an interface to the 

building control system must be developed to enable on-line analysis. This is a cumbersome

process and it does not scale well because all of the data must be obtained at a single point. A 

better approach would be to develop algorithms that can be embedded in commercial controllers 

so that the fault detection can be done as close to the source of the fault as possible. Only the 

result of the analysis needs to be conveyed to an operator or supervisory controller. 

AHU Performance Assessment Rules (APAR) is a diagnostic tool that uses a set of expert rules 

derived from mass and energy balances to detect common faults in air-handling units. Control 

signals are used to determine the mode of operation for the AHU. A subset of the expert rules 

corresponding to that mode of operation is then evaluated to determine if there is a mechanical

fault or a control problem. VAV box Performance Assessment Control Charts (VPACC) is a 

diagnostic tool that uses a statistical quality control measures to detect faults or control problems 

in VAV boxes. This report describes the results of a research study to determine the effectiveness 

of these tools in detecting commonly found mechanical faults and control problems. 

The research involved a complementary set of laboratory experiments using commercial AHU

and VAV box controllers under both normal operating conditions and operation with known 

faults, computer simulations, and emulations using the NIST Virtual Cybernetic Building 

Testbed (VCBT). The APAR and VPACC tools were both found to be successful at finding a 

wide variety of faults. It was also found that some faults could not be detected under certain 

operating conditions because the control system was able to mask the problem or because sensor 

data needed to detect the fault is not commonly available in commercial systems. Both tools 

appear to be suitable for embedding in commercial control products. 

Key words: BACnet, building automation and control, direct digital control, energy management

systems, fault detection and diagnostics, cybernetic building systems 

Acknowledgments

This work was supported in part by the California Energy Commission Public Interest Energy 

Research (PIER) Program. The authors wish to acknowledge the assistance of Peter G. 

Loutzenhiser from Iowa State University and Daniela Marquez from University of Michigan

who helped collect and process some of the data used in this report. 
Table of Contents 

1 Introduction............................................................................................................................. 1 

2 Validation of Fault Models..................................................................................................... 2 

2.1 Air Handling Unit Faults ................................................................................................ 3 

2.1.1 Supply Air Temperature Sensor Offset................................................................... 3 

2.1.2 Stuck Open Recirculating Air Damper................................................................... 8 

2.1.3 Leaking Heating Coil Valve ................................................................................. 10 

2.2 Variable-Air-Volume Box Faults ................................................................................. 14 

2.2.1 Stuck Open VAV Box Damper............................................................................. 14 

2.2.2 Stuck Open VAV Box Reheat Valve.................................................................... 19 

2.3 Conclusions................................................................................................................... 21 

3 FDD for Air Handling Units................................................................................................. 21

3.1 System Description ....................................................................................................... 21

3.2 AHU Performance Assessment Rules (APAR)............................................................ 23 

3.2.1 Operational and Design Data Requirements......................................................... 26 

3.2.2 Detecting and Diagnosing Faults.......................................................................... 27 

3.2.3 Threshold Selection .............................................................................................. 27

3.3 Instrumentation Accuracy Requirements...................................................................... 28 

3.4 Results from AHU Laboratory Experiments ................................................................ 28 

3.5 Results from APAR Analysis of AHU Simulation Data Using HVACSIM+

............... 33 

3.5.1 Normal Operation ................................................................................................. 34

3.5.2 Simulated Faults.................................................................................................... 34

3.6 Results from AHU Emulation in the VCBT................................................................. 39 

3.6.1 Faults Implemented............................................................................................... 40

3.6.2 Normal Operation ................................................................................................. 41

3.6.3 APAR Analysis & Results.................................................................................... 43 

3.7 APAR User Interface .................................................................................................... 51

4 FDD for VAV ....................................................................................................................... 55 

4.1 VAV box Performance Assessment Control Charts - VPACC .................................... 55 

4.1.1 Control Charts....................................................................................................... 55

4.1.2 System Description ............................................................................................... 57

4.1.3 CUSUM Applied to VAV Box Diagnostics......................................................... 58 

4.1.4 Point requirements................................................................................................ 59

4.1.5 Threshold Selection .............................................................................................. 60

4.1.6 Instrumentation Accuracy Requirements.............................................................. 60 

4.2 Results from VAV Box Laboratory Experiments......................................................... 60 

4.2.1 Faults Implemented............................................................................................... 60

4.2.2 Normal Operation ................................................................................................. 61

4.2.3 Control Charts....................................................................................................... 62

4.3 Results from VAV Box Simulation Using HVACSIM+

............................................... 65 

4.3.1 Faults Implemented............................................................................................... 66

4.3.2 Normal Operation ................................................................................................. 67

4.3.3 Control Charts....................................................................................................... 67

4.4 Results from VAV Box Emulation in the VCBT ......................................................... 71 

4.4.1 Faults Implemented............................................................................................... 72
4.4.2 Normal Operation ................................................................................................. 73

4.4.3 Control Charts....................................................................................................... 73

5 Summary and Future Work................................................................................................... 83 

6 References............................................................................................................................. 85 

Attachment A: Nomenclature for Table 1 .................................................................................... 86 
1 INTRODUCTION 

Control of building systems is becoming increasingly more intelligent and complex. The control

infrastructure necessary for the operation of various building systems provides a distributed 

platform that is powerful and flexible enough to perform fault detection and diagnostics (FDD). 

Most of today’s emerging FDD tools are stand-alone software products that are not integrated 

with the building control system. Thus, trend data files must be processed off-line, or an interface 

to the building control system must be developed to enable on-line analysis. 

The purpose of the research effort described in this report is to develop, test, and demonstrate 

FDD methods that can detect common mechanical faults and control errors in air-handling units 

(AHUs) and variable-air-volume (VAV) boxes. The tools are intended to be sufficiently simple 

that they can be embedded in commercial building control systems and rely upon only sensor

data and control signals that are commonly available in commercial building automation and

control systems. 

AHU Performance Assessment Rules (APAR) is a diagnostic tool that uses a set of expert rules 

derived from mass and energy balances to detect common faults in air-handling units. Control 

signals are used to determine the mode of operation for the AHU. A subset of the expert rules 

corresponding to that mode of operation is then evaluated to determine if there is a mechanical

fault or a control problem. VAV box Performance Assessment Control Charts (VPACC) is a 

diagnostic tool that uses a statistical quality control measures to detect faults or control problems 

in VAV boxes. VPACC can be applied to most VAV box control strategies. Fault thresholds are 

determined by statistical analysis of a database of “normal operation” data. This report describes 

the development and the results of a research study to determine the effectiveness of these tools

in detecting commonly found mechanical faults and control problems. 

The FDD tools for AHUs and VAV boxes are being developed with distinct approaches because 

of the nature of the systems. VAV boxes are simple devices with a limited number of operation 

modes and possible faults. Because the building industry is sensitive to the first cost, the VAV 

boxes typically have little instrumentation and controllers with limited capability. However, 

VAV boxes are very numerous in a typical HVAC system, resulting in a large amount of data to

be monitored for faults. AHUs are more complex and thus susceptible to more kinds of faults. 

They also tend to have more instrumentation and more capable controllers. The FDD tools for 

both systems are designed to be robust so that they can adapt to the variety of applications 

typical of their use. 

The research involved a complementary set laboratory of experiments using commercial AHU 

and VAV box controllers under both normal operating conditions and operation with known 

faults, emulations using the NIST Virtual Cybernetic Building Testbed (VCBT), and computer

simulations using HVACSIM+

. These three approaches compliment each other and provide a 

reasonable assurance that the results are representative of what can be expected in real building

systems. 

The VCBT is a simulation-emulation environment that combines simulations of a building and 

its HVAC system with the emulation of the actual commercial controllers. It provides a way to 

1 
conduct tests under a wide variety of carefully controlled conditions and to compare the results 

of several different commercial products. Details of the VCBT design and operation are 

documented in NISTIR 6818, Using the Virtual Cybernetic Building Testbed and FDD Test 

Shell for FDD Tool Development [2]. Emulation provides a test environment that is closer to a

real building because it uses real building controllers but, like simulation, it also provides 

carefully controlled and reproducible conditions. Because emulation is done in real time it takes 

much longer than simulation, making it more difficult to test a broad range of faults and 

conditions in a limited time. 

The emulation also provides a controlled testbed for verifying the capabilities of FDD tools that 

are used to interface with control systems in real buildings. It will also be used in the future as a 

platform for testing the FDD tools when they are embedded in commercial controllers. 

Simulation provides a way to expand the variety of weather and fault conditions without the realtime constraints of the experimental and emulation studies. The simulations were conducted 

using HVACSIM+

[4]. The results from the laboratory experiments, emulations, and simulations 

were compared for a subset of test cases as a way to verify that the results are reasonable. 

The APAR and VPACC tools were both found to be successful at finding a wide variety of faults

including stuck or leaking dampers and control valves, sensor drift, and improper control 

sequencing. It was also found that some faults could not be detected under certain operating 

conditions because the control system was able to mask the problem or because sensor data 

needed to detect the fault is not commonly available in commercial systems. Both tools appear to 

be suitable for embedding in commercial control products. 

2 VALIDATION OF FAULT MODELS 

Numerous fault models for AHU and VAV box sensors and controlled devices have been 

implemented in HVACSIM+

. Bushby et al. [2] provide detailed descriptions of the manner in 

which the faults were introduced in the simulation code. In this section, simulation data produced 

by three of the AHU faults and two of the VAV box faults implemented in HVACSIM+ and 

described by Bushby et al. [2] will be compared with data collected at the Iowa Energy Center 

Energy Resource Station (designated as the ERS throughout the remainder of this report) to 

ensure that the characteristics and trends observed in the simulation data are representative of

real systems. The complete set of faults simulated is provided in Table 3.5 and includes sensor 

faults, stuck and leaking valve faults, and stuck and leaking damper faults for the AHU. The 

faults that are validated are a supply air temperature sensor offset fault, a stuck open recirculation 

air damper fault, and a leaking heating coil valve fault. The VAV box faults simulated are a 

stuck open or closed damper and a stuck open or closed reheat valve. The faults that are

validated are a stuck open damper fault and a stuck open reheat valve fault. The set of faults that 

are validated is considered to be representative of the complete set since one fault of each type 

(sensor, damper, valve for the AHU; damper and valve for the VAV box) is examined and the 

implementations in HVACSIM+

 of the faults that are not validated are identical to those that are 

validated. 

2 
2.1 Air Handling Unit Faults 

Simulation models for three AHU faults have been validated using data collected at the ERS, 

namely, a supply air temperature sensor offset fault, a stuck open recirculation air damper fault, 

and a leaking heating coil valve fault. The subsections that follow provide descriptions of the 

fault implementations in the simulation code and at the ERS. This is followed by a comparison 

of the trends in the simulation data and ERS data using select temperatures and control signals. 

Simulation data consisting of temperatures, humidities, pressures, flow rates, control signals and

other similar types of outputs were produced for summer, winter, and fall weather data profiles, 

each two weeks in length. The weather data were extracted from the "Weather Year for Energy 

Calculation" (WYEC) tape for Washington, D.C. for the months of February, July, and October. 

The methodology described by Nall et al. [7] was used to select the weather data. The internal 

loads due to occupants, lighting, and equipment were the same for all weather conditions in the 

simulations. The ERS data were collected only for summer and winter weather conditions. The 

internal loads for the ERS were the same for both sets of weather conditions, although the 

schedules of the loads were slightly different. The schedule differences are not important when

examining the trends due to the presence of faults. The comparisons of the data utilize weather 

conditions where the impact of a given fault is most clearly seen. 

2.1.1 Supply Air Temperature Sensor Offset 

The simulated fault causes the supply air temperature offset to increase linearly from 0 oC to 4 oC 

over a two-week period. The effect of the fault was studied by comparing data produced with the 

fault embedded in the simulation, to simulation data produced under normal operating 

conditions. 

To implement the supply air temperature sensor offset fault at the ERS, the linearization 

parameter that establishes the y-intercept in the supply air temperature sensor calibration 

equation was modified to produce a fixed positive offset of 2.8 oC in AHU-B (i.e., the sensor 

value is higher than the actual supply air temperature). The effect of the fault was studied by 

comparing the fault data to data produced by an identical unit (referred to as AHU-A) operating 

under normal conditions on the same day. 

The comparison of the faulty operation to normal operation was done using simulation and ERS 

data produced under winter weather conditions because the effect of the fault is most evident 

under these conditions. During winter conditions, AHUs often satisfy the cooling supply air

temperature set point by modulating the mixing box dampers while the heating valve and cooling 

valve remain closed. In this control mode, the supply air temperature is slightly higher than the 

mixed air temperature, with the difference attributed to a temperature rise across the fan (on the 

order of 1 oC to 2oC) and, in the simulation case, numerical error. For the data collected at the

ERS, numerical error is replaced by measurement error. 

When the offset is introduced to the supply air temperature sensor value, the AHU must

compensate by lowering the actual supply air temperature. If the AHU remains in the free

cooling mode, this will be accomplished by introducing more outdoor air, thereby lowering the 

mixed air temperature. If the sensor offset is significant, the impact of the fault will be apparent 

from the difference in the supply air temperature and the mixed air temperature. The simulation 

was run using the outdoor air temperature profile labeled February 12 to February 25, 2001 as 

input (The year label for the outdoor air temperature data used in the simulation represents the 

year when the simulations were run, and not the year that the weather data were actually 

3 
collected.) The ERS data considered were collected on January 24, 2002. Plots of the outdoor air 

temperature for the simulation and ERS data sets corresponding to these time periods are shown 

in Figures 2.1 (simulation) and 2.2 (ERS). These plots will be used to help describe the behavior 

of the AHUs in the presence of this and other faults where the outdoor air temperature directly 

impacts the conditions that are observed. 

Figure 2.3 shows simulation results for the difference in the supply air temperature and mixed air 

temperature for faulty and normal operation. For normal operation, the difference is generally 

less than 0.5 oC. This difference is slightly less than what is typically seen in real equipment; 

however, diagnostic tools must routinely deal with uncertainties in measurements and models

and the degree of this difference will not compromise the data. The fault case, on the other hand, 

shows the difference increasing linearly as the offset of the supply air temperature grows to 4 oC

over the two-week simulation. 

Figure 2.4 shows results from the ERS for the difference in the supply air temperature and mixed 

air temperature for faulty and normal operation. For all ERS results presented, the AHUs operate 

in the occupied mode from 6:00 to 18:00. During this time period, AHU-A operates normally 

and the difference in the supply and mixed air temperatures is approximately 2.5 oC. AHU-B,

however, has a difference of approximately 5.0 oC in the supply and mixed air temperatures, or

twice that for normal operation. 

The impact of the fault can also be observed in the operation of the mixing box dampers. Figure 

2.5 shows the outdoor air damper control signal for the simulation fault case and for normal 

operation. The plot indicates that the dampers are commanded to provide additional outdoor air 

to be introduced for the fault case. As described previously, for the ambient conditions in the 

simulation, the additional outdoor air will lower the mixed air temperature and compensate for 

the fault (i.e., enable the supply air set point to be maintained). The effect becomes more

pronounced as the offset increases over the two-week period. A similar result is found in the 

ERS data shown in Figure 2.6. In this case, the outdoor air damper control signal for the fault 

case (AHU-B) and normal operation (AHU-A) are nearly the same early in the occupied period 

because the outdoor air temperature is very low, ranging from –12 oC to 0 oC over the first 6 

hours of occupancy. At these temperatures, small amounts of outdoor air are sufficient to 

compensate for the fault. The impact of the fault becomes evident when the outdoor temperature

increases in the afternoon. 

4 
Time

0 50 100 150 200 250 300 350

Temperature, o

C

-15

-10

-5

0

5

10

, hrs

Figure 2.1: Outdoor air dry-bulb temperature used for simulations for the period of 

February 12 to February 25, 2001 

Time, hrs

02468 10 12 14 16 18 20 22 24

Temperature, o

C

-15

-10

-5

0

5

10

Figure 2.2: Outdoor air dry-bulb temperature at the ERS on January 24, 2002 

5 
Time

0 50 100 150 200 250 300 350

Temperature, o

C

-4

-2

0

2

4

6

8

10

12

14

Sensor Offset Fault

Normal Operation

 , hrs

Figure 2.3: Simulation data showing the difference between the supply air temperature and 

the mixed air temperature for the supply air temperature offset fault and normal operation 

Time, hrs

0 2 4 6 8 10 12 14 16 18 20 22 24

Temperature, o

C

-4

-2

0

2

4

6

8

10

12

14

Sensor Offset Fault

Normal Operation

Figure 2.4: ERS data showing the difference between the supply air temperature and the 

mixed air temperature for the supply air temperature offset fault (AHU-B) and normal 

operation (AHU-A) 

6 
Time, hrs

0 50 100 150 200 250 300 350

Outdoor Air Damper

Contr

ol Signal, % Open

0

10

20

30

40

50

60

70

80

90

100

Sensor Offset Fault

Normal Operation

Figure 2.5: Simulation data showing the outdoor air damper control signal for the supply 

air temperature offset fault and normal operation 

Time, hrs

0 2 4 6 8 10 12 14 16 18 20 22 24

Outdoor Air Damper

Contr

ol Signal, % Open

0

10

20

30

40

50

60

70

80

90

100

Sensor Offset Fault

Normal

Operation

Figure 2.6: ERS data showing the outdoor air damper control signal for the supply air 

temperature offset fault (AHU-B) and normal operation (AHU-A) 

7 
2.1.2 Stuck Open Recirculating Air Damper 

This fault was simulated by setting the position of the motor driven actuator final control element 

for the recirculation air damper equal to one, causing the damper to stay open throughout the 

simulation. To implement the stuck open recirculation air damper fault at the ERS, the power 

supply for the damper was disconnected at the field panel. When there is no power supplied to 

the damper motor, the spring return on the damper motor causes the damper to go to the full 

open position. The damper remains at the 100 % open position for the duration of the test. Note 

that the outdoor air damper and exhaust air damper operate normally in the presence of this fault. 

The fault causes excess return air when the AHU operates in either the free cooling mode

(dampers modulate to maintain supply air temperature set point) or the mechanical cooling with 

100 % outdoor air mode. For winter conditions, the AHU operates much of the time in the free 

cooling mode and the fault effectively raises the mixed air temperature above what would be 

expected at a given outdoor air damper position. To compensate, the AHU must increase the 

control signal to the outdoor air damper to introduce additional outdoor air to the system. The 

simulation was run using the February 12 to February 25, 2001 outdoor air temperature profile as 

input. The ERS data considered were collected on January 31, 2002. Plots of the outdoor air 

temperature for the simulation and ERS data sets corresponding to these time periods are 

presented in Figures 2.1 (simulation) and 2.7 (ERS). The impact of the fault for the simulation 

case is seen in Figure 2.8. The plot shows the outdoor air damper control signal for the fault case 

and for normal operation. The plot indicates that the dampers are commanded to provide

additional outdoor air to be introduced for the fault case. The impact is the same as observed for 

the supply air temperature offset fault depicted in Figure 2.5, although the impact of the damper

fault is more pronounced. 

