The errors and CUSUMs are only calculated during occupied periods. During unoccupied
periods, the errors are not computed, and the CUSUMs are reset to zero. The first hour of the
occupied period is treated the same as the unoccupied period, to allow steady state conditions to
develop.

2.2.5 Point requirements

Most of the points required by VPACC are already available in the local VAV box controller:
room temperature, cooling setpoint, heating setpoint, airflow rate setpoint, 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 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.

2.2.6 Parameters

For each process error to which CUSUM analysis is to be applied, there is a set of parameters
that must be known and/or specified. These are the expected value of the process error (%), the
process error standard deviation (¢ ), the slack parameter (k), and the alarm limits for the S and T
CUSUMs (hs and hy). For the purposes of this study, the expected value and standard deviation
of the process error were determined by analysis of a short period of fault-free operation from a
particular data source. CUSUM analysis was performed for each error using an expected value
and standard deviation representative of the VAV boxes from each site. These parameters will be
referred to as the VPACC statistical parameters throughout the remainder of this paper. The
slack parameter k = 3 and alarm limits hs = hy = 900 are the same for all data sources. The
values for hs, hr, and k were determined heuristically based on results of previous work [1]. To
exceed the alarm limit value using one min data, an error that is five standard deviations from the
mean would have to persist for 7.5 hours. When a CUSUM does exceed the alarm limit, it is
reset to zero and the calculations resume. Each CUSUM is also reset to zero during unoccupied
periods (and during the first hour of occupancy, to allow steady state conditions to develop).
Thus, the severity of a fault can be established from the number of alarms over a period of time.

2.2.7 Special Cases
2.2.7.1 No Discharge Air Temperature Sensor

Many VAV boxes are equipped with a discharge air temperature sensor, which VPACC needs to
calculate the reheat coil differential temperature error. If a discharge air temperature sensor is not
available, a simplified version of VPACC could be used, implementing the airflow rate error, the
absolute value of the airflow rate error, and the temperature error only. In this case, a leaking
reheat coil valve (or, in the case of electric reheat, staged reheat enabled “on” in the cooling
mode) would not be detected unless it was so extreme that the VAV box was unable to maintain
the zone temperature at the set point, thereby causing alarms due to excessive values of Sr.

18


4.2 ERS

The supply air temperature from the AHU controller was made available to each of the VAV box
controllers via the control network and was used as the entering air temperature in the calculation
of reheat coil differential temperature error. To establish the VPACC statistical parameters, three
days of normal operation data were collected at one-minute sampling intervals from four VAV
box controllers during the cooling season. The data were processed off-line and yielded the
parameters in Table 4.2. The mean calculated for each of the errors was used for % in the
VPACC algorithm, and the standard deviation was used for 6. During testing of various fault
conditions, online inspection of the output from VPACC showed the output to be consistent with
what was expected. That is, the dominant CUSUM value (or values) for each data set was
appropriate for the implemented fault.

Table 4.2 VPACC statistical parameters for the ERS data sets

Error Mean Standard Deviation
Qerror 3.30x10° m/s (7.00 CFM) 1,32x107 m/s (28.0 CFM)
Terror 0.59 °C (1.07 °F) 0.36 °C (0.65 °F)

AT error 1.11 °C (2.00 °F) 0.34 °C (0.62 °F)

Results obtained for this data set are shown in Table 4.3. Twenty-seven days of normal operation
data were processed with VPACC with no false alarms. The reheat coil valve stuck partially
open fault was implemented for four days with different severities. Significant differences
between the entering and discharge air temperatures produced 17 alarms of Sr. The failed
differential pressure sensor produced large negative airflow errors, leading to 9 alarms of Tg and
Sig). Similarly, the stuck open damper fault produced large airflow errors, the signs of which
were determined by the loads on the zone.

The unstable airflow fault was implemented for four days with different severities and produced
6 alarms of Sig. There was one day of testing that did not produce any alarms because the fault
was not severe enough. Figure 4.9 shows the airflow error for one day of testing when the fault
was more severe. On this particular day, the standard deviation of Qerror Was 0.11 m/s (233
CFM), which is more than eight times the standard deviation of the data for normal operation.
VPACC output is shown in Figure 4.10. The fault is apparent from Sig, but not from Sg or Tg.