Time, hrs

02468 10 12 14 16 18 20 22 24

Temperature, o

C

-15

-10

-5

0

5

10

Figure 2.7: Outdoor air dry-bulb temperature at the ERS on January 31, 2002 

8 
Time, hrs

0 50 100 150 200 250 300 350

Outdoor Air Damper

Contr

ol Signal, % Open

0

10

20

30

40

50

60

70

80

90

100

Stuck Open Damper Fault

Normal Operation

Figure 2.8: Simulation data showing the outdoor air damper control signal for the stuck 

open recirculation air damper fault and normal operation

Time, hrs

0 2 4 6 8 10 12 14 16 18 20 22 24

Outdoor Air Damper

Contr

ol Signal, % Open

0

10

20

30

40

50

60

70

80

90

100

Normal Operation

Stuck Open Damper Fault

Figure 2.9: ERS data showing the outdoor air damper control signal for the stuck open 

recirculation air damper fault (AHU-A) and normal operation (AHU-B) 

9 
2.1.3 Leaking Heating Coil Valve 

This fault was implemented in the simulation code by setting the leakage parameter for the 

heating coil valve to several different values, causing leakage through a three-way valve of 

different severities. For the data considered here, a 10 % leakage fault was simulated. To 

implement the leaky hot-water valve fault at the ERS, a manual bypass valve was opened 

partially to allow hot water to be diverted around the automatic three-way bypass valve that 

normally controls water flow to the heating coil. An ultrasonic flow measurement device was

used to determine the water flow through the heating coil. The fault was implemented to allow a 

leakage rate of approximately 0.019 L/s to 0.044 L/s (0.3 gal/min to 0.7 gal/min) through the

coil, with a digital resolution of 0.0063 L/s (0.1 gal/min; all measurements were made in 

gal/min). The leakage rate is approximately 2 % to 3 % of full flow through the heating coil. 

The hot-water leakage fault imposes an additional cooling load on the AHU. During the free 

cooling mode, the mixed air and supply air temperatures may differ significantly due to the 

presence of this fault. During the mechanical cooling modes (with minimum outdoor air or 100 

% outdoor air), the additional load may force the cooling valve to saturate at the full open

position depending on the thermal load. The impact of the fault at the ERS was quantified by 

establishing a fixed leakage rate and allowing the AHU to reach quasi steady-state condition in

which the entering and leaving air and water temperatures and flow rates were stabilized. The 

unit was run with 100 % return air to help stabilize the entering air temperature. The temperature 

rise across the heating coil was then measured for fixed inlet water conditions to the coil and a

fixed airflow rate across the coil. The fault characterization data from the ERS are provided in 

Table 2.1 to document the severity of the leaking valve fault in comparison to sensor faults with 

fixed offsets. The first column of Table 2.1 contains the leakage rate through the coil, the second 

column the airflow rate across the coil, and the final column the temperature rise across the coil.

The heating water pump that circulates water to the heating coil operated continuously 

throughout the test and produced a constant flow rate of 1.65 L/s. Nearly all the water bypassed 

the heating coil, the exception being the small amount of leakage in column 1 of Table 2.1. The 

entering water temperature to the coil ranged from 60.5 oC to 62.2 oC during the fault 

characterization tests. 

Table 2.1: ERS fault characterization data for AHU-B for a leaking heating coil valve fault 

Water Leakage 

Rate (L/s) 

Airflow 

Rate (m3

/s) 

Temperature Rise 

Across Coil (oC) 

0.032 0.80 3.1

0.032 1.61 2.1

0.044 0.80 4.6

0.044 1.62 2.8

The simulation was run using the February 12 to February 25, 2001 outdoor air temperature 

profile as input. The ERS data considered were collected on January 29, 2002. Plots of the 

outdoor air temperature for the simulation and ERS data sets corresponding to these time periods

are presented in Figures 2.1 (simulation) and 2.10 (ERS). 

10 
Time

02468 10 12 14 16 18 20 22 24

Temperature, o

C

-15

-10

-5

0

5

10

, hrs

Figure 2.10: Outdoor air dry-bulb temperature at the ERS on January 29, 2002 

Figure 2.11 shows simulation results for the outdoor air damper control signal for faulty and 

normal operation. The fault causes the AHU to operate in the mechanical cooling with 100 % 

outdoor air mode for most of the simulation period; however, there is a short period of time

between hours 10 and 20 of the simulation during which the AHU operates in the free cooling 

mode. This time period will be discussed because the ERS AHU operates in the free cooling 

mode throughout the duration of this fault. During the period when the AHU operates in the free

cooling mode, the simulation control signal to the outdoor air damper for the fault case is

considerably larger than the normal operation case, indicating that a much greater amount of 

outdoor air is being introduced to the system to compensate for the fault. The amount of outdoor

air introduced throughout the simulation is greater for the fault case than for normal operation. 

Figure 2.12 shows the outdoor air damper control signals for the AHUs at the ERS. The fault is 

implemented in AHU-B. Although the difference is not large, the control signal to AHU-B is

consistently higher than that for AHU-A. The same effect was seen in the simulation data. 

11 
Time

0 50 100 150 200 250 300 350

Outdoor Air Damper

Contr

ol Signal, % Open

0

10

20

30

40

50

60

70

80

90

100

Leaking Valve Fault

Normal Operation

, hrs

Figure 2.11: Simulation data showing the commanded outdoor air damper control signal 

for the leaking heating coil valve fault and normal operation 

Time, hrs

0 2 4 6 8 10 12 14 16 18 20 22 24

Outdoor Air Damper

Contr

ol Signal, % Open

0

10

20

30

40

50

60

70

80

90

100

Leaking Valve Fault

Normal Operation

Figure 2.12: ERS data showing the commanded outdoor air damper control signal for the 

leaking heating coil valve fault (AHU-B) and normal operation (AHU-A) 

12 
Time, h

0 50 100 150 200 250 300 350

Temperat

ure, o

C

-15

-10

-5

0

5

10

15

20

25

30

35

Normal Operation

Leaking Valve Fault

rs

Figure 2.13: Simulation data showing the difference between the supply air temperature and the 

mixed air temperature for the leaking heating valve fault and normal operation 

The impact of the fault is more pronounced when the difference in the supply air temperature and

mixed air temperature is considered during the free cooling mode. Simulation results showing 

the difference in the supply air and mixed air temperatures for faulty and normal operation are 

plotted in Figure 2.13. For normal operation the difference is generally less than 0.5 oC. For the 

fault case, however, the difference ranges from 6 oC to 24 oC. During the period noted previously 

when the AHU operates in the free cooling mode, the temperature difference is 18 oC to 19 oC. 

This temperature difference is too large when one considers that the supply air set point is 14 oC

to 16 oC during this time period. The implication is that the mixed air temperature is less than 

0oC, and typical AHUs have freeze protection thermostats that shut down the unit when the 

mixed air temperature drops below 2 oC to 3 oC. To make the simulation data more realistic, the

severity of the leakage should be reduced. 

Figure 2.14 shows results from the ERS for the difference in the supply air temperature and 

mixed air temperature for faulty and normal operation. AHU-A operates normally and 

thedifference in the supply and mixed air temperatures is approximately 2.5 oC during the 

occupied period. AHU-B, however, has a difference of approximately 7.0 oC in the supply and 

mixed air temperatures, or nearly three times that for normal operation. The faulty operation 

corresponds to a water leakage rate of approximately 0.019 L/s and an airflow rate of 

approximately 0.69 L/s. These conditions are slightly outside the range of those in Table 2.1; 

however, a slightly smaller temperature rise across the coil might have been anticipated. 

Nonetheless, the trend observed in the ERS data is consistent with that seen in the simulation. 

The primary difference is the larger impact observed in the simulation data. 

13 
Time, hrs

02468 10 12 14 16 18 20 22 24

Temperat

ure, o

C

-15

-10

-5

0

5

10

15

20

25

30

Normal Operation

Leaking Valve Fault

Figure 2.14: ERS data showing the difference between the supply air temperature and the 

mixed air temperature for the leaking heating valve fault (AHU-B) and normal operation 

(AHU-A) 

2.2 Variable-Air-Volume Box Faults 

The simulation models for two VAV box faults have been validated using data collected at the 

ERS. The models validated are a stuck open VAV box damper fault and a stuck open reheat 

valve fault. A schematic of the test room layout at the ERS is provided in Figure 2.15 to orient 

the reader to the matched pairs of test rooms in the facility. Data from the VAV boxes serving

the test rooms are used to validate the simulation models. 

2.2.1 Stuck Open VAV Box Damper 

This fault was simulated by setting the position of the motor driven actuator final control element 

for the VAV box damper equal to one, causing the damper to stay open throughout the 

simulation. The model includes a VAV box for each of three zones. The fault was implemented 

in Zone 1. The effect of the fault was studied by comparing data produced with the fault 

embedded in the simulation, to simulation data produced under normal operating conditions. 

The fault implemented at the ERS was a failed differential pressure reading, not a stuck open 

VAV box damper fault. The fault was implemented by disconnecting the tubing leads for the 

differential pressure transducer. The differential pressure is used to compute the airflow rate 

through the VAV box. With the tubing leads disconnected, the sensed airflow rate is zero. The 

controller tries to maintain the flow rate at a set point, and sensing there is no airflow rate, 

14 
Figure 2.15: Floor plan of the ERS showing matched pairs of test rooms. 

sends a signal to the damper to open further. After a few minutes, the damper is 100 % open and 

remains open throughout the test. This fault has the same impact on the zone as the stuck open 

VAV box damper; however, two of the signatures of the fault are different. First, the airflow rate

that is input to the controller is different (Note, however, that the actual airflow rate should be 

approximately the same since in both cases the damper is fully open). For the failed differential 

pressure measurement, the input is always zero. For the stuck open damper fault, the correct 

reading is obtained. Second, the control signal to the damper will be different under most 

conditions. For the failed differential pressure measurement, the control signal to the damper will 

always be saturated at its maximum value after the first few minutes of the fault. For the stuck

15 
open damper fault, the control signal to the damper will be saturated at its minimum position 

unless the loads on the zone are such that maximum cooling is required. 

The effect of the damper being 100 % open (either because it is stuck or because the airflow

sensor has failed) is to overcool the zone. In response, the airflow set point will be reduced to its 

minimum value. The fault prevents the damper from responding and, if the degree of overcooling 

is great enough, the controller will transition to the heating mode. The simulation was run using 

the February 12 to February 25, 2001 outdoor air temperature profile as input. The ERS data 

considered is for the South test rooms of the facility and was collected on January 25, 2002. To 

be consistent with the source of the data, the terminology zone will be used when referring to the 

simulation data and the test room name (e.g. South A, West B) will be used when referring to the 

ERS data. 

The impact of the fault for the simulation case is seen in Figure 2.16. The plot shows the airflow 

rate to the zone for the fault case and for normal operation. As expected, the airflow rate for the 

fault case is higher than the normal case throughout the simulation. Figure 2.17 shows the 

volumetric airflow rate for side-by-side south-facing zones with matched equipment and subject 

to the same external and internal loads for the fault case (South A) and normal operation (South 

B). Note that due to the nature of the fault in South A, the volumetric airflow rate was computed 

by subtracting the sum of the airflow rates to the other test rooms on AHU-A from the supply

airflow rate. The effect is the same as that seen in the simulations; the test room subjected to the 

fault experienced larger airflow rates for the duration of the faulty operation. 

Another impact of the fault is seen in the use of reheat energy at the zones. Figure 2.18 shows the 

control signal to the reheat coil valve for Zone 1 for the fault case and normal operation. The 

overcooling caused by the excess air supplied to the zone for the fault case must be offset with

reheat energy. Figure 2.18 shows that for the fault case, the control signal to the reheat valve is 

greater than that for normal operation throughout the simulation. 

Figure 2.19 shows a similar effect in the data obtained at the ERS. In this case, the control signal 

to the reheat valve for South A (failed differential pressure measurement) is greater than that for 

South B except for a short period of time in the morning. As in the simulation case, the fault 

causes South A to be overcooled requiring extra reheat energy to compensate. 

16 
Time, hrs

0 50 100 150 200 250 300 350

Air

flow Rat

e, m3/s

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Normal Operation

Stuck Open VAV Damper Fault

Figure 2.16: Simulation data showing the volumetric airflow rate to Zone 1 for the stuck 

open VAV box damper fault and normal operation 

Time, hrs

0 2 4 6 8 10 12 14 16 18 20 22 24

Air

flow Rat

e, m3/s

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Differential Pressure

Measurement Fault

Normal Operation

Figure 2.17: ERS data showing volumetric airflow rate to the South rooms for the failed 

differential pressure measurement (South A) and normal operation (South B) on January 

25, 2002 

17 
 Time, hrs

0 50 100 150 200 250 300 350

Reheat Coil V

alve

Control Signal, % Open

0

10

20

30

40

50

60

70

80

90

100

Normal Operation Stuck Open VAV

Damper Fault

Figure 2.18: Simulation data showing the control signal to the reheat coil valve for Zone 1 

for the stuck open VAV box damper fault and normal operation 

Time, hrs

0 2 4 6 8 10 12 14 16 18 20 22 24

Reheat Coil Val

v

e

Control Signal, % Open

0

10

20

30

40

50

60

70

80

90

100

Differential Pressure

Measurement Fault

Normal

Operation

Figure 2.19: ERS data showing the control signal to the reheat coil valve for the South 

rooms for the failed differential pressure measurement (South A) and normal operation 

(South B) on January 25, 2002 

18 
2.2.2 Stuck Open VAV Box Reheat Valve 

The simulated fault was implemented by setting the position of the motor driven actuator final 

control element for the VAV box reheat coil control valve equal to one, causing the valve to stay 

open throughout the simulation. The fault was implemented in the VAV box serving Zone 1. To 

implement the stuck open VAV box reheat valve fault at the ERS, a control voltage from a 

source independent of the control system was applied to the reheat coil valve actuator in West A. 

Unlike the simulation case in which the valve was stuck 100 % open, the fault implemented at 

the ERS had a small magnitude. The valve was stuck only slightly open, producing a flow rate 

through the valve of approximately 1.7e-2 L/s, whereas maximum flow is approximately 20 

times this value. The effect of the fault is the same (i.e., the valve can not open or close) from the 

standpoint of the trends that are observed in the data and, therefore, the data can be used to 

validate the simulation model. Beyond this use, however, the more subtle effect of the fault

implemented at the ERS will pose a greater challenge when the data are used to test the ability of 

diagnostic tools to detect the presence of the fault. 

Depending on the zone (test room) conditions and the severity of the fault, the stuck reheat valve 

either creates an additional cooling load that the air-handling unit must try to remove, or it 

prevents the valve from modulating to provide additional heating energy to the zone. In the first 

case, the controller increases the airflow rate to the zone in an attempt to compensate for the 

fault. If the fault is severe, the zone temperature will gradually increase beyond the zone set 

point. In the second case, the zone temperature will tend to gradually decrease below the zone set 

point. The impact of the fault at the ERS was quantified by establishing a fixed input voltage to 

the reheat valve and a fixed airflow rate through the VAV box, and then measuring the entering 

and leaving water and air temperatures, and the water and air flow rates. The fault 

characterization data from the ERS are provided in Table 2.2 to document the severity of the 

fault. Column one of Table 2.2 lists the water leakage rate, column two the room airflow rate,

and column three the temperature rise across the reheat coil. The entering water temperature to 

the reheat coil ranged from 57.4 oC to 58.7 oC during the fault characterization testing. Table 2.2

indicates that, as expected, the temperature rise of the air moving across the reheat coil increases

as the water leakage rate increases. 

19 
Time, hrs

0 50 100 150 200 250 300 350

Airfl

ow Rat

e, m3/s

0.0

0.1

0.2

0.3

0.4

0.5

0.6

Stuck Open Reheat Valve Fault

Normal Operation

 

Figure 2.20: Simulation data showing the volumetric airflow rate for Zone 1 for the 

stuck open VAV box reheat valve fault and normal operation 

The simulation was run using the July 12 to July 25, 2001 outdoor air temperature profile 

as input. The ERS data considered were collected on August 30, 2001. Figure 2.20 shows 

simulation results for faulty and normal operation. The fault causes the damper to open 

fully in an effort to maintain the zone temperature at the set point. Hence, the airflow rate 

for the fault case is consistently higher than the airflow rate for normal operation. 

Figure 2.21 shows a similar effect for the data from the ERS. The hot water flow rate

through the coil was approximately 0.022 L/s. From Table 2.2, a temperature rise across 

the coil of approximately 5 oC is expected. From Figure 2.23 the actual airflow rate is

roughly 40 % higher than the value in Table 2.2 (approximately 0.4 m3

/s compared to

0.28 m3

/s), so the temperature rise across the coil would be somewhat less if all other 

inlet conditions were unchanged. 

20
Time

0 2 4 6 8 10 12 14 16 18 20 22 24

Airflow Rate, m3/s

0.0

0.1

0.2

0.3

0.4

0.5

0.6

Normal Operation

Stuck Open Reheat

Valve Fault

, hrs

Figure 2.21: ERS data showing the volumetric airflow rate for the West rooms for the

stuck open reheat coil valve (West A) and normal operation (West B) on August 30, 2001 

2.3 Conclusions 

Data collected at the ERS have been used to validate HVACSIM+ simulation data with

embedded AHU and VAV box faults. The AHU faults considered are a supply air 

temperature sensor offset fault, a stuck open recirculation air damper fault, and a leaking 

heating coil valve fault. The VAV box faults considered are a stuck open damper fault 

and a stuck open reheat valve fault. Although the severity of the faults and the manner in 

which they were implemented in some cases differed between the data sets (e.g., linearly 

increasing temperature sensor offset rather than a fixed offset), the trends seen in the 

simulation data were consistent with what was expected and with what was observed in 

the ERS data. The validation exercise did reveal that the severity of the leaking heating 

coil valve fault should be reduced in the simulation model to avoid unrealistic conditions 

associated with excessively low mixed air temperatures. 

3 FDD FOR AIR HANDLING UNITS 

The fault detection tool described in this section was developed for application to single

duct variable-volume or constant-volume air handlers with hydronic heating and cooling 

coils with airside economizers. The rules that are used for FDD focus on temperature 

control in an AHU. Hence, the system description will be restricted to components and 

control strategies directly related to temperature control. Figure 3.1 is a schematic 

diagram of a typical single duct variable-air-volume (VAV) air handling unit (AHU). 

3.1 System Description 

The AHU controller typically controls the supply temperature to maintain a setpoint 

temperature at a location in the supply duct downstream of the supply fan. Outdoor air 

enters the AHU and is mixed with air returned from the building. The mixed air passes 

over the heating and cooling coils, where if necessary, it is conditioned prior to being 

21
supplied to the building. The typical operating sequence for AHUs consists of four

primary modes of operation during occupied periods for maintaining the supply air 

temperature and the ventilation at preset levels. The relationship of the four operating 

modes to the control of the heating coil valve, the cooling coil valve and the mixing box 

dampers is shown in Figure 3.2. Sequencing logic determines the mode of operation as

dictated by various thermal relationships including the internal and external loads on the 

zones served by the AHU. 

C

C

Return

Fan

Recirculation

Air Damper

Normally Open

Exhaust Air Damper

Normally Closed

Cooling

Coil

Heating

Coil

Filter

H

C

Supply

Fan

T & H

T & H

Return Air

Damper

Motor

Air Handling Controller

Outputs

Inputs

Outdoor Air Damper

Normally Closed

From

Zones

To

Zones

T

Outdoor Air Temperature

& Humidity Sensors T

Mixed

Air

Supply

Air

Figure 3.1: Schematic diagram of a single duct air-handling unit 

Mode 1 Mode 2 Mode 3 Mode 4

Control Sig

nal (%)

100

0

Heating

Coil Valve

Cooling

Mixing Box Coil Valve

Dampers

Mode 1 Mode 2 Mode 3 Mode 4

Control Sig

nal (%)

100

0

Heating

Coil Valve

Cooling

Mixing Box Coil Valve

Dampers

Figure 3.2: Typical operating modes of an air-handling unit 

22
In the heating mode (Mode 1 in Figure 3.2), the heating coil valve is controlled to 

maintain the supply air temperature at the heating set point and the cooling coil valve is

closed. The outdoor air damper is positioned to allow the minimum outdoor air necessary

to satisfy ventilation requirements. As cooling loads increase, the AHU transitions from 

heating to “free” cooling (Mode 2). In this mode, the heating and cooling coil valves are 

closed and the mixing box dampers are modulated to maintain the supply air temperature 

at cooling set point. As the loads continue to increase, the mixing dampers eventually

saturate with the outdoor air damper fully open and the AHU changes modes again to 

mechanical cooling. When the AHU is operating in the mechanical cooling mode, the 

cooling coil valve modulates to maintain the supply air temperature at cooling set point,

the heating coil valve is closed, and the outdoor air damper is either fully open or at its 

minimum position. There are several different types of economizer controls, one of the 

economizer control logic uses a comparison of the outdoor and return air temperatures or 

enthalpies to determine the proper position of the outdoor air damper such that 

mechanical cooling requirements are minimized. Hence, the third primary mode (Mode 

3) of operation is mechanical cooling with 100 % outdoor air and the fourth primary 

mode (Mode 4) of operation is mechanical cooling with minimum outdoor air. 

3.2 AHU Performance Assessment Rules (APAR) 

The basis for the fault detection methodology is a set of expert rules used to assess the 

performance of the AHU. The tool developed from these rules is referred to as APAR 

(AHU Performance Assessment Rules). APAR uses control signals and occupancy 

information to identify the mode of operation of the AHU, thereby identifying a subset of 

the rules that specify temperature relationships that are applicable for that mode. The two 

main mode classifications are occupied and unoccupied. For occupied periods, the mode

is further categorized as described in the previous paragraph. For convenience, the 

operating modes are summarized below: 

• Mode 1: heating 

• 

• 

• 

• 

Mode 2: cooling with outdoor air 

Mode 3: mechanical cooling with 100 % outdoor air 

Mode 4: mechanical cooling with minimum outdoor air 

Mode 5: unknown 

Because the direct digital control (DDC) output to the actuators of the coil valves and the 

mixing box dampers are known, the mode of operation can be ascertained. Although not 

depicted in Figure 3.2, a fifth mode of operation referred to “unknown” operation has 

been defined and listed above. The unknown mode applies to the case in which the AHU 

is running in an occupied mode, but none of the control output relationships defined for

Modes 1-4 are satisfied. The unknown mode could be associated with mode transitions 

and/or with faulty operation such as simultaneous heating and cooling. 

 

Once the mode of operation has been established, rules based on conservation of mass

and energy can be used along with the sensor information that is typically available for 

controlling the AHUs. For example, normal operation in the mechanical cooling mode

with 100 % outdoor air (Mode 3) dictates that the outdoor and mixed air temperatures 

23
must be approximately equal. Defining Toa and Tma as the outdoor air and mixed air

temperatures, respectively, the rule (defined as Rule 10) is written as 

Rule 10: |Toa - Tma| > εt 

where εt is a threshold that depends on the uncertainty (or accuracy) of the 

measurements. The rules are written such that a fault is indicated if a rule is true. In the 

example above, the rule states that the outdoor and mixed air temperatures are not the 

same (i.e., if true, a fault has occurred). 

House et al. [5] provide a detailed description of the 28 APAR rules and the reasoning

behind them. For this reason, the rules are simply listed in Table 3.1 without detailed 

explanation. Table 3.1 groups the rules according to mode of operation. As indicated in 

the column heading for the rule expression, a true expression is indicative of a fault. 

Nomenclature for the rule set is provided in the Appendix to this report. Table 3.2 

presents the rules as related groups and indicates the sensors and control signals used to 

evaluate each rule. The first group of rules treats the relationship of temperatures in the 

coil subsystem of the AHU. For these four rules, only the relational operator in the rules

change from one mode to another. A typical rule from this subgroup requires the supply 

air temperature to be lower than the sum of the mixed air temperature and the 

temperature rise across the supply fan in the mechanical cooling modes. There are also 

groups of rules treating the mixing box subsystem, the zone subsystem, economizer

operation, comfort requirements, and controller logic/tuning. Hence, although there are 

28 rules, in reality only a small number of temperature and control signal relationships 

are used to define the rules. 

With the possible exception of the mixed air temperature, this information is generally

available for most AHUs controlled with a DDC system. If one or more sensors are not

available, certain rules will no longer be applicable. For instance, in the absence of a 

mixed air temperature sensor, nine rules listed in Table 3.2 (Rules 1, 2, 7, 10, 11, 16, 18, 

26, and 27) will be eliminated from consideration in APAR. Conversely, the presence of 

additional sensors would expand the rule set and provide an opportunity to either detect 

more faults, or to detect faults during modes of operation in which they would normally 

be hidden. For instance, if a temperature sensor was installed between the heating and 

cooling coils, leakage through the heating valve could be detected during the mechanical 

cooling modes, whereas normally it would be masked in these modes. 

24
Table 3.1: APAR Rule Set 

Mode Rule # Rule Expression (true implies existence of a fault)

1 Tsa < Tma + ∆Tsf - εt

2 For |Tra - Toa| ≥ ∆Tmin: |Qoa/Qsa - (Qoa/Qsa)min | > εf

 

3 |uhc – 1| ≤ εhc and Tsa,s – Tsa ≥ εt

Heating 

4 |uhc – 1| ≤ εhc

5 Toa > Tsa,s - ∆Tsf + εt

6 Tsa > Tra - ∆Trf + εt

Cooling with 

Outdoor Air

7 |Tsa - ∆Tsf - Tma| > εt

8 Toa < Tsa,s - ∆Tsf - εt

9 Toa > Tco + εt

10 |Toa - Tma| > εt

11 Tsa > Tma + ∆Tsf + εt

12 Tsa > Tra - ∆Trf + εt

13 |ucc – 1| ≤ εcc and Tsa – Tsa,s ≥ εt

Mechanical 

Cooling with 

100 % Outdoor 

Air 

14 |ucc – 1| ≤ εcc

15 Toa < Tco - εt

16 Tsa > Tma + ∆Tsf + εt

17 Tsa > Tra - ∆Trf + εt

18 |Tra - Toa| ≥ ∆Tmin 

19 |ucc – 1| ≤ εcc and Tsa – Tsa,s ≥ εt

Mechanical 

Cooling with 

Minimum 

Outdoor Air 

20 |ucc – 1| ≤ εcc

21 ucc > εcc and uhc > εhc and εd < ud < 1 - εd

22 uhc > εhc and ucc > εcc

23 uhc > εhc and ud > εd

Unknown 

Occupied 

Modes 

24 εd < ud < 1 - εd and ucc > εcc

25 | Tsa – Tsa,s | > εt

26 Tma < min(Tra , Toa) - εt

27 Tma > max(Tra , Toa) + εt

All Occupied

Modes 

28 Number of mode transitions per hour > MTmax

25
Table 3.2: Summary of Rule Relationships 

Tsa Tra Tma Toa Tsa,s ∆Tsf ∆Trf Tco ucc uhc ud

1 1   

7 2   

11 3   

16 4   

2 1   

18 4   

26 1, 2, 3, 4   

27 1, 2, 3, 4   

25 1, 2, 3, 4  

3 1   

13 3 

19 4 

4 1 

14 3 

20 4 

5 2 

8 3 

6 2   

12 3   

17 4   

9 3  

15 4  

10 3  

21 -   

22 -  

23 -  

24 -  

28 -   

Controller Logic/Tuning: Rules are related and

identify periods of operation associated with 

controller problems, such as simultaneous 

heating and cooling, and excessive mode

changes.

Coil Subsystem: The relational sign (<, >, etc.)

changes based on the mode of operation.

Comfort Requirements: The first four rules 

indicate comfort is sacrificed (with Rules 3, 13, 

and 19 indicating the system is out of control), 

whereas the latter three rules indicate comfort

could soon be sacrificed (system is out of 

control). 

The relational sign (<, >, etc.) changes based

on the mode of operation.

Zone Subsystem: Rules are identical.

Economizer: The relational sign (<, >, etc.)

changes based on the mode of operation.

Mixing Box Subsystem: Rules are related

through calculation of outdoor air fraction. If 

Rule 26 or 27 is satisfied, the outdoor air

fraction will be negative or greater than unity.

Sensors and Control Signals Rule Mode * Relationship Between Grouped Rules

* The dash symbol indicates either an unknown mode or multiple modes of operation. 

3.2.1 Operational and Design Data Requirements 

APAR uses the following occupancy information, setpoint values, sensor measurements, 

and control signals: 

• Occupancy status; 

• Supply air temperature set point; 

• Supply air temperature; 

• Return air temperature; 

• Mixed air temperature; 

• Outdoor air temperature; 

• Cooling coil valve control signal; 

• Heating coil valve control signal; 

• Mixing box damper control 

signal; 

• Return air relative humidity 

(for enthalpy-based economizers 

only) 

• Outdoor air relative humidity 

(for enthalpy-based economizers 

only). 

26
In addition to the operational data listed above, certain design data are needed to 

implement the rules. The required design data are: 

Minimum and maximum values of control signals for the heating coil valve, cooling 

coil valve and mixing box dampers for normalizing the control signals; 

• 

• 

• 

• 

• 

• 

• 

• 

• 

• 

Percentage outdoor air necessary to satisfy ventilation requirements; 

Changeover temperature from mechanical cooling with 100 % outdoor air to 

mechanical cooling with minimum outdoor air (or equivalent condition for enthalpybased economizer); 

Description of sequencing/economizer cycle strategy. 

The description of the sequencing/economizer cycle strategy is used to verify that the 

rules are suitable to a particular AHU installation. 

3.2.2 Detecting and Diagnosing Faults 

APAR can be used to detect faults; however, a specific set of faults that can be identified

has not been established. Rather, any fault that causes a rule to be satisfied would be 

detected and additional effort would be necessary to isolate the source of the problem. 

Faults that could potentially be identified by the rule set include the following: 

Stuck or leaking mixing box dampers, heating coil valves, and cooling coil valves; 

Temperature sensor faults; 

Design faults such as undersized coils; 

Sequencing logic errors; 

Central plant faults affecting the hot or chilled water supply conditions at the AHU 

coils; 

Inappropriate operator intervention. 

The operating point, severity of a fault, and threshold selection for the rules will 

obviously influence when a particular rule is satisfied. Threshold selection is discussed 

next. 

3.2.3 Threshold Selection 

In addition to the sensor, control signals, and setpoint information, other parameters must 

be specified for APAR. For instance, estimates of the temperature rise across the supply 

fan (and return fan, if one exists) must be provided, a reasonable default is 1.1 oC. A 

model-based value correlated to the airflow rate or the control signal to the fan could be 

used as the basis for this estimate; however, some amount of training data would likely be 

necessary to establish the correlation. Thresholds used in evaluation of rules such as εt

 in 

Rule 10 must also be specified. A value of 1.7 oC is typically used for all rules involving 

the comparison of temperatures. These threshold values (and several others used in 

APAR) is currently determined heuristically. A more rigorous approach that is being 

considered would involve determining the uncertainty associated with each temperature 

measurement and then combining the uncertainty terms to produce the most conservative

representation of each rule. Such an approach would tend to produce rules that are both

27
sensitive to faults and robust against false alarms. As an example, the threshold in Rule 

10 would be determined from the expression 

εt = εToa + εTma 

where εToa and εTma are the uncertainties associated with the measurement of the outdoor

and mixed air temperatures. Typical values of other threshold parameters used in APAR 

are listed below (see rule set in Table 3.1 and nomenclature in Attachment A): 

• εt = 1.7 oC 

• εf = 0.3

• εcc = εd = εhc = 0.02

• ∆Tsf = ∆Trf = 1.1 oC 

• ∆Tmin = 5.6 oC 

• (Qoa/Qsa) min = 0.1 

• MTmax = 6

3.3 Instrumentation Accuracy Requirements 

APAR uses existing sensor points in the building automation system (BAS) to perform 

the fault detection calculations. The typical industrial grade sensors that are already 

installed for control purposes have sufficient accuracy. Laboratory grade instruments are 

not required. 

3.4 Results from AHU Laboratory Experiments

In Section 2.1 the implementation procedures for three AHU faults that were examined at 

the ERS were described. Two to three days of data were typically collected for each of 

the faults (supply air temperature sensor offset, stuck open recirculation air damper, and 

leaking heating coil valve) for both summer and winter conditions. The temperature

sensor offset fault was implemented with a 1.7 oC offset (stage 1 fault) and a 2.8 oC offset 

(stage 2 fault). The leaking heating coil valve fault was implemented with a leakage rate 

of 0.019 L/s (stage 1 fault), 0.032 L/s (stage 2 fault), and 0.044 L/s (stage 3 fault). For 

the summer conditions, the occupied period was 8:00 AM to 6:00 PM, whereas 

occupancy began at 6:00 AM for the winter conditions. 

Results obtained from processing the ERS data with APAR are presented in Table 3.3 

(summer conditions) and Table 3.4 (winter conditions). The first column in the tables

represent the operation state of the AHU. Where applicable, the stage of the fault is

provided in the second column. The AHU being tested, the date, the number of hours the 

fault was active, the APAR rule satisfied and the number of hours satisfied are presented 

in columns three through six. The operation state is either normal or one of the three 

faults. Other operational problems could arise during testing; however, with the exception 

of a few hours of problems noted in the discussion below, there is no indication that they 

did. 

28
Table 3.3 shows that APAR did not produce any false alarms for normal operation; 

however, the faults went undetected during the summer conditions with the exception of

a single hour of the supply air temperature sensor fault. Although the faults have a 

significant effect on the operation of the AHU, the AHU is able to compensate for the 

faults and maintain the supply air temperature at the required set point. For instance, the

leaking heating coil valve becomes an additional cooling load on the AHU and the chilled

water valve simply opens further to offset this load. Without a sensor to indicate the

temperature of the heating coil, or a model to predict the supply air temperature based on 

mixed air and chilled water conditions and the control signal to the cooling coil valve,

there is no evidence to indicate the presence of the fault, unless it is so severe that the 

supply temperature cannot be maintained at its cooling set point. As described earlier, 

APAR does not use models to predict temperatures in the AHU did not have a sensor 

located between the heating and cooling coils. Thus, for summer conditions, this fault 

was difficult for APAR to detect. Similar explanations can be made for why APAR was 

unable to detect the other two faults during summer conditions. 

It should be noted that the fault detected on July 12, 2001 was likely a false alarm. The 

rule satisfied (Rule 18) indicates that insufficient outdoor air was entering the AHU.

Examination of the airflow rates in the data set indicates that this is not the case. 

Table 3.3: APAR results for ERS data and summer conditions 

Operation Stage AHU Date Number of

Hours with 

Faults 

Rules Satisfied 

(Number of hours 

violated) 

Normal NA B July 14, 2001 0 

B July 15, 2001 0 

Supply Air Temperature 1 B July 12, 2001 1 18 (1) 

Offset 2 B July 13, 2001 0 

Stuck Open Recirculation B July 16, 2001 0 

Air Damper

NA 

B July 17, 2001 0 

Leaking Heating Coil 2 B July 18, 2001 0 

Valve 3 B July 19, 2001 0 

Table 3.4 presents APAR results for winter conditions. The data set produced was more

extensive than that for summer conditions and includes 10 days of normal operation data 

for AHU A and 6 days for AHU B. APAR did not generate false alarms when it 

processed the normal operation data. 

The results are also quite encouraging for the fault cases, which are described one at a 

time. First consider the supply air temperature sensor offset fault. The fault was 

implemented and detected in both AHUs. Detection was due to Rule 7 being satisfied. 

As shown in Table 3.1, Rule 7 is satisfied if the mixed air temperature and supply air 

temperature differ significantly (∆Tsf = 1.1 oC and εt

 = 1.7 oC in Rule 7) while the AHU 

operates in “free” cooling mode. Figure 2.4 shows the difference in these temperatures 

for AHU-A (normal operation) and AHU-B (stage 2 sensor offset fault) for January 24, 

29
Table 3.4: APAR results for ERS data and winter conditions 

Operation Stage AHU Date

Number of 

Hours with 

Faults 

Rules Satisfied 

(Number of hours 

violated) 

A Jan. 22, 2002 0 

B Jan. 22, 2002 0 

A Jan. 23, 2002 0 

A Jan. 24, 2002 0 

A Jan. 25, 2002 0 

A Jan. 26, 2002 0 

A Jan. 27, 2002 0 

A Jan. 28, 2002 0 

A Jan. 29, 2002 0 

B Jan. 30, 2002 0 

B Jan. 31, 2002 0 

B Feb. 1, 2002 0 

A Feb. 2, 2002 0 

B Feb. 2, 2002 0 

A Feb. 3, 2002 0 

Normal NA

B Feb. 3, 2002 0 

B Jan. 23, 2002 10 7 (10) 1 A Jan. 30, 2002 8 7, 25 (6, 2) *Supply Air Temperature 

Offset 

2 B Jan. 24, 2002 11 7 (11) 

B Jan. 25, 2002 11 8, 10 (4, 11) 

B Jan. 26, 2002 8 8, 10 (3, 8) 

Stuck Open Recirculation 

Air Damper NA 

A Jan. 31, 2002 11 8, 10 (11, 11) 

2 B Jan. 27, 2002 4 8, 11 (4, 3) 

3 B Jan. 28, 2002 11 24, 28 (5, 6) 

1 B Jan. 29, 2002 11 7 (11) 

Leaking Heating Coil 

Valve 

2 A Feb. 1, 2002 9 7, 25, 27 (8, 1, 1) 

* 7, 25 (6, 2) denotes that Rule 7 was satisfied for 6 h and Rule 25 was satisfied for 2 h. 

2002. The impact of the fault is evident in Figure 2.4 and the presence of a fault is 

correctly identified by APAR. Recall that APAR does not perform diagnostics; it simply 

suggests possible explanations why a rule might be satisfied. A temperature sensor error 

is one possible reasons provided by APAR to explain why Rule 7 might be satisfied. 

Table 3.4 indicates that Rule 25 is satisfied for 2 hours when the sensor offset fault is 

implemented in AHU-A. This rule is satisfied when the supply air temperature is not 

maintained at the set point. In fact, the fault occurred because the AHU was not running, 

but the problem was actually caused by the outdoor air damper. It failed to open when it 

was initially commanded to do so, and then opened abruptly, drawing in excessive 

outdoor air and causing the mixed air temperature to drop significantly. The cold air 

tripped the freezestat and shut the unit down. 

30
The next fault considered is the stuck open recirculation air damper. The fault was 

introduced and detected in both AHUs for winter conditions. Table 3.4 indicates that 

Rules 8 and 10 were satisfied due to the effects of the fault. Both rules apply to operation 

in the mechanical cooling with 100 % outdoor air mode. Rule 8 is satisfied if the outdoor 

air temperature is low enough to enable “free” cooling with outdoor air. Rule 10 is 

satisfied if the outdoor air temperature and mixed air temperature differ significantly. For 

winter conditions, the fault causes warmer recirculation air to be mixed with outdoor air, 

resulting in a mixed air temperature that is significantly higher than expected (Operation 

with 100 % outdoor air is expected to result in a mixed air temperature that is nearly 

equal to the outdoor air temperature.). The impact of the fault can be seen in Figure 3.3.