Table 4.3 VPACC results for the ERS data sets

Number of Alarms
Operation ater Si T, S; T; S, T, Ry

2 2 rr T | Sar | Tar | Sia
Normal 27 0 0 0 0 0 0
Reheat Coil Valve Stuck Partially Open 4 0 0 0 0 17 0 0
Failed DP Sensor 3 0 9 0 0 0 9
Unstable Airflow 4 0 0 0 0 0 0 6
Damper Stuck Open 2 4 7 0 ) 0 0 11

35


2.1.2.2 Detecting and Diagnosing Faults

APAR does not search for the existence of a specific set of faults. Rather, any fault that causes a
tule to be satisfied would be detected and additional effort would be necessary to isolate the
source of the problem. Emulation, simulation, and laboratory experimentation from previous
work [1] as well as the current study demonstrate that the rule set can identify the following
faults:

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

Design faults such as undersized coils;

Controller programming errors related to tuning, setpoints, and sequencing logic;
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.

2.1.2.3. Threshold Selection

In addition to the sensor, control signals, and setpoint information, there are other parameters
that 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 °C (2.0 °F).
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 & in Rule 10 must also
be specified. Another approach might be to calculate the threshold values based on the

uncertainty of each sensor or actuator value. As an example, the threshold in Rule 10 would be
determined from the expression

& = €f,,+ €f,,

where ép,, and é,, are the uncertainties associated with the measurement of the outdoor and
mixed air temperatures. If a threshold is too great, the associated fault(s) must be relatively
severe to be detected. If, on the other hand, a threshold is too small, normal variation in
operating conditions may result in false alarms. These threshold values were determined
heuristically for each site of the sites in this study.

2.1.3. Instrumentation Accuracy Requirements

APAR uses existing sensor points in the control system to perform the fault detection
calculations. Previous work [1] demonstrated that the typical industrial grade sensors that are
already installed for control purposes have sufficient accuracy. Laboratory grade instruments are
not required. Higher quality sensors that have been installed and calibrated properly will allow
the use of tighter thresholds (less severe faults can be detected) than lower quality sensors, or
those that have been poorly calibrated or installed.

12


counter exceeds the maximum number of mode switches, the binary value representing Rule 28
is set to a value of “on” — indicating a fault, otherwise it is set to a value of “off”.

A stand-alone program, BACnet Data Source (BDS) [4], was installed on a workstation
connected to the VCBT control network and configured to trend the raw data (temperatures,
valve positions, etc.) as well as intermediate values and results of the APAR algorithm (mode
and rule status). The raw data was read once per minute, while the APAR values were read once
every five minutes. The EWMAs were not trended, as they could easily be recreated from the
raw data and mode status if necessary. For every fault detected by APAR, the raw data was
examined to verify that the FDD results matched the conditions.

3.1.3 VCBT Fault Descriptions
3.1.3.1 Supply Air Temperature Sensor Drift

Supply air temperature sensor drift is introduced as a sensor offset for a range of 0.0 °C to +4.0
°C (0.0 °F to +7.2 °F), applied linearly over a three week emulation period. If a controller
maintains the measured supply air temperature at the set point, a negative sensor offset would
result in a decreased actual supply air temperature. A positive sensor offset would result in an
increased actual supply air temperature.

3.1.3.2 Return Air Temperature Sensor Drift

Return air temperature sensor drift is introduced as a sensor offset for a range of 0.0 °C to +4.0
°C (0.0 °F to +7.2 °F), applied linearly over a three week emulation period. The return air
temperature sensor is used to control the economizer operation for AHU-A and -C. A negative
sensor drift for the return air temperature with no change to the humidity reading would result in
an increased return air enthalpy. A positive sensor drift for the return air temperature with no
change to the humidity reading would result in a decreased return air enthalpy. Depending on
weather conditions, this fault may cause inappropriate economizer control.

3.1.3.3 Mixed Air Temperature Sensor Drift

Mixed air temperature sensor drift is introduced as a sensor offset for a range of 0.0 °C to +4.0°C
(0.0 °F to +7.2 °F), applied linearly over a three week emulation period. If a controller maintains
the measured mixed air temperature at the set point, a negative sensor offset would result in a
decreased actual mixed air temperature. A positive sensor offset would result in an increased
actual mixed air temperature.

3.1.3.4 Outdoor Air Temperature Sensor Drift

Outdoor air temperature sensor drift is introduced as a sensor offset for a range of 0.0 °C to +4.0
°C (0.0 °F to +7.2 °F), applied linearly over a three week emulation period. The outdoor air
temperature sensor is used to control the economizer operation for AHU-A, -B, and -C. A
negative sensor drift for the outdoor air temperature with no change to the humidity reading
would result in an increased outdoor air enthalpy. A positive sensor drift for the outdoor air
temperature with no change to the humidity reading would result in a decreased outdoor air
enthalpy. Depending on weather conditions, this fault may cause inappropriate economizer
control.