For the first several hours of the day, the outdoor air temperature is well below the supply 

air temperature set point of 12.8 oC. Hence, outdoor air alone can provide the necessary

cooling load. The figure also reveals that the mixed and outdoor air temperatures differ 

by 4.0 oC or more over the entire day. This is more than twice the allowable difference 

established by the rule threshold of 1.7oC. 

The final fault considered is the leaking heating valve fault. Table 3.4 indicates that 

Rules 8 and 11 are satisfied for the January 27, 2002 data. Both rules apply to the 

mechanical cooling with 100 % outdoor air mode. As shown in Figure 3.4, the outdoor 

air temperature is sufficiently low to enable cooling with outdoor air for much of the 

morning. Hence, Rule 8 is satisfied. Rule 11 is satisfied when the supply air temperature 

is significantly higher than the mixed air temperature. This should only happen in the 

heating mode. Although not shown in Figure 3.4, the mixed air temperature is nearly the 

same as the outdoor air temperature over the entire day because the unit operates with 

100 % outdoor air. By comparing the outdoor air temperature and supply air temperature, 

it is evident that Rule 11 is satisfied for several hours in the morning. APAR lists a 

leaking heating coil valve as one of the possible explanations for Rules 8 and 11 being 

satisfied. 

The leaking heating valve fault data for January 29, 2002 and February 1, 2002 result in 

Rule 7 being satisfied for most of those two days. Recall that Rule 7 is satisfied if the

supply and mixed air temperatures differ significantly while the unit operates in the

cooling with outdoor air mode. The presence of a leaking valve fault is one of the reasons

this type of operational behavior might be observed. The data for February 1, 2002 also 

include one hour during which Rules 25 and 27 were satisfied. These rules were satisfied 

because a freezestat trip occurred similar to the one described previously. 

The data from January 28, 2002 resulted in several hours of violations of Rules 24 and 

28. The control of AHU-B was very unstable that day. In particular, the oscillation of the 

dampers was extreme and persistent over a 5 hour to 6 hour period. Eventually the

dampers stabilized, but they did so at a partially open position. Simultaneously the 

cooling coil valve was modulating to maintain the supply air temperature at the set point. 

Rule 28 was satisfied due to excessive mode switching caused by the unstable behavior. 

Rule 24 was satisfied due to the simultaneous modulation of the dampers and the cooling 

coil valve. 

31
Time (hh:mm)

 00:00 04:00 08:00 12:00 16:00 20:00 00:00

Temperature, o

C

-5

0

5

10

15

20

25

Mixed Air Temperature

Outdoor Air Temperature

Occupied Period

Figure 3.3: Stuck open recirculation damper fault for winter conditions (AHU-B, 

January 25, 2002) 

Time (hh:mm)

 00:00 04:00 08:00 12:00 16:00 20:00 00:00

Temperature, o

C

-5

0

5

10

15

20

25

30

Occupied Period

Supply Air

Temperature Set Point

Outdoor Air

Temperature

Supply Air

Temperature

Figure 3.4: Leaking heating coil valve fault for winter conditions (AHU-B, January 

27, 2002) 

32
3.5 Results from APAR Analysis of AHU Simulation Data Using HVACSIM+

Simulation, as described in section 2, was used to test and refine APAR. The simulation 

consisted of a single floor in a commercial office building with a VAV AHU and three 

VAV boxes. The faults selected for simulation are those that typically occur in AHUs and

include sensor drifts, stuck or leaking dampers, and stuck or leaking valves. Each 

simulation was run for a two-week period, including a range of fault types and fault 

severities. 

In Table 3-5 the first four columns list the fault identification number along with 

descriptions of the component affected, the type of error simulated, and the respective 

fault condition. The last three columns of Table 3.5 list the results of the APAR analysis

for heating season, swing season, and cooling season. In this table, a dash denotes that no 

fault was detected. 

Table 3.5: Fault Description and APAR Detection Results for Simulated Data 

Fault APAR Rule Violated 

ID Component Fault Type 

Fault 

Condition Heating Swing Cooling 

0 Fault free none none - - - 

1 Supply air temp. sensor zero drift (0 to -4) oC 7 5,7 - 

2 zero drift (0 to +4) oC 7 - - 

3 Return air temp. sensor zero drift (0 to -4) oC - - 18 

4 zero drift (0 to +4) oC - - 26 

5 Mixed air temp. sensor zero drift (0 to -4) oC 7 7,10,26 10,18,26 

6 zero drift (0 to +4) oC 7 7,10 10,18,27 

7 Outdoor air temp. sensor zero drift (0 to -4) oC - 10 10,27 

8 zero drift (0 to +4) oC - 5,10,26 10 

10 Outdoor air damper stuck closed - 10 - 

15 Recirculating air damper stuck closed - - 18,19,25

16 leakage 10 % - - -

18 leakage 40 % - 10 - 

19 Cooling coil valve stuck closed - 13,25 12,13 

20 leakage 10 % 1 1,5,7,24,25 -

21 leakage 25 % 28 1,5,7,25,28 1,5,7, 25 

22 leakage 40 % 28 1,5,7,24,25,28 1,5,7,17,25

23 Heating coil valve stuck closed - - - 

24 leakage 10 % 7,8,11 8 -

25 leakage 25 % 8,11 8,11 19,20,25

26 leakage 40 % 8,11 8,11,24 19,25 

27 VAV box damper stuck open - - - 

28 stuck closed - 28 - 

29 VAV reheat valve stuck open - 25 -

30 stuck closed - - - 

33
3.5.1 Normal Operation 

The simulation was carried out using weather data representing cooling, heating, and 

swing seasons. Specifically, two weeks of actual weather data from the “Weather Year

for Energy Calculation” tape were selected from the months of July, October, and 

February, respectively. 

The APAR tool produced no false alarms for normal data, regardless of the season tested. 

This result confirms that the system is not violating any of the applied rules. The 

simulated control system tested here operates on a simple control loop. First, the outdoor 

and return air enthalpies are compared to determine whether it is beneficial to operate the

economizer. If the answer is yes, the damper is modulated based on an algorithm that 

uses the supply air temperature measurement as an input. Other methods of control that 

use the mixed air, outdoor air, or return air conditions to modulate the damper position

were not tested. This has a large impact on both the response of the system to simulated 

faults as well as the ability of APAR to detect a fault as is presented in the discussion 

below. 

3.5.2 Simulated Faults 

The following subsections will first describe the faults tested, provide a brief explanation 

of the expected outcome, and then the actual test results will be presented and discussed. 

3.5.2.1 Supply Air Temperature Sensor Drift (Faults 1 & 2) 

The sensor drift for the supply air temperature sensor was introduced as a sensor offset 

for a range of (0 to -4) oC, applied linearly over the two-week simulation period. The

effect of the negative sensor offset would result in a increased recirculation air 

temperature as the control logic seeks to lower the supply air temperature to meet the 

setpoint. However, if the supply air setpoint target is met, the actual supply air 

temperature would be lower and the VAV box controls would have to compensate. First, 

the VAV box dampers would close, and if the temperature to the zone is still too low,

then the reheat coil would be activated. This type of fault would typically be masked in a 

system without diagnostic capabilities and results in increased energy use. 

The simulation uses the supply air temperature as a direct input to the system control; 

therefore this fault has a significant impact on the system operation even at low fault 

levels. For all seasons, the controller works to reverse the decreasing temperature 

reading for Fault 1 (Figure 3.5). In heating season this is achieved by damper 

modulation, closing the damper in response to the negative sensor drift of Fault 1 and as a

result the mixed air temperature is too high relative to the supply air temperature reading 

that triggers a fault. Similarly, in swing season negative sensor drift is detected. In 

cooling season, the economizer is called on earlier and the cooling coil valve signal is 

reduced to bring the supply air temperature to the setpoint. The actual supply air 

temperature will be too high and may make it difficult to meet the zone setpoint 

temperatures but APAR detects no fault. However, if the VAV box damper is not able to 

maintain the zone temperature, a reset of the supply air temperature setpoint to a lower 

setting will be triggered. 

34
Figure 3.5: Heating Season Simulation Data Showing the Change to Damper 

Position and Mixed Air Temperature Due to Fault 1 

A second simulation was carried out to introduce a sensor offset in the range of (0 to +4) 

oC. For Fault 2, a positive sensor offset would prompt the control logic to lower the 

supply air temperature. The VAV box controls would work to compensate. First, the 

VAV box dampers would open. As the severity of this type of fault is increased, the 

thermal comfort in the room would be compromised and the condition of the high zone 

temperature and open dampers would trigger a reset of the supply air temperature to a 

lower setpoint. 

In simulation, Fault 2 was detected for heating mode when the damper controller opens to 

reduce the supply air temperature, causing the mixed air temperature to drop out of range. 

For swing season there is a missed alarm, no fault is detected though the damper responds

by opening and the mixed air temperature drops. In cooling season, the economizer is 

called on later and the cooling coil valve signal is increased to bring the supply air 

temperature to the setpoint. The actual supply air temperature will be too low and will 

require reheat at VAV box but APAR detects no fault. 

3.5.2.2 Return Air Temperature Sensor Drift (Faults 3 & 4) 

Return air temperature sensor drift was introduced as a sensor offset for a range of (0 to 

+4) oC, also applied linearly over the two-week simulation period. In this simulation, the

return air temperature measurement is used in the determination for economizer use, but 

not for damper control. Therefore, for heating and swing season, where the cooling load 

35
conditions are such that the economizer is meeting the cooling load, the control is 

unaffected. The change in the measurement of the return air temperature does not trigger

an APAR rule violation. In cooling season, the outdoor air enthalpy is higher and 

therefore the perceived reduction in return air temperature causes the economizer to be 

called on later but with the low outdoor air fraction, APAR warns that energy may be 

wasted by not using economizer (Rule 18). This type of fault would not affect thermal

comfort, as the supply air temperature will be met by mechanical cooling. However, 

additional cooling load is being introduced to the building and thereby resulting in 

increased energy use. 

A second simulation, Fault 4, was carried out to introduce a sensor offset in the range of 

(0 to –4) oC. The effect of negative sensor drift for the return air temperature with no 

change to the humidity reading would result in a reduced recirculation air enthalpy. This 

false reading is input to the economizer control and depending on the operating 

conditions, the controller may open the recirculation air damper while reducing the 

outdoor air damper position to the minimum setting. The system may stop use of the 

economizer due to the false readings and switch to mechanical cooling with minimum

outdoor air. 

For Fault 4, control in heating and swing season is unaffected and the increase in return 

temperature does not trigger an APAR rule violation. For cooling season, the economizer 

use is extended and APAR detects that the mixed air temperature reading is lower than 

outdoor air temperature or return air temperature and triggers violation of Rule 26. This 

type of fault would also not effect thermal comfort as the supply air temperature will be 

met, but energy use is increased. 

3.5.2.3 Mixed Air Temperature Sensor Drift (Faults 5 & 6) 

Mixed air temperature sensor drift was introduced as a sensor offset for a range of (0 to 

+4) oC, also applied linearly over the two-week simulation period. The mixed air 

temperature measurement is also used in the determination for economizer use, but not 

for damper control. In this case, control is unaffected regardless of season. However, for 

heating and swing season the change in the measurement of the mixed air temperature 

relative to the supply air temperature reading does trigger an APAR rule violation (Rule 

7). In Mode 3, it is expected that the mixed air temperature will closely track the outdoor 

air temperature and that the mixed air temperature will be greater than the outdoor or 

return air temperatures. Because these statements were not true, Rules 10 and 26 were 

violated in the swing and cooling seasons. A related Rules 18 is also violated in cooling 

season. 

 

For Fault 6, a simulation was carried out to introduce a sensor offset in the range of(0 to –

4) oC which shows that a similar pattern exists due to the positive sensor drift. Once

again control is unaffected, but the relationships between the temperature measurements

cause several rule violations. 

36
3.5.2.4 Outdoor Air Temperature Sensor Drift (Faults 7 & 8) 

Outdoor air temperature sensor drift was introduced as a sensor offset for a range of (0 to 

–4) oC, also applied linearly over the two-week simulation period. The outdoor air 

temperature measurement is used in the determination for economizer use, but not for

damper control. Therefore, for heating season, the control is unaffected and the change 

in the measurement of the outdoor air temperature does not trigger an APAR rule

violation. In swing season, control is still unaffected because the economizer is still in

use, but the perceived difference between the outdoor air temperature and the mixed air 

temperature is too great when considering that the outdoor air damper is fully opened.

APAR detects this condition and determines that Rule 10 has been violated. In cooling 

season, the perceived outdoor air enthalpy is lower and therefore the perceived reduction 

in outdoor air temperature causes the economizer to be cut off later. For swing and 

cooling season, APAR detects the Rule 10 violation and also determines that in cooling 

season the mixed air temperature is reading higher than the return air temperature and the 

outdoor air temperature, a violation of Rule 27. 

A second simulation, Fault 8, was carried out to introduce a sensor offset in the range of 

(0 to +4) oC. In this simulation, control in heating and swing season is unaffected as the 

outdoor air temperature sensor reading increases. For cooling season, the economizer use 

is reduced because of the perception that the outdoor air temperature has increased. 

APAR is able to detect this fault in all cases except for heating mode. In cooling and 

swing season there is at least one day where the damper is commanded to be fully open 

and APAR detects that difference between the outdoor air temperature and the mixed air 

temperature is not reasonable, violating Rule 10. 

3.5.2.5 Outdoor Air Damper Fault (Fault 10) 

This fault was introduced to the outdoor air damper by setting the control signal for the 

motor-driven actuator to the stuck closed position, causing the damper to stay at the 

minimum open position throughout the simulation. The result of this is that no 

economizer action can be taken and mechanical cooling will have to be used exclusively. 

The control system response for this fault was varied. In heating season, the damper 

opening is increased from 40 % open to 80 % open. In swing season, the damper 

opening was also increased, and over several days the cooling coil is activated. In

cooling season, the control system calls for mechanical cooling with minimum outdoor 

air. Therefore, the system is insensitive to the fault. APAR was only able to detect a rule 

violation for Fault 10 in swing season because it expects the mixed air temperature and 

the outdoor air temperature to be roughly equal for conditions of 100 % outdoor air. In 

heating season, the damper is not called to be fully open and in cooling season, the 

economizer cycle is off. 

3.5.2.6 Recirculating Air Damper (Faults 15-16) 

This fault was introduced to the recirculation air damper by setting the control signal for 

the motor-driven actuator to three different levels: stuck open, 40 %, 10 % leakage, and 

stuck closed. 

37
The stuck closed recirculation air damper fault causes the damper position to prevent air 

from being recirculated. For the case of free cooling with 100 % outdoor air, the 

recirculation damper is already closed. For conditions that the outdoor enthalpy is higher 

than the return air enthalpy, the system is not operating under the economizer cycle, the 

outdoor air damper would be set to its minimum, and the fault would cause the 

recirculation damper to be closed, hence the airflow may be reduced. This was only 

evident during cooling season where Faults 18, 19 and 25 were triggered. In the twoweek simulations for both heating and swing season, the stuck closed recirculation

damper had no effect on the control system response and was not detected by APAR. 

The simulation runs at 40 % leakage and 10 % leakage are meant to test the ability of the 

fault detection and diagnostic tool to detect lower level faults. In the case that the return

air enthalpy is greater than that of the outdoor air, additional cooling will be required.

However, the 10 % and 40 % leakage simulations caused no notable change to the 

control, regardless of season. Furthermore, the only rule violated was during swing 

season when it was detected that the outdoor air temperature did not match the mixed air 

temperature. This fault does not apply to the summer conditions when the economizer

cycle is not used. 

3.5.2.7 Cooling Coil Valve Faults (Faults 19-22) 

This fault was introduced to the cooling coil valve by setting the control signal for the 

motor-driven valve actuator to four different levels: stuck closed, 10 % leakage, 25 % 

leakage, and 40 % leakage. The various leakage rates are meant to test the sensitivity of 

the fault detection tool. 

The cooling coil is needed to maintain the supply air temperature setpoint when cooling 

with outdoor air is insufficient. The stuck closed cooling coil valve fault causes the valve 

position to prevent cold water from being circulated through the cooling coil. This fault 

is easily detected when the cooling load is high, primarily swing season and cooling

season. Looking at the APAR results, it is shown that Fault 19 is not detected during 

heating season. This is logical because the heating season cooling load is met by use of 

the economizer having low outdoor temperatures. As the seasonal temperatures are 

higher for swing season and cooling season, the cooling coil valve is called open. APAR 

also identifies that the control system is not meeting the cooling load in swing season and 

cooling season while the cooling coil signal is not at a maximum.

During swing season and cooler outdoor air conditions, the economizer can provide 

sufficient cooling. Therefore, having a leaking cooling coil valve will cause the supply

air temperature to drop and the VAV box reheat coil would have to be activated to bring 

the temperature up to the zone setpoint. This represents a tremendous waste of energy as 

the cooling coil and reheat coil are working against each other. For the leaking cooling 

coil faults, the control system calls for an end to economizer operation and activation of 

the heating coil valve. In all cases, control of the heating coil becomes unstable for fault 

21 and 22. Various rule violations are reported, showing that many of the expected

temperature relationships are not being met. In addition the system is incapable of

38
maintaining the supply air temperature setpoint. This type of fault could go undetected in 

a system because unless the leak is larger than the heating capacity of the reheat coil, the 

zone temperature will be maintained. 

3.5.2.8 Heating Coil Valve Faults (Faults 23-26) 

These faults were introduced to the heating coil valve by setting the control signal for the 

motor-driven valve actuator to four different levels: stuck closed, 10 % leakage, 25 % 

leakage, and 40 % leakage. The various leakage rates are meant to test the sensitivity of 

the fault detection tool. 

The heating coil valve is not activated due to cold weather conditions, rather the heating 

coil valve is opened when the system is at risk of freezing or if a cooling coil leak exists.

Therefore, Fault 23, a stuck heating coil valve causes the valve position to prevent hot

water from being circulated through the heating coil but affects no change to the control 

of the air handling unit and furthermore was not detected by the APAR fault detection

tool. 

In contrast, even a 10 % valve leakage caused the dampers to be fully open, during

heating and swing season to make use of the economizer cycle and additionally called for 

the cooling coil valve to be activated. For cooling season, the economizer is off and the 

cooling coil must meet the full cooling load. As the severity of the fault increased to 25 

%, the control system response during heating and swing season shows a further opening 

of the cooling coil valve. For the cooling season, the cooling coil was at its maximum 

open position and the supply air temperature was not maintained, which is noted by the

violation of Rule 19. 

3.5.2.9 VAV Box Faults (Faults 27-30) 

Although the VAV box faults are downstream of the air handling unit, APAR was used to 

evaluate the operational data to determine whether the tool was robust enough to pick up 

any faults. APAR is not designed or intended to reliably detect faults in the attached 

VAV boxes, but may be useful in identifying when a problem exists outside of the air 

handling unit. The results shown in Table 3.5 show that in swing season there were two 

faults that caused an APAR rule violation. The first evidence of faulty operation came

from the VAV box damper stuck closed. It shows Rule 28 was violated. It is not evident 

whether the operating point caused the excessive mode switches, or if in fact the closed 

damper reduced the airflow to a level that the system began to fluctuate between modes 

of operation in an effort to maintain the supply set point. The second evidence of faulty 

operation was detected for the reheat valve stuck open. In this case we see that the

system is unable to maintain the supply temperature setpoint. 

3.6 Results from AHU Emulation in the VCBT 

Emulation, using the VCBT as described in section 2, was used in addition to simulation, 

to test and refine APAR. The emulation consisted of two floors in a commercial office

building with a VAV AHU and three VAV boxes for each floor. 

39
3.6.1 Faults Implemented 

The faults tested in emulation were selected to represent common faults that had potential 

to be detected using typical sensor measurements. 

3.6.1.1 Supply Air Temperature Sensor Drift 

The occurrence of sensor drift for the supply air temperature sensor was introduced as a

sensor offset for a range of (0 to –4) oC, applied linearly over the two-week simulation 

period. A negative sensor offset would prompt the control logic to raise the supply air 

temperature. The VAV box controls will work to compensate. First, the VAV box 

dampers will open. As the severity of this type of fault is increased, the thermal comfort 

in the room is compromised and the condition of the high zone temperature and open 

dampers will trigger a reset of the supply air temperature to a lower setpoint. 

3.6.1.2 Outdoor Air Temperature Sensor Drift 

The outdoor air temperature sensor is an input to the economizer control. It is used to 

calculate the outdoor air enthalpy which is compared to the recirculation air enthalpy. 

The effect of negative sensor drift for the outdoor air temperature with no change to the

humidity reading would result in an increased outdoor air enthalpy. 

3.6.1.3 Stuck Outdoor Air Damper 

This fault was introduced to the outdoor air damper by setting the control signal for the 

motor-driven actuator to the stuck closed position, causing the damper to stay at the 

minimum open position throughout the simulation. The result of this is that no 

economizer action can be taken and mechanical cooling will have to be used exclusively. 

3.6.1.4 Stuck Recirculation Air Damper 

This fault was introduced to the recirculation air damper by setting the control signal for 

the motor-driven actuator to stuck open. This allows excess return air to be recirculated 

and mixed with the outdoor air in the free cooling mode or mechanical cooling with 100 

% outdoor air. For conditions that the outdoor enthalpy is lower than the return air 

enthalpy, the control system will have to respond by further opening the outdoor air 

damper. 

3.6.1.5 Stuck Cooling Coil Valve 

This fault was introduced to the cooling coil valve by setting the control signal for the 

motor-driven valve actuator to four different levels: stuck closed, 10 % leakage, 25 % 

leakage, and 40 % leakage. During swing season and cooler outdoor air conditions, the 

economizer can provide sufficient cooling. Therefore, having a leaking cooling coil 

valve will cause the supply air temperature to drop and the VAV box reheat coil would 

have to be activated to bring the temperature up to the zone setpoint. This represents a 

tremendous waste of energy as the cooling coil and reheat coil are working against each 

other. This type of fault could go undetected in a system because unless the leak is larger 

than the heating capacity of the reheat coil, the zone temperature will be maintained. The 

10 %, 25 %, and 40 % leakage rates are meant to test the sensitivity of the fault detection

tool. 

40
3.6.2 Normal Operation 

Normal data was collected for the two AHUs in heating season, swing season, and 

cooling season. These data represents the “no fault” case and is used as a benchmark for

comparison to faulty data. One week of normal data was collected in real time for each 

of these cases. Expert knowledge of the system combined with the trending data from the 

input signals to the control system was used to verify the system response. In Figure 3.6,

the top graph shows the input temperatures during typical operation in heating season for 

Controller A. This data show the natural fluctuations in outdoor air temperature, and the 

ability of the control system to maintain the temperature setpoint within an allowable

threshold. The lower graph in the Figure 3.6 shows the control system response to 

control the cooling coil, heating coil and outdoor air damper position. Here it is evident 

that even in heating season, the heating coil valve is closed and the building cooling load 

is met by use of the enthalpy-based economizer. The economizer determines the 

enthalpy of the return air and outdoor air to determine whether it is advantageous to use 

outdoor air to provide the needed cooling. For the weather conditions selected to 

represent heating season, we see that a 40 % to 60 % damper position is selected and the

system is “free” cooling which is designated as Mode 2 (Fig 3.2). 

For Controller B, identical weather data is used. The only difference is the control logic 

used to maintain the setpoint temperature; Controller A controls the system based on the 

supply air temperature reading while Controller B controls based on the outdoor, return, 

and mixed air conditions. For Controller B, the supply air and the mixed air temperature 

are almost the same over the range of operating conditions and for the occupied periods,

there is significantly less fluctuation of the damper position. 

For swing season, the outdoor air no longer provides sufficient cooling to meet the

buildings cooling load. This weather data triggers the control system to open the outdoor 

air damper to maximum when the outdoor air enthalpy is less than the return air enthalpy, 

and in addition, the cooling coil valve position is opened as required. The data shown in 

Figure 3.7 for controller B is similar to data from controller A. The last season evaluated 

was the cooling season. No fault cases were evaluated in cooling season, however, it was 

41
Figure 3.6: Fault-free Heating Season Data for Controller A 

 

 Figure 3.7: Fault-free Cooling Season Data for Controller B

42
Figure 3.8: Fault-free Swing Season Data for Controller B

necessary to check that the cooling coil was not undersized or oversized for the loads. By 

evaluating Figure 3.8, it is evident the cooling coil is operating within an appropriate

range. Three no fault cases and 24 fault cases were processed for each control system 

using APAR, the results of which will be presented in the next section. 

3.6.3 APAR Analysis & Results 

The rules and thresholds specified in APAR are defined based on heuristic knowledge 

and not training data, but data from normal operation can be used as a means to test the 

assumptions that define normal, fault-free operation. It was anticipated that all of the

Normal data evaluated with APAR would be classified according to the mode of 

operation and deemed fault free for all cases. This was not the case. Table 3.6 shows the 

rule violations associated with the normal operation. For swing season, Rule 28 was 

violated. This rule states that there are too many mode switches, which in this case was a

switch from Mode 2 (free cooling) to Mode 3 (Mechanical cooling with maximum 

outdoor air). The control system is at a point where the cooling coil is fluctuating around 

the threshold that defines Mode 3, specifically, that the cooling coil signal must be

greater than 0.01 on the range of 0 to 1. As this is the benchmark for normal operation, it

was anticipated that this same fault might appear in other test runs as will be discussed in 

this section. 

43
Table 3.6: APAR Results for Normal Data 

Control System A Control System B 

Heating Season No rule violations No rule violations 

Swing Season 28 No rule violations 

Cooling Season No rule violations No rule violations 

Five fault cases were evaluated by applying APAR rules to the output for each of the 

controllers (section3.6.1). The results are listed in Tables 3.7 and 3.8 and are discussed in 

detail below for each specific fault case. The fault numbers listed refer to Table 3.5 while 

the rule violations refer to Table 3.1. 

Table 3.7: APAR Results for Heating Season Fault Data 

Heating Season Rule Violations 

Fault Number Control System A Control System B 

Fault 01 (Supply Temp. Sensor Drift) 7 7,25 

Fault 14 (Stuck Open Return Air Damper) No rule violations 7,25 

Fault 20 (Cooling Coil Valve Leakage,10 %) 1,25 7,25 

Table 3.8: APAR Results for Swing Season Fault Data 

Swing Season Rule Violations 

Fault Number Control System A Control System B 

Fault 07 (Outdoor Temp. Sensor Drift) 28 25,28 

Fault 10 (Stuck Closed Outdoor Air Damper) 10 10,28 

The first fault case evaluated in heating season is that of supply air temperature drift, fault 

1. This particular fault clearly highlights the difference in operation for the two control 

systems. The fault data collected over the one-week period is plotted in Figure 3.9 for

Controller A and Figure 3.10 for Controller B. Both controllers are receiving the same

input date showing a range of (0 to -4) oC sensor drift for the supply air temperature 

sensor. However, in Controller A, the system uses the supply air temperature reading as 

an input to the control logic which responds by closing the outdoor air damper in an 

effort to raise the supply air temperature. This in turn causes the mixed air temperature to 

rise while the supply air temperature set point is maintained, as seen in Figure 3.9.

Controller B, shown in Figure 3.10 operates using control logic similar to the pure

simulation case. Here we know that the control system does not use the supply air 

temperature as an input to the control logic. Instead the damper position is determined 

based on the outdoor, return, and mixed air conditions. Because this is a simulated fault, 

there are only natural variations in the outdoor, return, and mixed air temperatures, and 

the damper position shows no change when compared to the normal operation. APAR 

results are good, with a detection of the fault made in both cases by day two, when the 

fault level is approximately 1.5 oC . 

44
Figure 3.9: Fault 1 Heating Season Data for Controller A 

Figure 3.10: Fault 1 Heating Season Data for Controller B 

45
Figure 3.11: Fault 14 Heating Season Data for Controller A 

The second fault case evaluated in heating season is a stuck recirculation air damper, 

Fault 14. In this fault, warm return air is recirculated back into the system regardless of 

the enthalpy of the return air. Figure 3.11 shows the temperature plot and control signal 

plots for Controller A. Here it is noted that as the outdoor temperature rises, the damper

is opened and reaches the full open position halfway through the occupied period. 

Comparatively, the no fault case only has roughly a 60 % open outdoor air damper

position for the same conditions. Subsequently, as the supply air temperature keeps

rising and exceeds the setpoint temperature, the control system shifts into Mechanical 

cooling with 100 % outdoor air. The cooling coil must compensate for the additional 

heat returned to the system. The results of APAR for this case, as listed in Table 3.5 

show no rule violations. In this case the system is able to compensate for the fault and

the rule is masked. 

46
Figure 3.12: Fault 14 Heating Season Data for Controller B 

For controller B, a similar response is attained for fault 14. As the outdoor temperature 

rises, the damper is opened and reaches the full open position early in the occupied 

period. Comparatively, the no fault case only has roughly a 60 % open outdoor air 

damper position for the same conditions. Subsequently, as the supply air temperature 

keeps rising and exceeds the setpoint temperature, the control system shifts into

Mechanical cooling with 100 % outdoor air. The cooling coil must compensate for the 

additional heat returned to the system. However, it is evident in Figure 3.12 that once the

system calls for the cooling coil response, we see a cycling room temperature as the 

cooling coil alternates the valve position. This appears to be a tuning issue. The results of 

APAR for this case show two faults. First, a Rule 7 violation identifies that there is too

large a difference between the supply and mixed air temperatures. Secondly, a Rule 25 

violation identifies that the difference between the supply air temperature setpoint and the 

supply air temperature measurement is too great. 

47
Figure 3.13: Fault 20 Heating Season Data for Controller A 

The third fault case evaluated in heating season is that of stuck cooling coil valve, Fault 

#20. The response for Controller A is shown in Figure 3.13. Under the fault condition,

the 10 % valve leakage causes the damper to change from 50 % open to its minimum 

position. This provides 15 % outdoor air and 85 % return air. In the top graph we see 

that the mixed air temperature closely tracks the return air temperature. However, the 

temperature of the supply air is lower than the supply air temperature setpoint and the 

heating coil must be activated at a level of approximately 15 %. APAR results show 

Rule 1 and Rule 25 violations for Controller A. The Rule 1 violation shows that the 

supply and mixed air temperature do not correlate in the heating mode. This is the 

expected alarm. In addition the Rule 25 violation shows that the control system was 

unable to maintain the setpoint. 

Controller B finds the same rule violations, but applied to Mode 2 because the controller 

is cooling with outdoor air. The response for Controller B is not shown because the 

control logic does not control based on supply air temperature and therefore behaves as if 

there is no fault. The only symptom that appears is that the supply air temperature drops 

as a result of the cooling coil leak. In a real world application, the reheat coil at the VAV 

boxes would have to compensate by adding heat. This represents very expensive and 

inefficient operation, as use of the reheat coil is an expensive way to counteract the fault. 

48
Figure 3.14: Fault 7 Swing Season Data for Controller B 

 

The first fault case evaluated in swing season is that of outdoor air temperature drift, fault 

07. Both controllers are receiving the same input date showing a range of (0 to -4) oC

sensor drift for the outdoor air temperature sensor. However, in Controller A, the system

uses the supply air temperature reading as an input and therefore a false reading at the 

outdoor air temperature is not expected to be detected. The results however, show a rule 

28 violation. This violation states that the allowable number of mode transitions per hour 

has been exceeded. This is a tuning issue. For Controller B the system control is 

determined based on the condition of the outdoor, return, and mixed air, similar to the 

pure simulation case. The fault data collected Controller B over the one-week period is 

plotted in Figure 3.14. It shows that as the fault level increases, the system begins to 

have difficulty maintaining the setpoint. This triggers a violation of rule 25, where the 

supply air temperature is not equal to the supply air setpoint. In addition, for selected 

ranges of operation APAR detected excessive mode switching. 

49
Figure 3.15: Fault 10 Swing Season Data for Controller B 

The second fault case evaluated in swing season is that of stuck closed outdoor air 

damper, fault 10. When the outdoor air damper is stuck closed, only the minimum 

ventilation of 15 % enters the building. The rest is made up of return air, which has a 

higher temperature than the outdoor air for most of the simulation days. This fault 

effectively disables the economizer from making use of free cooling. As a result, the 

damper is commanded open earlier than for the no fault condition, and there is increased 

cooling coil activity to bring the return air temperature to the supply air temperature 

setpoint, as seen in Figure 3.15. APAR results for this data show one rule violation, Rule 

10 for both controllers. This rule states that there is an unexpected difference in the 

mixed air temperature and the outdoor temperature. In Mode 3, mechanical cooling with 

100 % outdoor air, the outdoor air should be roughly equal to the mixed air. APAR finds 

that this is not the case for Controller A and Controller B, triggering faults in each case.

In addition, for Controller B, a Rule 28 violation signals excessive mode switching. 

Upon further analysis, it is determined that the cause of this is the fluctuation of the 

cooling coil valve when the load is slightly larger than the economizer can handle with 

free cooling. This mode switching violation is one that has proven recurrent in swing 

season data. 

50
3.7 APAR User Interface 

The user interface is instrumental to the successful application of an FDD method. This 

section will present FDD_AHU1.1, which is a front end to the APAR code which was 

developed by the Centre Scientifique et Technique du Bâtiment [3]. It enables batch 

processing of system data and in the future will be improved to process on-line streaming 

data. To evaluate the user interface and test the implementation of APAR, archived AHU 

data from Montgomery College was processed and the output compared to output 

produced by a separate implementation of APAR [5]. 

The test case used to develop this interface uses data from two air-handling units. Data 

are imported as a text file and processed to detect and diagnose faults. Figure 3.16 shows

the files available for import. The naming convention for the imported data files is 

significant because the first two characters in the name represent the type of air-handling

unit being evaluated. In this example, it is air-handler 1, denoted by “a1”. The 

corresponding rules differ slightly depending on the AHU number because some units 

use an enthalpy-based economizer while others use a temperature-based economizer.

The information regarding the type of air-handlers being evaluated is selected in the

initial software setup. 

Figure 3.16: FDD_AHU Import Data Figure 3.17: FDD_AHU Fault List 

This software is designed to implement the APAR rules, which define temperature 

relationships in the AHU that are dependent on the mode of operation. If a rule is 

satisfied, a fault is indicated, but there is no diagnosis of the exact nature of the fault. 

Within the program, users are capable of enabling or disabling rules. S specific faults can

be deselected so the corresponding rule is not evaluated and therefore does not trigger an 

alarm. This feature, shown in Figure 3.17, is useful when service has already been called 

regarding a specific fault because it gives users the ability to acknowledge receipt of the 

alarm status and remove it from evaluation as the rest of the system is checked. 

51
Once the data is imported and the evaluation criteria are selected, the analysis can be run

by clicking the “go” button. This processes the data for the week and generates a color 

coded one-week calendar that summarizes the operation of a given AHU for that week

(see Figure 3.18). Three levels of granularity can be shown within this one-week 

calendar. Days with no faults are white. Days with 1hour to 4 hours of faults are colored 

beige. Finally, days with more than 5 hours of faults are colored red. It should be pointed 

out that the APAR rules are evaluated hourly, so the presence of two faults indicates that 

there were two hours of operation during that day that represented faulty operation. This 

user interface has the capability to present information both temporally (a number of days 

of operation is shown rather than a snapshot of current operation) and spatially (a number 

of AHUs can be shown simultaneously to understand how their operation compares). 

igure 3.18: Summary of Fault Detection Results for One-week Data 

igure 3.19 illustrates results for one day (Monday) selected from week 12 of year 2000. 

F

F

Here a color coded key shows the periods of faulty, unknown, and fault free operation 

along with the mode of operation associated with a particular hour within a day. The first 

row lists the time of day while the second row shows the dominant fault cause by 

number. Fault causes are provided in a list at the bottom of the window. Also shown in 

this window are the mode of operation and operating condition for each hour of the day. 

Note that “multiple modes” of operation indicates that the mode switched at least once in 

that hour. If the mode switches too often (more than six times for all data considered 

52
Figure 3.19: Summary of Fault Detection Results and Possible Causes

Figure 3.20: Temperature graph 

53
Figure 3.21: Control signal graph 

here), a fault is indicated. Otherwise, hours that have multiple modes of operation are 

indicated to be normal. The three buttons to the right side of the window, “Details”, 

“Graphic T” and “Graphic CMD” provide a greater depth of information for the user. 

The details window, not shown here, is a text file listing the specific hours where faults 

were detected and the associated causes. The “Graphics T” window (see Figure 3.20) 

shows a temperature versus time plot for several key sensors while the “Graphic CMD”

window (see Figure 3.21) shows the relevant control signals. This is intended as a 

troubleshooting tool for users and is also useful in debugging the software to ensure that 

the logic is applied correctly. 

54
4 FDD FOR VAV 

4.1 VAV box Performance Assessment Control Charts - VPACC 

The primary purpose of heating, ventilating, and air-conditioning (HVAC) equipment in 

commercial buildings is to provide a comfortable and healthy environment for occupants. 

Variable-air-volume (VAV) air handling systems are common for conditioning air and 

delivering the air to occupied zones. VAV boxes are an integral part of such systems and 

are the final piece of equipment that air passes through prior to reaching the occupants. 

As such, it is important to ensure that these devices operate correctly. 

The challenges presented in detecting and diagnosing faults in VAV boxes are similar to 

those encountered with other pieces of HVAC equipment. Generally there are very few 

sensors, making it difficult to ascertain what is happening in the device. Limitations 

associated with controller memory and communication capabilities further complicate the 

task. The number of different types of VAV boxes and lack of standardized control 

sequences add a final level of complexity to the challenge. This set of constraints is

counterbalanced by the fact that VAV boxes are much more numerous than other pieces 

of HVAC equipment. For instance, buildings may have ten to fifteen times more VAV 

boxes than air-handling units. Hence, maintenance staffs would clearly benefit from a

tool that assisted them in monitoring VAV box operation. 

The needs and constraints described above have led to the development of VAV Box 

Performance Assessment Control Charts (VPACC), a fault detection tool that uses a 

small number of control charts to assess the performance of VAV boxes. The underlying 

approach, while developed for a specific type of VAV box and control sequence, is 

general in nature and can be adapted to other types of VAV boxes. This section describes 

the basic concept of control charts and their use for determining when control processes

have gone “out of control”. The specific control charts developed and implemented in 

VPACC are then presented for a single duct pressure-independent throttling VAV box 

with reheat. Results obtained by applying VPACC to data obtained from real VAV boxes 

at the ERS, from computer simulation, and from real-time emulation using the VCBT are 

then described. Finally, conclusions and future work pertaining to the testing of VPACC 

are presented. 