22


Abstract

Building HVAC equipment routinely fails to satisfy performance expectations envisioned at
design. Such failures often go unnoticed for extended periods of time. Additionally, higher
expectations are being placed on a combination of different and often conflicting performance
measures, such as energy efficiency, indoor air quality, comfort, reliability, limiting peak
demand on utilities, etc. To meet these expectations, the processes, systems, and equipment used
in both commercial and residential buildings are becoming increasingly sophisticated. 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 diagnostics (FDD).

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 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 statistical quality control measures to detect faults or control problems in VAV boxes.

This report describes a research study of embedding APAR and VPACC in HVAC equipment
controllers in a laboratory setting. APAR was embedded in several air handling unit controllers
and evaluated in an emulation environment, while VPACC was embedded in several VAV box
controllers and evaluated in a laboratory environment. APAR and VPACC were both found to
be successful at finding a wide variety of mechanical and control faults. 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 U.S. Department of Energy Office of Energy Efficiency
and Renewable Energy. In addition, this project would not have been possible without the
assistance of many individuals. Thanks are due to Cheol Park and Michael Galler of the
National Institute of Standards and Technology for their assistance with the emulation aspect of
the project and Xiaohui Zhou of the Iowa Energy for his assistance with the laboratory aspect of
this project.


4.8) plus the deadband (1.1 °C (2.0 °F)) by more than a specified threshold (1.7 °C (3 °F)), a
fault has been detected. The fault status data collected from the AHU-B controller indicates that
this fault was detected on this particular day of operation.

33


2.2.7.2 Pressure Dependent

In some VAV boxes, the damper is controlled directly in response to zone temperature without
an intermediate determination of an airflow setpoint. Qerror and IQerror! do not exist for a pressure
dependent VAV box. In this case, a stuck damper may go undetected. In the case where the zone
is overcooled, the reheat coil valve will open (or staged reheat will be enabled “on” if electric
reheat is employed) and compensate for the fault, masking its existence. In the case where the
zone is undercooled, the rising zone temperature may create alarms due to excessive values of Sr.

2.2.7.3 No Reheat

Some VAV boxes do not have reheat capabilities. Others do not have reheat available part of the
year because a two pipe hydronic system is being used for chilled water at that time. Since the
VAV box cannot take any control action to increase zone temperature, a negative temperature
error does not necessarily indicate a fault. In this situation, only the Sr CUSUM will be
calculated for Terror.

2.2.7.4 Dual Duct

In a dual duct VAV box, there is no reheat coil (and no electric reheat). Instead there are two air
inlets, namely, a cold deck and a hot deck. Each air inlet has a damper and differential pressure
sensor. For this arrangement, two airflow errors (Qerror,hot 20d Qerror,cold) and two absolute value
airflow errors (IQerror, hot! and !Qerror, cold!) Will be calculated. No ATerror Will be calculated as there
is no reheat capability.

19


3.2.2.4 Unstable Flow Control

The fault is implemented by altering a component of the control logic that limits the rate of
increase and decrease of the airflow control output. The fault causes the VAV box damper to
oscillate, thereby producing airflow rates that oscillate about the set point.

25


3.1.1.2.2 Temperature Control

AHU-B uses a single PI control loop to determine a temperature control signal to maintain the
supply air temperature at a fixed set point. Depending on the magnitude of the signal, it is
mapped to one of three outputs which control the heating coil, cooling coil, and mixing-box
dampers. Additional logical sets the-position of the other two outputs appropriately depending
on which one is active. For example, if the heating coil valve is active, the cooling coil valve
will be fully closed and the mixing box damper will be set to the minimum position (which
depends on the occupancy status), The outdoor air temperature is compared to a fixed
changeover temperature to determine whether to enable or disable economizer operation.