4.1.1 Control Charts 

Control charts are common tools for monitoring control processes wherein a measured 

quantity is compared to upper and lower limits that define allowable (or fault free) 

operation. If the measured quantity falls outside these limits, the process is said to be “out 

of control”. The limits are typically defined using statistical parameters and, therefore, 

control charts are often referred to as statistical quality control charts. 

There are many different types of control charts. VPACC implements an algorithm

known as a CUSUM (cumulative sum) chart. The basic concept behind CUSUM charts is 

to accumulate the error between a process output and the expected value of the output.

Large values of the accumulated error are indicative of an out of control process. With

55
the process output at sampling time i denoted xi, the estimate of the expected value 

denoted x , and the estimate of the process standard deviation denoted by , the 

normalized process output is given by: 

σˆ

σ = ˆ

x - x

z i

i (1) 

The normalized process output is used to compute two cumulative sums defined as 

follows: 

S max [ 0, z - k S ] (2) i = i + i-1

T max [ 0, - z - k T ] (3) i = i + i-1

where k is a slack parameter that must be specified. Positive values of z greater than k 

cause the sum S to increase and the sum T to decrease (or remain at zero). Negative 

values of z whose absolute values are greater than k cause the sum T to increase and the 

sum S to decrease (or remain at zero). A process is said to be out of control when either S 

or T exceeds a threshold value defined by the parameter h. Figure 4-1 [6] presents 

normalized data and the S and T cumulative sums for k = 0.5 and h = 5. The first 20 data 

points come from a random normal distribution with a mean value of zero and a standard

deviation of unity. The mean value is then increased to 0.25, 0.5, 0.75 and 1.0 for

subsequent sets of 20 data points. Note that S exceeds the threshold value of h after about 

68 data points. Because the mean value increases above 0, the cumulative sum T remains 

below the threshold. 

Sample Number

0 20 40 60 80 100

z

-3

-2

-1

0

1

2

3

Sample Number

0 20 40 60 80 100

C

u

m

ulative Sums

0

5

10

15

20

T

S

Upper Chart Limit

Figure 4.1: A simple CUSUM control chart indicating an “out of control” process. 

 

CUSUM charts are generally considered to be effective for detecting gradual shifts in the

process mean. The most commonly used control charts are Shewhart and Shewhart-type 

charts. Shewhart charts are effective for detecting large, sudden changes in the process 

mean. Generally Shewhart chart limits are set at values of x ± 3σˆ . In terms of the 

56
normalized parameter z, the chart limits are z = ± 3 . Shewhart charts were not 

investigated as part of this study; however, it is interesting to note that the basic CUSUM 

and Shewhart charts are equivalent if the CUSUM parameters k and h are selected as k =

3 and h = 0. 

 Coil

o

Hot Water 

Return

 Coil

o

Hot Water 

Return

4.1.2 System Description 

Figure 4-2 is a schematic diagram of a typical single duct variable-air-volume (VAV) box 

with hydronic reheat. The diagram depicts a damper that is used to modulate airflow to

the zone and a control valve that modulates hot water flow to the reheat coil. Several 

sensors are also shown in Figure 4-2. The zone thermostat measures the air temperature 

in the zone. The differential pressure transducer is used to measure the flow rate of air 

into the zone. Finally, the discharge air temperature sensor measures the temperature of

the air stream entering the zone. This sensor is used to provide diagnostic information 

rather than for control purposes. The VAV box controller reads the sensor information,

computes control outputs for the damper and reheat valve, and transmits these signals to

the appropriate actuators.

Room

H

C

DP

Hydronic

Reheat

VAV Box Controller

Thermostat

T

Damper

Hot Water Supply

M

M

Supply Air

Room

H

C

DP

Hydronic

Reheat

VAV Box Controller

Thermostat

T

Damper

Hot Water Supply

M

M

Supply Air

Figure 4.2: Schematic diagram of a single duct pressure-independent VAV box with 

hydronic reheat. 

Control systems for pressure-independent VAV boxes commonly use a cascade control 

strategy to maintain the zone temperature at the set point value. A typical control 

sequence is shown graphically in Figure 4-3. A heating set point and a cooling set point 

are specified. As the zone temperature increases above the cooling set point, the airflow 

57
rate to the zone increases proportionally. This is accomplished by resetting the setpoint

value of the airflow rate and modulating the damper to achieve this flow rate. As the zone 

temperature decreases below the cooling set point, the damper gradually decreases until it 

is providing the minimum flow rate necessary for ventilation. If the room temperature

continues to decrease and reaches the heating set point, the reheat valve will begin to 

open. The airflow rate can also be varied in the heating mode, with the airflow increasing 

as the temperature decreases. Alternatively, a higher fixed airflow rate may be specified 

for heating operation to improve the distribution of the warm air. In Figure 4-3, it is 

assumed that a fixed airflow rate associated with the ventilation requirement of the room 

is provided in the heating mode. 

igure 4.3: Damper and valve control sequence as a function of room temperature 

1.3 CUSUM Applied to VAV Box Diagnostics 

box control strategy. However, a 

from its value during normal operation, which can be detected by a CUSUM chart. 

Airflow

Setpoint

Heating

Set

Point

Cooling

Set

Point

Room

Temperature

Valve

Position

Fully Maximum

Open

Fully

Closed

Airflow 

Setpoint

Valve

Heating

Set

Point

Cooling

Set

Point

Room

Temperature

Valve

Position

Minimum

Fully

Open

Fully

Closed

Valve

F

for a single duct pressure-independent VAV box with hydronic reheat.

 

4.

The previous section described one particular VAV

wide variety of control strategies are employed by controller manufacturers, most of 

which use a cascaded control loop to maintain the zone temperature and zone airflow rate 

at setpoint values. In order to make VPACC independent of the control strategy used in a 

particular controller/VAV box application, three generic errors were identified: the

airflow rate error, the temperature error, and the reheat coil differential temperature error. 

As long as the VAV box controller has an airflow setpoint, as well as heating and cooling 

temperature setpoints, VPACC will function independently of the control strategy used. 

Each fault considered in this study will result in a deviation of one or more of these errors

58
The airflow rate error, Qerror, is defined as 

Qerror = Qactual - Qsetpoint

Qactual is the actua point is the airflow rate setpoint. The CUSUM of 

is error will detect stuck damper faults, differential pressure sensor faults, and unstable

e error, Temperror, is defined as 

If Temproom > CSP 

Temperror = 0 : If HSP <

l airflow rate and Qset

th

airflow faults. 

The temperatur

Temperror = Temproom – CSP : 

 Temproom < CSP 

Temproom is the ro cooling setpoint, and HSP is the heating 

tpoint. The CUSUM of the temperature error will detect damper faults, valve faults,

erature error, ∆Terror, is defined as 

∆Terror = 0 : If uhc ≠ 0 

DAT is the discha erature (the temperature of the air leaving the reheat coil),

AT is the entering air temperature (the temperature of the air entering the reheat coil),

ulated during occupied periods. During 

noccupied periods, the errors are not computed, and the CUSUMs are reset to zero. The 

ost of the points required by VPACC are already available in the local VAV box 

cooling setpoint, heating setpoint, airflow rate setpoint, 

Temperror = HSP - Temproom : If Temproom < HSP 

om temperature, CSP is the

se

and temperature sensor faults. The specific definition of temperature error used in this 

report is based on the control sequence described above. Various other commonly used 

control sequences may require changes to the definitions of heating setpoint, cooling 

setpoint, and temperature error. 

The reheat coil differential temp

∆Terror = DAT – EAT : If uhc = 0 

rge air temp

E

and uhc is the control signal to the reheat coil valve. The CUSUM of the reheat coil 

differential temperature error will detect a leaking valve fault. Although this error is only 

used to detect one fault, this is a fault that highlights the advantages of automated FDD. 

Without VPACC, the local controller masks this fault from the occupants by increasing 

the airflow rate into the space. There will be no “too hot” or “too cold” complaints, so a 

significant energy penalty may be accrued. 

The errors and CUSUMs are only calc

u

first hour of the occupied period is treated the same as the unoccupied period, to allow 

steady state conditions to develop. 

4.1.4 Point requirements 

M

controller: room temperature,

actual airflow rate, and occupancy status. Entering air temperature is typically not 

available, so supply air temperature (available over the control network from the AHU 

controller) could be used. Many VAV boxes are equipped with a discharge air 

temperature sensor, which VPACC needs in order to calculate the reheat coil differential 

59
temperature error. If a discharge air temperature sensor is not available, a simplified 

version of VPACC could be used, implementing the airflow rate error and the

temperature error only. In this case, a leaking reheat coil valve would not be detected. 

4.1.5 Threshold Selection 

Two different sets of thresholds are used in the CUSUM method. Each of the three errors 

as a slack parameter associated with it, which serves to filter out the variation of that 

AV box

pes, sizes, and control strategies. Data taken during normal operation will be used to 

Accuracy Requirements

PACC uses existing points in the building automation system to perform all 

lready installed for control purposes 

he ERS has two test VAV air-handling systems, referred to as AHU-A and AHU-B,

l of that found in commercial 

ets include normal operation data and data for three faults, namely, a failed

ifferential pressure sensor, a hydronic reheat coil valve stuck partially open, and an 

essure Sensor

his fault was introduced during an unoccupied period by disconnecting both tubing 

fault is expected to cause the VAV box

h

error during normal operation. In addition, the S (positive) and T (negative) cumulative 

sums of each error have alarm limits, which identify when a fault has occurred. 

In order to determine threshold values, data must be collected for a variety of V

ty

determine slack parameters, and data for operation with various faults will be used to 

determine alarm limits. 

4.1.6 Instrumentation

V

calculations. The industrial grade sensors that are a

have sufficient accuracy. Laboratory grade instruments are not required. 

4.2 Results from VAV Box Laboratory Experiments 

T

each serving four zones. The HVAC equipment is typica

buildings. The VAV boxes are single duct throttling units having both hydronic and 

electric reheat capabilities. They were operated with hydronic reheat to produce the 

current data sets. The VAV boxes are well instrumented; many more points are 

monitored than would commonly be available in a commercial building. During testing

the HVAC equipment was operated in an occupied mode from 8:00 am to 5:00 pm and 

unoccupied otherwise. For normal operation, the AHU supply air temperature is 12.8 oC

(55 oF) and the static pressure is 300 Pa (1.2 in w.g.). Data were collected at 1 min 

intervals. 

The data s

d

unstable control loop. The procedure for implementing the faults and their expected 

impacts are described below. 

4.2.1 Faults Implemented

4.2.1.1 Failed Differential Pr

T

leads to the differential pressure sensor. The

damper to go to the full open position because the flow sensor will indicate an airflow 

rate of zero and the PI loop will attempt to correct for this condition. 

60
4.2.1.2 Hydronic Reheat Coil Valve Stuck Partially Open 

This fault was implemented by applying a control voltage from an independent source to

e hydronic coil valve actuator. The severity of the fault was adjusted periodically to 

0.1 GPM) to 0.00003 m3/s

Water Entering Leaving Airflow Entering Air 

rature

)) 

Leaving Air 

Temperature 

( oC (oF)) 

th

produce flow rates through the coil varying from 0.0063 m3

/s (

(0.5 GPM). The fault causes the discharge air temperature to be significantly higher than 

the entering air temperature for control conditions that indicate the reheat valve is closed. 

The impact of the fault was quantified by providing a fixed airflow rate to the VAV box 

and measuring the air temperature rise across the reheat coil. The data are summarized in 

Table 4.1. 

Table 4.1: Hydronic Reheat Coil Valve Stuck Partially Open 

Leakage 

Rate 

Water

Temperature 

Water

Temperature 

Rate 

(m3

/s 

Tempe

( oC (oF

(m3

/s 

(GPM)) 

( oC (oF)) ( oC (oF)) (CFM)) 

* 

7.6x10-6 ) )

(0.12) 

57.4 (135.3) 32.8 (91.1) 0.28 

(598) 

13.9 (57.1 16.0 (60.8

1.5x10-5

(0.23) 

57.8 (136.0) 33.5 (92.3) 0.28 14.3 (57.8) 18.2 (64.8) 

(602)

2.1x10-5 57.7 (135.8) 36.1 (97.0) 13.2 (55.8) 18.4 (65.1) 

(0.34) 

0.28 

(599)

2.8x10-5 58.0 (136.4) 38.8 (101.8) 12.6 (54.6) 18.5 (65.3) 

(0.45) 

0.29 

(605)

3.4x10-5 58.7 (137.6) 41.0 (105.8) 12.9 (55.3) 19.6 (67.2) 

(0.54) 

0.29 

(604)

3

/s (600 CFM as establish

rflow p

*A fixed supply airflow rate of approximately 0.28 m ) w ed

by setting the minimum and the maximum ai arameters for the VAV box to 0.28 

3

/s (600 CFM). 

4.2

Th ented by changing the integral coefficient of the controller used for 

airflow control. The parameter referred to as the “reset action” by the manufacturer was 

0.5 to 15.0. The fault caused the VAV box damper to 

rgy Center (IEC). The first set is

ormal operation only. The second set is a combination of normal and faulty operation. 

e

m

.1.3 Unstable Control Loop 

e fault was implem

adjusted from the normal value of

cycle continuously with a cycling period of approximately 2 min and 15 s. The impact 

should be similar to that of unstable static pressure control for the supply fan, although 

the impact should be localized to a single VAV box. 

4.2.2 Normal Operation 

Two sets of data were collected from the Iowa Ene

n

Normal operation data were collected from four VAV box controllers once per minute 

during the occupied periods of eight different days. The airflow rate error, th

61
temperature error, and the reheat coil differential temperature error were calculated for 

or Mean Standard Deviation 

each one-minute sample. The mean and standard deviation were calculated for each 

error. 

Table 4.2: Error Statistics For Normal Operation. 

Err

Qerror 0 m /s (0 CFM) 3 0.002 m3/s (5 CFM) 

Temperror 0.026 oC (0.1 oF) 0.026 oC (0.1 oF) 

∆Terror 0.044 oC (0.8 oF) 0.038 oC (0.7 oF) 

ed faults were use e CUSUM charts

4.2.3 Control Charts 

Data collected from the implement d to creat of the 

irflow rate error, the temperature error, and the reheat coil differential temperature error. 

using Equation 1 and the mean and standard deviation values 

on

e room cause the room temperature to increase. The controller responds by increasing 

value of Qactual is zero. This VAV box has a 

inimum Qsetpoint of 0.09 m3

/s (200 CFM) (to meet ventilation requirements). The PI 

a

The errors were normalized

from the “normal operation” data collected previously. The slack parameter k in 

Equations 2 and 3 was set equal to a value of 3. Plots showing various operating 

parameters, followed by the CUSUM charts for each zone were generated for each date 

data was collected. Detailed results are presented for one day of typical normal operation 

(no faults) in one VAV box and one day of operation with a failed differential pressure 

sensor in one VAV box, followed by a summary of all VPACC results from the ERS. 

Figure 4.4 shows the results for one day of normal operation (no faults) from a single 

VAV box. When the occupied period begins, the internal and external cooling loads

th

Qsetpoint. The damper is modulated to maintain the Qactual close to the Qsetpoint. Since the 

room temperature is above the HSP the reheat coil valve stays closed. (Note that the 

damper is closed when the control signal is 0 %, the reheat coil valve is closed when the

control signal is 100 %). As a result, the Qerror is very small and the Qerror CUSUM is 

negligible as well. The control algorithm maintains room temperature close to the

cooling setpoint, so the Temperror and Temperror CUSUM are also negligible. There is a 

small difference between entering air temperature and discharge air temperature due to 

measurement errors from the temperature sensors. The use of the slack parameter results 

in zero values for the ∆Terror CUSUM. 

Figure 4.5 shows results for one day of operation with a failed differential pressure 

sensor. As expected, the measured

m

algorithm used to determine damper position quickly saturates at full open due to the 

Qerror of 0.09 m3

/s (200 CFM). The Qerror CUSUM steadily increases, ending up with a 

value of 20 000 by the end of the occupied period. The fully open damper causes the 

room temperature to fall below the heating setpoint. At this point, the controller opens 

the reheat valve, which brings the room temperature back up to the heating setpoint. At

first, the Temperror and Temperror CUSUM register the effect of dropping room

temperature. As the reheat valve regains control of the room temperature, the Temperror

and Temperror CUSUM decrease to normal levels. By midday, is enough of a cooling 

load to maintain the room temperature above the HSP without reheat. As the reheat valve 

62
0 500 1000 1500

0

0.05

0.1

0.15

0.2

Time (min)

Air Flow (m3/s)

Airflow Set

Airflow Actual

0 200 400 600

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

CUSUM: Airflow

Time (min) from beginning of occupied period

CUSUM

Pos (S)

Neg (T)

0 500 1000 1500

12

14

16

18

20

22

24

Time (min)

Temperature ( oC)

CSP

HSP

RT

EAT

DAT

0 200 400 600

-1

-0.5

0

0.5

1

CUSUM: Temperature

Time (min) from beginning of occupied period

CUSUM

Pos (S)

Neg (T)

0 500 1000 1500

40

50

60

70

80

90

100

Time (min)

Control Signal (0 % to 100 %)

Dam per

HC Valve

0 200 400 600

-1

-0.5

0

0.5

1

CUSUM: DELTAT

Time (min) from beginning of occupied period

CUSUM

Pos (S)

Neg (T)

Figure 4.4: Interior A 8/31/01 Normal Operation. 

63
0 500 1000 1500

0

0.1

0.2

0.3

0.4

Time (min)

Air Flow (m 3/s)

Airflow Set

Airflow Actual

0 200 400 600

0

0.5

1

1.5

2

x 10

4 CUSUM: Airflow

Time (min) from beginning of occupied period

CUSUM

Pos (S)

Neg (T)

0 500 1000 1500

12

14

16

18

20

22

24

26

Time (min)

Temperature ( oC)

CSP

HSP

RT

EAT

DAT

0 200 400 600

-140

-120

-100

-80

-60

-40

-20

0

CUSUM: Temperature

Time (min) from beginning of occupied period

CUSUM

Pos (S)

Neg (T)

0 500 1000 1500

60

70

80

90

100

Time (min)

Control Signal (0 % to 100 %)

Dam per

HC Valve

0 200 400 600

-1

-0.5

0

0.5

1

CUSUM: DELTAT

Time (min) from beginning of occupied period

CUSUM

Pos (S)

Neg (T)

Figure 4.5: South A 8/28/01 Failed DP. 

64
modulates in response to room temperature, the discharge air temperature increases and 

decreases accordingly. Since the control signal to the reheat valve is not “full close”, the 

∆Terror and ∆Terror CUSUM are reset to zero. 

Table 4.3 shows a range of values for alarm limits for each CUSUM indicated. These 

values will detect all of the faults in the data set without any false alarms. As long as the 

alarm limit for each S and T function is set between the minimum and maximum limits, 

every fault will be detected with zero false alarms. “Not used” indicates that the limit 

was not needed to detect any of the particular faults that were implemented, although the 

limit may be needed to detect other faults. These preliminary results do not constitute a 

rigorous statistical approach. Before definitive alarm limits can be established, much 

additional data needs to be collected across a range of VAV box types, 

cooling/heating/swing seasons, size ranges, and controller manufacturers. It may be 

necessary to adjust the alarm limits based on one or more of these parameters. For

example, there might be a set of alarm limits for small VAV boxes, another for medium

boxes, and another for large boxes. 

Table 4.3: Alarm Limits 

Minimum Alarm Limit Maximum Alarm Limit 

Airflow positive (S) 3 180 

Airflow negative (T) 3 100 

Temperature positive (S) 120 950 

Temperature negative (T) 0 Not used 

DELTA T positive (S) 0 400 

DELTA T negative (T) 0 Not used 

Table 4.4 shows the peak values of each CUSUM chart for each run. The bold values 

show where each CUSUM identified exceeds the alarm limits from Table 4.3, detecting a

fault. 

4.3 Results from VAV Box Simulation Using HVACSIM+

Simulation, as described in the introduction of this report, was used to test and refine

VPACC. The simulation consisted of a single floor in a commercial office building with

a VAV AHU and three VAV boxes. The VAV boxes are modeled as single duct, 

throttling units with hydronic reheat coils. In order to leverage previous VAV box

modeling work, a pressure dependent system was used (the VAV box damper is 

positioned directly as a function of zone temperature, there is no airflow rate sensor or 

airflow rate setpoint). Although this type of system no longer represents a typical 

installation in the industry, its use here illustrates the flexibility, as well as some of the 

s other than the pressure independent system 

described in 4.1.2. 

limitations, of adapting VPACC to system

65
Table 4.4: CUSUM Chart Peak Values 

Date Zone Fault S T S T S T

/30/01 East A Normal Operation 1

CFM CFM TEMP TEMP ∆T ∆T

8 -1 100 0 0 0 

8/31/01 East A Normal Operation 3 -3 30 0 0 0 

9/1/01 East A Normal Operation 2 -2 0.6 0 0 0 

8/28/01 Interior A Normal Operation 0 0 120 0 0 0 

8/30/01 Interior A Normal Operation 0.5 0 0 0 0 0

8/31/01 Interior A Normal Operation 1 0 0 0 0 0

9/1/01 Interior A Normal Operation 0 0 0 0 0 0

8/28/01 South A Failed DP 20 000 0 0 -120 0 0

8/30/01 South A Failed DP 20 000 0 0 -150 0 0

8/31/01 South A Unstable Airflow 200 -25 200 0 0 0

9/1/01 South A Unstable Airflow 30 -100 0 0 0 0

8/28/01 West A Stuck HC Valve 180 0 950 0 400 0 

8/30/01 West A Stuck HC Valve 8 -1 9 0 2400 0 

8/31/01 West A Stuck HC Valve 10 -5 9 0 4000 0 

9/1/01 West A Normal Operation 3 -1 0 0 0 0 

4.3.1 Faults Implemented

amper S k Open 

by g the VAV box damper actuator position to the full

irf the maximum, and zone temperature will 

ooling load is small enough, the zone erature will drop below 

using t hydronic reheat valve to op mpt to maintain

one temperature at the setpoint. 

ault was introduced by setting the VAV box damper actuator to the minimum 

osition. The zone airflow will go to the minimum value, and zone temperature will tend 

ill not be seen when the

Hydronic Reheat Coil Valve Stuck Closed 

his fault was introduced by setting the VAV box hydronic reheat coil valve actuator 

osition to zero. If there is a heating load, the fault is expected to cause the zone 

mperature to drop. 

4.3.1.1 VAV Box D tuc

This fault was introduced settin

open position. The zone a low will go to 

decrease. If the zone c temp

the heating setpoint, ca he en in an atte

z

4.3.1.2 VAV Box Damper Stuck Closed 

This f

p

to increase for cooling conditions. The impact of the fault w

VAV box operates in the heating mode because, according to the simulated control 

strategy, the damper will already be set to the minimum position. 

4.3.1.3 VAV Box Hydronic Reheat Coil Valve Stuck Open 

This fault was introduced by setting the VAV box hydronic reheat coil valve actuator 

position to full open. The zone temperature is expected to rise above the cooling setpoint, 

causing the damper actuator to open in an attempt to maintain zone temperature at the set 

point. 

4.3.1.4 VAV Box

T

p

te

66
4.3.2 Normal Operation 

The simu ny season. Th int n co in loads the

se e z g th a o g m

o fully greater th ini fo e ent occupied

g of t only rence between one season and 

to V u of the cooling load in each of the zones. 

f e esented in this section. Two weeks of 

n g f or “n mal operation” case, and for each 

u c m at coi fferen l

tu e a ute d a poi The ean and 

d we d for eac e st stical parameters of these 

r ed rom the Io r Cent

s me th da re to nerate e control 

4 o t ria s inh n ulation. 

o ar

le t ault ere u to c te C UM c f e

tu and fferential t erature erro The ors were 

ormalized using Equation 1 and the mean and standard deviation values from the 

ed from the ERS. The slack parameter k in Equations 2 

nd 3 was set equal to a value of 3.

the proportional control algorithm combine to produce 

damper control signal that closely mirrors the zone temperature. This control algorithm

cooling setpoint, so the Temperror and the 

igure 4.7 shows the results for one day of the reheat coil valve stuck open fault (fault

is fully open, the zone

The ∆Terror CUSUM also increases to a value of 10 000 by the end of the 

ccupied period, because the reheat coil valve control signal is zero, and there is a

eratures. The Temperror

lation can use weather data from a e er al ol g

model u s for th ones are large enou h that e VAV box

)

es re in c olin ode

(reheat c

re

il valve closed, airflow rate an m mum r th ire

period

another

ardless

the VA

the season, so that

boxes is the magnit

he

de

diffe

Results rom one season only, fall, ar pr

simulatio data were enerated for the no ault, or

of the fo r faults des ribed above. The te perature error and the rehe l di tia

tempera re error w re calculated for e ch one-min at nt. m

standard eviation re calculate h error. Th ati

errors we e compar to results f wa Ene gy er (IEC) as described in 4.2. 

The stati tical para ters calculated for e IEC ta we used ge th

charts in .3.3 due t he small process va tion erent i sim

4.3.3 C ntrol Ch ts

Data col cted from he implemented f s w sed rea US harts o th

tempera re error the reheat coil di emp r. err

n

“normal operation” data collect

a

Figure 4.6 shows the results for one day of normal operation (no faults) from the three 

VAV boxes in the simulation. When the occupied period begins, the internal and 

external cooling loads on the room cause the room temperature to increase. The

controller responds by increasing the damper control signal. The combination of a

pressure dependent VAV box and

a

maintains room temperature close to the

associated CUSUM statistics are negligible. There is a small difference between entering 

air temperature and discharge air temperature due to heat gain in the supply ducts and the 

thermal mass effects of the reheat coil. The use of the slack parameter results in zero

values for the ∆Terror CUSUM. 

F

implemented in zone 1 only). Since the reheat coil valve

temperature rises well above the cooling setpoint. The airflow control signal is increased 

to 100 % in response to the room temperature. As shown in Figure 4.8, the Temperror

CUSUM steadily increases, ending up with a value of 20 000 by the end of the occupied 

period.

o

difference between the VAV box entering and discharge air temp

and ∆Terror CUSUMs are reset to zero during each unoccupied period. 

67
 (m

0.6

0.4

Ctrl Sign

0.8

1 Z2 ud

Z2 uhc

0.2

A

0.2

0.4

Ctrl S

1.1 1.2 1.3 1.4 1.5 1.6

x 10

5

15

20

25

30

Time (s )

Temperature ( oC)

CSP

HSP

Zone 1

Zone 2

Zone 3

1.1 1.2 1.3 1.4 1.5 1.6

x 10

5

0

10

20

30

40

50

Time (s)

Temperature ( oC)

SAT

DAT Z1

DAT Z2

DAT Z3

0.8

1

3/s)

Z1

0.6

0.8

1

al

Z1 ud

Z1 uhc

1.1 1.2 1.3 1.4 1.5 1.6

x 10

5

0

0.2

Time (s )

Ai

1.1 1.2 1.3 1.4 1.5 1.6

x 10

5

0

0.2

Time (s)

1 Z2

rf

0.4 low

1.1 1.2 1.3 1.4 1.5 1.6

x 10

5

0

0.2

0.4

0.6

0.8

Time (s )

Airflow (m 3/s)

1.1 1.2 1.3 1.4 1.5 1.6

x 10

5

0

0.2

0.4

0.6

Time (s)

Ctrl Signal

0.4

0.6

0.8

1

irflow (m 3/s)

Z3

0.6

0.8

1

ignal

Z3 ud

Z3 uhc

1.1 1.2 1.3 1.4 1.5 1.6

x 10

5

0

Time (s )

1.1 1.2 1.3 1.4 1.5 1.6

x 10

5

0

Time (s)

Figure 4.6 Simulation - normal operation - process variables and control signals 

68
1.1 1.2 1.3 1.4 1.5 1.6

x 10

5

15

20

25

30

Time (s )

Temperature ( oC)

CSP

HSP

Zone 1

Zone 2

Zone 3

1.1 1.2 1.3 1.4 1.5 1.6

x 10

5

0

10

20

30

40

50

Time (s)

Temperature ( oC)

SAT

DAT Z1

DAT Z2

DAT Z3

1.1 1.2 1.3 1.4 1.5 1.6

x 10

5

0

0.2

0.4

0.6

0.8

1

Time (s )

Airflow (m 3/s)

Z1

1.1 1.2 1.3 1.4 1.5 1.6

x 10

5

0

0.2

0.4

0.6

0.8

1

Time (s)

Ctrl Signal

Z1 ud

Z1 uhc

1.1 1.2 1.3 1.4 1.5 1.6

x 10

5

0

0.2

0.4

0.6

0.8

1

Time (s )

Airflow (m 3/s)

Z2

1.1 1.2 1.3 1.4 1.5 1.6

x 10

5

0

0.2

0.4

0.6

0.8

1

Time (s)

Ctrl Signal

Z2 ud

Z2 uhc

1.1 1.2 1.3 1.4 1.5 1.6

x 10

5

0

0.2

0.4

0.6

0.8

1

Time (s )

Airflow (m 3/s)

Z3

1.1 1.2 1.3 1.4 1.5 1.6

x 10

5

0

0.2

0.4

0.6

0.8

1

Time (s)

Ctrl Signal

Z3 ud

Z3 uhc

Figure 4.7 Simulation - reheat coil valve stuck open – process variables and control

signals

69
450 500 550 600 650 700 750 800 850 900

-2.5

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

2.5

x 10

4 CUSUM: Tem perature

Time (min)

CUSUM

Zone 1

Zone 2

Zone 3

450 500 550 600 650 700 750 800 850 900

-1.5

-1

-0.5

0

0.5

1

1.5

x 10

4 CUSUM: DELTA T

Time (min)

CUSUM

Zone 1

Zone 2

Zone 3

Figure 4.8 Simulation - reheat coil valve stuck open - CUSUM charts 

70
Table 4.5 shows a range of values for alarm limits for each CUSUM indicated. These 

values will detect all of the faults in the data set (that can be detected) without any false 

alarms. 

Table 4.5: Alarm Limits 

Minimum Alarm Limit Maximum Alarm Limit 

Temperature positive (S) 0 4000 

Temperature negative (T) 0 Not used 

DELTA T positive (S) 0 10 000 

DELTA T negative (T) 0 Not used 

As long as the alarm limit for each S and T function is set between the minimum and 

maximum limits, every fault that can be detected will be detected with zero false alarms. 

“Not used” indicates that the limit was not needed to detect any of the particular faults 

that were implemented, although the limit may be needed to detect other faults. These 

preliminary results do not constitute a rigorous statistical approach. Before definitive 

alarm limits can be established, much additional data needs to be collected across a range 

of VAV box types, cooling/heating/swing seasons, size ranges, and controller 

manufacturers. It may be necessary to adjust the alarm limits based on one or more of 

these parameters. For example, there might be a set of alarm limits for small VAV 

boxes, another for medium boxes, and another for large boxes. 

In order to leverage work done previously by NIST [4], this study used a model that was 

originally written primarily to develop FDD tools for air handling units. Some

limitations of this model regarding its use as a zone/VAV box simulator have become

apparent. The first limitation was mentioned earlier: the internal loads for each zone are 

so great relative to the shell loads that all seasons are qualitatively the same. Each zone is 

in cooling mode for the entire occupied period. For this reason, the reheat coil valve

stuck closed fault is not detected by the CUSUM method, since it has no impact on 

system operation during cooling mode. Another limitation is that the VAV boxes are 

modeled as pressure dependent boxes, meaning the damper is controlled directly in

response to zone temperature without an intermediate determination of an airflow 

setpoint. A Qerror does not exist for a pressure dependent VAV box. The damper stuck 

open fault is a fault which is masked by the controls. As the zone cools below the 

heating setpoint, the reheat coil valve opens to provide heating to the zone. Without a 

Qerror CUSUM, this fault cannot be detected. 

Table 4.6 shows the peak values of each CUSUM chart for each run. The bold values 

show where each CUSUM identified exceeds the alarm limits from Table 4.5, detecting a

fault. 

.4 Results from VAV Box Emulation in the VCBT 

Emulation, using the VCBT as described in the introduction of this report, was used in 

addition to simulation, to test and refine VPACC. The emulation consisted of a

commercial office building with a VAV AHU and three VAV boxes for each floor. The 

VAV boxes were modeled as single duct, throttling units with hydronic reheat coils. On 

4

71
one of the floors, one manufacturer’s commercially available zone controllers were used 

to implement a pressure dependent control strategy as described in 4.3. On another floor, 

a different manufacturer’s commercially available zone controllers were used to

endent control strategy as described in 4.1.2.

t Pe Values

STEMP TTEM T∆T

implement a pressure indep

Table 4.6: CUSUM Char ak

Fault P S∆T

Normal operation 0 0 0 0 

Damper stuck fully open 0 0 0 0 

Damper stuck fully closed 4000 0 0 0

Reheat coil valve stuck fully open 20 000 0 10 000 0 

Reheat coil valve stuck fully closed 0 0 0 0 

4.4.1 Faults Implemented 

The faults implemented in the emulation are essentially the same as those considered in 

the simulation with the exception of a few details. The description below was included to 

illustrate those exceptions, as well as in the interest of completeness and to enable each

section to stand alone to the extent possible. 

4.4.1.1 VAV Box Damper Stuck Open 

alve Stuck Open 

his fault was introduced by setting the VAV box hydronic reheat coil valve actuator 

mperature at the set point. 

losed

This fault was introduced by setting the VAV box damper actuator position to the full

open position. The zone airflow will go to the maximum, and zone temperature will 

decrease. If the zone cooling load is small enough, the zone temperature will drop below 

the heating setpoint, causing the hydronic reheat valve to open in an attempt to maintain

zone temperature at the setpoint. 

4.4.1.2 VAV Box Damper Stuck Closed 

This fault was introduced by setting the VAV box damper actuator to the minimum 

position. The zone airflow will go to the minimum value, and zone temperature will tend 

to increase for cooling conditions. The impact of the fault will not be seen when the 

VAV box operates in the heating mode because the reheat coil valve will still modulate to 

heat the air supplied to the zone, thereby controlling the zone temperature.

4.4.1.3 VAV Box Hydronic Reheat Coil V

T

position to full open. The zone temperature is expected to rise above the cooling setpoint, 

causing the damper actuator to open, and airflow to increase, in an attempt to maintain

zone te

4.4.1.4 VAV Box Hydronic Reheat Coil Valve Stuck C

This fault was introduced by setting the VAV box hydronic reheat coil valve actuator 

position to zero. If there is a heating load, the fault is expected to cause the zone 

temperature to drop. Under cooling conditions, this fault has no impact on system 

operation. 

72
4.4.2 Normal Operation 

The emulation was run using weather data for “heating season” (winter), “swing season” 

(fall), and “cooling season” (summer). One week of fault free emulation data was 

generated for each of the seasons. The airflow error (pressure independent VAV boxes

nly), temperature error, and the reheat coil differential temperature error were calculated

r each one-minute data point. An aggregate mean and standard deviation were 

t-case results for the three seasons considered,

the values shown in Table 4.7 were used as parame rs the M me o .

Table 4.7: Error Statistics For Normal Operation

Mean Standard 

o

fo

calculated for each error. Based on wors

te for CUSU th d

.

Deviation 

e Independent

Rate Error /s M

Pressur

Airflow 0 m3 (0 CF ) 0.02 m3

/s (40 CFM)

Temperature Error 0.1 oC (0.2 oF) 0.1 oC (0.2 oF) 

Reheat Coil Differential Temperature Error 0.0 oC (0.0 oF) 1.1 oC (2.0 oF) 

Pressure Dependent

Temperature Error 0.0 oC (0.0 oF) 0.4 oC (0.7 oF) 

Reheat Coil Differential Temperature Error -1.1 oC (-2.0 oF) 1.4 oC (2.5 oF) 

4.4.3 Control Charts 

A subset of the data collected from the emulation is presented here for illustrative

purposes. The faults described in 5.4.1 were implemented in the following seasons: VAV 

box damper stuck open – heating season and swing season, VAV box damper stuck 

closed – swing season, VAV box hydronic reheat coil valve stuck open – swing season 

and cooling season, VAV box hydronic reheat coil valve stuck closed – heating season. 

ne to two days of data were generated for each fault. Data collected from the 

M charts of the temperature error and the 

e heating load decreases, the room temperature rises. 

decreasing the airflow

e internal and external 

error

aintains room temperature close to the cooling setpoint, so the Temperror is 

O

implemented faults were used to create CUSU

reheat coil differential temperature error. The errors were normalized using Equation 1 

and the mean and standard deviation values from the “normal operation” data collected 

previously. The slack parameter k in Equations 2 and 3 was set equal to a value of 3. 

Plots showing various operating parameters are followed by the CUSUM charts for three

zones on each date data were collected. 

Figure 4.9 shows the results for one day of heating season normal operation (no faults)

from the three VAV boxes in the pressure independent part of the emulation. When the 

occupied period begins (120 min into the emulation), the room temperatures are below

the heating setpoint, so the reheat coil valves open and the airflow rates increase. As the 

control actions take effect and th

The controllers respond by closing the reheat coil valves and

setpoint and airflow for each room. As the day progresses th

cooling loads increase. The controllers respond by increasing the airflow setpoint and 

airflow for each room. The airflow control loop in the zone controller maintains the 

airflow rate close to the airflow setpoint, so the Q is negligible. This control 

algorithm m

73
also negligible. There is little difference between entering air temperature and discharge 

nd the

eating load decreases, the room temperature rises. The controllers respond by closing 

ternal and external cooling loads 

eratures increase above the cooling 

tpoint and the controllers respond by increasing the damper position

re small, so the room temperatures remain below the cooling 

per control signals remain valu

room temperatures throughout the day. This thm 

tin he Tem

here is little difference between entering air temperature and discharge air temperature,

egligible. Consequently e CUSUM statistics are maintained well 

holds. 

igure 4.11 shows one day of data from the swing season for the reheat coil valve stuck 

in the pressure independent VAV boxes (fault implemented in 

re dependent VAV boxes (fault implemented in 

one 1 only). The behavior of zones 2 and 3 shows the same trends as in Figure 4.10 and 

air temperature, so the ∆Terror is also negligible. Consequently the CUSUM statistics are

maintained well below the fault thresholds. 

Figure 4.10 shows the results for one day of heating season normal operation (no faults) 

from the three VAV boxes in the pressure dependent part of the emulation. When the 

occupied period begins, the room temperatures are below the heating setpoint, so the 

reheat coil valves open as described in 4.3. As the control actions take effect a

h

the reheat coil valves. As the day progresses the in

increase. If the loads are large enough, the room temp

se of each VAV box.

In this case, the loads a

setpoint and the dam at their minimum es. The reheat coil

valves maintain the control algori

maintains room temperature close to the hea g setpoint, so t perror is negligible. 

T

so the ∆Terror is also n th

below the fault thres

F

open fault as implemented

zone 1 only). The behavior of zones 2 and 3 shows the same trends as in Figure 4.9 and 

any differences are due to the different season. Since the reheat coil valve in zone 1 is 

fully open, the zone temperature rises well above the cooling setpoint. The airflow rate 

setpoint and airflow control signal are increased to their maximum values in response to 

the room temperature. Figure 4.12 shows that the Temperror CUSUM steadily increases, 

ending up with a value of 11 000 by the end of the occupied period of the day. The

∆Terror CUSUM also increases to a value of 2500 by the end of the occupied period of the

day, because the reheat coil valve control signal is zero, and there is a difference between 

the VAV box entering and discharge air temperatures. 

Figure 4.13 shows one day of data from the swing season for the reheat coil valve stuck 

open fault as implemented in the pressu

z

any differences are due to the different season. Since the reheat coil valve in zone 1 is 

fully open, the zone temperature rises well above the cooling setpoint. The damper 

control signal is increased to 100 % in response to the room temperature. Figure 4.14 

shows that the Temperror CUSUM steadily increases, ending up with a value of 2000 by 

the end of the occupied period of the day. The ∆Terror CUSUM also increases to a value 

of 2500 by the end of the occupied period of the day, because the reheat coil valve 

control signal is zero, and there is a difference between the VAV box entering and 

discharge air temperatures. 

74
CSP

50

SAT SP

600

)

0 200 400 600

Time (min)

0.6

 (m

0.6 ign

30

0 200 400 600

15

20

25

Time (min)

Temperature ( oC)

HSP

Z1

Z2

Z3

0 200 400 600

0

10

20

30

40

Time (min)

Temperature ( oC)

DAT Z1

DAT Z2

DAT Z3

0 200 400

0

0.1

0.2

0.3

Time (min

Airflow (m 3/s)

Z1 Airflow

Z1 Airflow SP

0

20

40

60

80

100

Ctrl Signal

Z1 ud

Z1 uhc

0 200 400 600

0

0.2

0.4

Time (min)

Airflow (m 3/s)

Z2 Airflow

Z2 Airflow SP

0 200 400 600

0

20

40

60

80

100

Time (min)

Ctrl Signal

Z2 ud

Z2 uhc

0.8

1

3/s)

Z3 Airflow

Z3 Airflow SP

60

80

100

al

Z3 ud

Z3 uhc

0 200 400 600

0

0.2

0.4

Time (min)

Airflow

0 200 400 600

0

20

40

Time (min)

Ctrl S

Figure 4.9 Emulation – pressure independent - normal operation - process variables 

and control signals 

75
0 200 400 600

15

20

25

30

Time (min)

Temperature ( oC)

CSP

HSP

Z1

Z2

Z3

0 200 400 600

0

10

20

30

40

50

Time (min)

Temperature ( oC)

SAT

DAT Z1

DAT Z2

DAT Z3

0 200 400 600

0

0.1

0.2

0.3

Time (min)

Airflow (m 3/s)

Z1

0 200 400 600

0

2

4

6

8

10

Time (min)

Ctrl Signal

Z1 ud

Z1 uhc

0 200 400 600

0

0.2

0.4

0.6

Time (min)

Airflow (m 3/s)

Z2

0 200 400 600

0

2

4

6

8

10

Time (min)

Ctrl Signal

Z2 ud

Z2 uhc

0 200 400 600

0

0.2

0.4

0.6

0.8

1

Time (min)

Airflow (m 3/s)

Z3

0 200 400 600

0

2

4

6

8

10

Time (min)

Ctrl Signal

Z3 ud

Z3 uhc

Figure 4.10 - Emulation – pressure dependent - normal operation - process variables

and control signals 

76
0 200 400 600

15

20

25

30

Time (min)

Temperature ( oC)

CSP

HSP

Z1

Z2

Z3

0 200 400 600

0

10

20

30

40

50

Time (min)

Temperature ( oC)

SAT SP

DAT Z1

DAT Z2

DAT Z3

0 200 400 600

0

0.1

0.2

0.3

Time (min)

Airflow (m 3/s)

Z1 Airflow

Z1 Airflow SP

0 200 400 600

0

20

40

60

80

100

Time (min)

Ctrl Signal

Z1 ud

Z1 uhc

0 200 400 600

0

0.2

0.4

0.6

Time (min)

Airflow (m 3/s)

Z2 Airflow

Z2 Airflow SP

0 200 400 600

0

20

40

60

80

100

Time (min)

Ctrl Signal

Z2 ud

Z2 uhc

0 200 400 600

0

0.2

0.4

0.6

0.8

1

Time (min)

Airflow (m 3/s)

Z3 Airflow

Z3 Airflow SP

0 200 400 600

0

20

40

60

80

100

Time (min)

Ctrl Signal

Z3 ud

Z3 uhc

Figure 4.11 Emulation – pressure independent - reheat coil valve stuck open – 

process variables and control signals

77
0 50 100 150 200 250 300 350 400 450

-6

-5

-4

-3

-2

-1

0

1

CUSUM: Airflow

CUSUM

Time (min) starting 1 h after beginning of occupied period

Zone 1

Zone 2

Zone 3

0 50 100 150 200 250 300 350 400 450

0

2000

4000

6000

8000

10000

12000

CUSUM: Tem perature

CUSUM

Time (min) starting 1 h after beginning of occupied period

Zone 1

Zone 2

Zone 3

0 50 100 150 200 250 300 350 400 450

0

500

1000

1500

2000

2500

3000

CUSUM: DELTA T

CUSUM

Time (min) starting 1 h after beginning of occupied period

Zone 1

Zone 2

Zone 3

Figure 4.12 Emulation – pressure independent - reheat coil valve stuck open - 

CUSUM charts (x axis shows elapsed time in minutes from 1 hour after beginning of

occupancy) 

78
0 200 400 600

15

20

25

30

Time (min)

Temperature ( oC)

CSP

HSP

Z1

Z2

Z3

0 200 400 600

0

10

20

30

40

50

Time (min)

Temperature ( oC)

SAT

DAT Z1

DAT Z2

DAT Z3

0 200 400 600

0

0.1

0.2

0.3

Time (min)

Airflow (m 3/s)

Z1

0 200 400 600

0

2

4

6

8

10

Time (min)

Ctrl Signal

Z1 ud

Z1 uhc

0 200 400 600

0

0.2

0.4

0.6

Time (min)

Airflow (m 3/s)

Z2

0 200 400 600

0

2

4

6

8

10

Time (min)

Ctrl Signal

Z2 ud

Z2 uhc

0 200 400 600

0

0.2

0.4

0.6

0.8

1

Time (min)

Airflow (m 3/s)

Z3

0 200 400 600

0

2

4

6

8

10

Time (min)

Ctrl Signal

Z3 ud

Z3 uhc

Figure 4.13 Emulation – pressure dependent - reheat coil valve stuck open – pr

variables and control signals 

ocess

79
0 50 100 150 200 250 300 350 400 450

0

500

1000

1500

2000

2500

CUSUM: Tem perature

CUSUM

Time (min) starting 1 h after beginning of occupied period

Zone 1

Zone 2

Zone 3

0 50 100 150 200 250 300 350 400 450

0

500

1000

1500

2000

2500

CUSUM: DELTA T

CUSUM

Time (min) starting 1 h after beginning of occupied period

Zone 1

Zone 2

Zone 3

Figure 4.14 Emulation – pressure dependent - reheat coil valve stuck open – 

CUSUM charts 

80
Tables 4.8 and 4.9 show a range of values for alarm limits for each CUSUM indicated. If 

the actual fault threshold (alarm limit) is less than the minimum shown, false alarms will 

be generated. If the threshold is greater than the maximum shown, some of the faults will 

not be detected. Therefore, using the values in Table 4.8 for the pressure independent

VAV boxes will allow the detection of all the faults in the data set without any false 

alarms. Using the values in Table 4.9 for the pressure dependent VAV boxes will detect 

all of the detectable faults in the data set without any false alarms. As explained in the 

following paragraph, some faults will not be detected for pressure dependent VAV boxes. 

“Not used” in Tables 4.8 and 4.9 indicates that the limit was not needed to detect any of 

the faults that were implemented. These preliminary results do not constitute a rigorous 

statistical approach. Before definitive alarm limits can be established, additional data 

need to be collected across a range of VAV box types, cooling/heating/swing seasons, 

size ranges, and controller manufacturers. It may be necessary to adjust the alarm limits 

based on one or more of these parameters. For example, there might be a set of alarm

limits for small VAV boxes, another for medium boxes, and another for large boxes.

Table 4.8: Pressure Independent VAV Box Alarm Limits 

Minimum Alarm Limit Maximum Alarm Limit 

Airflow positive (S) 30 3300

Airflow negative (T) 50 3200

Temperature positive (S) 0 10 000

Temperature negative (T) 1 3500

DELTA T positive (S) 20 2800

DELTA T negative (T) 0 Not used

Table 4.9: Pressure Dependent VAV Box Alarm Limits 

Minimum Alarm Limit Maximum Alarm Limit 

Temperature positive (S) 9 2000

Temperature negative (T) 60 1200

DELTA T positive (S) 25 2400

DELTA T negative (T) 8 Not used

Tables 4.10 and 4.11 show the peak values of each CUSUM chart for the pressure 

independent and pressure dependent VAV boxes respectively. The bold values show 

where each CUSUM identified exceeds the alarm limits from Tables 4.8 and 4.9,

nt VAV

.

Table 4.11 shows that the damper stuck open (heating and cooling seasons) and damper 

stuck closed (swing season) faults are not detected in the pressure dependent VAV boxes, 

but the reheat coil valve stuck open (swing and cooling seasons) and reheat coil valve 

detecting a fault. Table 4.10 shows that all of the faults in the pressure independe

boxes are detected

81
stuck closed (heating season) faults are detected. In the case of the damper stuck open 

fault, the control system masks the fault by operating the reheat coil valve to maintain 

room temperature between the heating and cooling setpoints. The pressure independent

VAV boxes can detect this fault when the Qerror CUSUM exceeds the alarm limits, but 

there is no Qerror for the pressure dependent VAV boxes, so the faults go undetected. 

When the damper stuck closed fault is implemented in the pressure dependent VAV 

boxes, the room temperature does rise above the cooling setpoint, but the rise is still 

within the normal range of +3 standard deviations of the normal operation data. Since the 

normal operation data for the pressure dependent VAV boxes shows more variation in 

room temperature than the data for the pressure independent system, the normal range is 

larger and the fault must be more severe before it can be detected. 

Table 4.10: Pressure Independent VAV Box CUSUM Chart Peak Values (bold 

signifies where an alarm limit has been exceeded) 

Fault SQ TQ STEMP TTEMP S∆T T∆T

Normal operation – heating 

season 

30 50 0 1 20 0

Normal operation – swing 6 8 0 0 0 0

season 

Normal operation – cooling 

ason

6 8 0 0 0 0

se

Damper stuck fully open – 3300 0 0 1 16 

heating season 

0

Damper stuck fully open – swing 3300 0 1 0.4 16 

season 

0

Damper stuck fully closed – 0 3200 350 0 1.1

swing season 

0

Reheat coil valve stuck fully 1 6 10 000 0 2800 0

open – swing season 

Reheat coil valve stuck fully 

open – cooling season 

2 2 11 000 0 2800 0

Reheat coil valve stuck fully

n

060 3500 0

closed – heating seaso

0

82
Table 4.11: Pressure Dependent VAV Box CUSUM Chart Peak Values (bold 

signifies where an alarm limit has been exceeded) 

ault STEMP TTEMP S∆T T∆T

ormal operation – heating 

eason

0 60 25 8

ormal operation – swing 

eason

900 0

ormal operation – cooling 

eason

000 0

F

N

s

N

s

N

s

Damper stuck fully open – 

h

0 14 17 0

eating season

Damper stuck fully open – swing 0

season 

4 17 0

Damper stuck fully closed –

swing season 

0 0 12 0

Reheat coil valve stuck fully open 2000 0 2500 0

– swing season 

Reheat coil valve stuck fully open 2300 0 2400 0

– cooling season 

Reheat coil valve stuck fully 0 1200 25 0

closed – heating season 

5 SUMMARY AND FUTURE WORK

a rule based DD or AH nd V CC, a istica

l based FDD tool for VAV boxes. APAR consists of a set of expert rules, 

rgy balances Control sig used de e th

ration, which identifies the subset of the rules to be evaluated. A 

by CSTB, a ows sim fie ssing HU data

et of process errors, valid for most VAV box control strategies, to 

nce. CUSUM charts, a statistic qu ontrol l, ar

rrors. Thresholds are determined by statistical analysis of a 

database of “normal operation” data. 

APAR was evaluated with laboratory, simulation, and emulation results. Using 

simulation data, APAR detected 10 of 20 faults during heating season, 14 of 20 faults 

during swing season, and 12 of 20 faults during cooling season. In addition, during 

swing season APAR was able to detect the presence of 2 VAV box faults. In the 

emulation, two systems (designated A and B) were used to generate data. Due to the 

real-time nature of the emulation, only a subset of the faults tested in simulation were 

studied. The emulation faults presented in this paper were those believed to have the

largest impact on system operation. Using emulation data, APAR detected 3 out of 5 

faults for Controller A, and 5 out of 5 for Controller B. Some of the missed faults could 

only be detected with additional sensors. For example, to detect a heating coil valve 

This report describes APAR,

quality contro

F tool f Us a PA stat l

derived from mass and ene

AHU’s mode of ope

. nals are to termin e

graphical interface, developed

VPACC uses a small s

ll pli d proce of A trend .

measure VAV box performa

used to evaluate the process e

al ality c too e

83
leakage in winter would require the installation of an additional sensor, such as a 

APAR

stem A

HU operation. In addition a false

 during normal operation was due to unstable control action taken by the system A 

tem B data, APAR as able to t all five

fully detected every harmful fault implemented. 

was evaluated using laboratory, simulation, and emulation results. The IEC

epresents normal o ration as we as three fau . VPAC

d all three faults. Two of the four faults from simulation were detected. One of 

VAV box damper st k open, w not detect ecause th

dent controls masked the fault. The other fault, the VAV box reheat coil 

cted because the controls never attempted to open the 

herefore, the fault had no impact on system operation. Two systems were 

: one was pressu independe the other s pressu

he pressure independent system detected all four of the faults considered, 

itions. ressure dep ndent im

o detect the VAV box damper stuck open fault because it was masked by

ent impl ation of VP C also d the VA

closed fault because of the small impact this fault had. When provided 

pendent VAV box VPACC was able to detect every fault 

false alarms. 

APAR and VPACC were evaluated using data from several different sources – laboratory 

d emulation. Consistent results across these

l buildings.

temperature sensor between heating coil and cooling coil. Using system A data,

was able to detect four of the five faults. The control strategy implemented by sy

caused the undetected fault to have no impact on A

alarm

AHU controller. Using sys

success

w detec faults. APAR 

VPACC

laboratory VAV box data r

detecte

pe ll lts C

the undetected faults, the

pressure depen

uc as ed b e

valve stuck closed, was not dete

valve; t

evaluated in the emulation

dependent. T

re nt, wa re

under a variety of weather cond

VPACC failed t

The p e plementation of 

the controls. The pressure depend

box damper stuck

ement AC misse V

with data from pressure inde

considered without any

es,

based AHUs and VAV boxes, simulation, an

diverse testing environments, in combination with the laboratory testing to validate the 

fault models, gives a high level of confidence that not only will the FDD tools perform 

well in real buildings, but also that the test conditions reflect the conditions in real 

buildings. 

In the next phase of the project, additional trend data from other buildings will be 

collected in order to refine both FDD tools and to determine robust thresholds for 

VPACC. Ultimately, the tools will be embedded in local AHU and VAV box controllers 

and evaluated in the VCBT and in rea

84
6 REFERENCES 

[1] Fasolo, P.S., and Seborg, D.E., 1992, “ Fault Detection in HVAC Control Systems

Using Statistical Quality Control Charts”, prepared for the California Institute for Energy 

Efficiency. 

[2] Bushby, S.T., Castro, N.S., Galler, M.A., Park, C., House, J.M., 2001, “Using the 

Virtual Cybernetic Building Testbed and FDD Test Shell for FDD Tool Development”, 

ISTIR 6818.

E. Flynn. 1981. Abbreviation of Climate Data for

uilding Thermal Analysis Programs Using Representative Samples of Various Lengths. 

N

 [3] Centre Scientifique et Technique du Bâtiment, 2000, FDD_AHU1.1 

[4] Park, C., Clark, D. R., and Kelly, G. E., An Overview of HVACSIM+

, A Dynamic

wBuilding/HVAC/Control Systems Simulation Program, Proceedings of the 1st Annual

Building Energy Simulation Conference, Seattle, WA, August 21-22 (1985). 

[5] House, J.M., Vaezi-Nejad, H., and Whitcomb, J.M., 2001, “An Expert Rule Set for

Fault Detection in Air-Handling Units,” ASHRAE Transactions, Vol. 107, Pt. 1: pp. 858-

871. 

[6] Ryan, Thomas P., 2000, Statistical Methods for Quality Improvement, 2nd Edition, 

Wiley and Sons, New York. 

[7] Nall, D. H., E. A. Arens, and L. 

B

ASHRAE Transactions Vol. 87, Pt. 1: 923-934. 

85
ATTACHMENT A: NOMENCLATURE FOR TABLE 1 

ma = mixed air temperature 

= changeover air temperature for switching between Modes 3 and 4 

T = supply air temperature set point 

Tsf = temperature rise across the supply fan 

Q /Q = outdoor air fraction = (T - T )/(T - T )

= normalized heating coil valve control signal [0,1] with uhc = 0 

indicating the valve is closed and uhc = 1 indicating it is 100 % 

e control signal [0,1] with ucc = 0

indicating the valve is closed and u = 1 indicating it is 100 % 

d

indicating the outdoor air damper is closed and ud = 1 indicating it 

is 100 % open 

εt = threshold parameter accounting for errors in temperature 

measurements 

 εf = threshold parameter accounting for errors related to airflows 

(function of uncertainties in temperature measurements) 

 εhc = threshold parameter for the heating coil valve control signal 

εcc = threshold parameter for the cooling coil valve control signal 

εd = threshold parameter associated with the mixing box damper control 

signal 

MTmax = maximum allowable number of mode transitions per hour

Tsa = supply air temperature 

T

Tra = return air temperature 

Toa = outdoor air temperature 

 Tco

sa,s

 ∆

∆Trf = temperature rise across the return fan 

∆Tmin = threshold on the minimum temperature difference between the 

return and outdoor air 

oa sa ma ra oa ra

(Qoa/Qsa)min = threshold on the minimum outdoor air fraction 

uhc

open 

ucc = normalized cooling coil valv

cc

open 

ud = normalized mixing box damper control signal [0,1] with u = 0 

86