3.1.1.3 AHU-C Control Strategy

3.1.1.3.1 Fan Control

The supply air fan speed is controlled to maintain the supply air pressure at a fixed set point.
The return air fan speed is controlled to maintain a constant difference between the supply and
return air flow rates. :

3.1.1.3.2 Temperature Control
AHU-C uses a separate PI control loop for each of three outputs which control the supply air

temperature: the heating coil, cooling coil, and mixing-box dampers. The heating coil and
cooling coil outputs are controlled to maintain supply air temperature at a fixed set point. The
mixing box dampers are controlled by comparing the outside air and the return air enthalpies. If
the return air enthalpy is greater than the outdoor air enthalpy, the mixing-box damper control
loop maintains mixed air temperature at its set point (also fixed). Interlocks, dead bands, and
time delays are incorporated to prevent simultaneous heating, cooling, and economizing.

3.1.2 Embedded APAR

APAR was embedded in the AHU controllers by adding additional logic to their control
programs to execute the APAR algorithm. The three AHU controllers are each from a different
manufacturer. .

The cooling coil valve, mixing box damper, and heating coil valve control signals are evaluated,
along with the occupancy status, to determine in which mode the AHU is operating. A binary
value (BV) is used to represent the status of each mode. Exponentially weighted moving
averages (EWMAs) are computed for these control signals, as well as for supply air temperature,
return air temperature, mixed air temperature, outdoor air temperature, supply air temperature
setpoint, return air humidity, return air enthalpy, outside air humidity, and outside air enthalpy.
EWMaAs are used to smooth the variation in the data because, unlike other types of moving
averages, no historical data need be stored in the controller’s memory - only the current value
and EWMA of each measurement need to be stored. The EWMAs are reset when a mode switch
occurs. A 60 minute timer starts when the logic recognizes that the AHU is operating in one of
the five defined modes of operation, and is reset when the AHU is no longer in that mode. When
the timer expires, the rules for that particular mode are evaluated. Each rule is represented by a
binary value, set to a value of “on” if the rule is satisfied — indicating a fault, otherwise it is set to
a value of “off”. Rule 28, regarding excessive mode switches, is a special case, since it is
evaluated independently of the modes. To evaluate Rule 28, a counter is incremented every time
a mode switch occurs. A timer resets the counter to zero every 60 minutes. If the value of the

21


(0% - 100 %)

Control Signi

‘Temperature (°C)

110 — oa

Economizer Control Signal
100 pligeee
90 ‘Cooling Coil Control Signal
80
70
60
50
40
30
20
10
‘Heating Coil Control Signal
° rae
oO 100 200 300 400 500
Time (min from beginning of occupancy)
Figure 4.7 VCBT AHU-B Economizer Control Logic Fault — Control Signals
35
“+ ~nixed air Temperature
30
25
ac Retum Air Temperature
as ‘Temperature
20
15
ac 2° DY Air Temperature
L ‘Supply Air Temperature Setpoint
10
oO 100 200 300 400 500

Time (min from beginning of occupancy)

Figure 4.8 VCBT AHU-B Economizer Control Logic Fault ~ Process Temperatures

34



assumed that a fixed airflow rate associated with the ventilation requirement of the room is
provided in the heating mode.

Valve Airflow
Position Setpoint
Fully | o___ oe. 4 _— Maximum
Open \
\
‘Valve
\
\ Airflow
\ Setpoint
. — Minimum
\
\
Fully __| \ L
Closed ' '
Room Heating Cooling Warmer
Temperature
Set Set

Point Point

Figure 2.5: Damper and valve control sequence as a function of room temperature for a
single duct pressure-independent VAV box with hydronic reheat.

2.2.4 CUSUM Applied to VAV Box Diagnostics

The previous section described one particular VAV box control strategy. However, a 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, four generic errors were identified: the airflow rate error, the absolute value of
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.
Common mechanical and control faults will result in a deviation of one or more of these errors
from its value during normal operation, which can be detected by a CUSUM chart.

The airflow rate error, Qerror, is defined as

Qerror = Qactuat - Qsetpoint (3)

where
Quctuat = measured airflow rate
Qsetpoint = airflow rate set point.

16


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 heating and
cooling coil valves and the mixing box dampers are known, the mode of operation can be
ascertained. Although not depicted in Figure 2.2, 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
must be approximately equal. Defining T,, and T,,, as the outdoor air and mixed air

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

Rule 10: !7gq- Tal > &

where &€ 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 if the outdoor and mixed air temperatures are not the
same (i.e., if true) a fault has occurred.

As a detailed description of the 28 APAR mules and the reasoning behind them is
available elsewhere [2], the rules are simply listed in Table 2.1 without detailed
explanation. Table 2.1 groups the rules according to mode of operation. As indicated in
the column heading for the rule expression, a true expression indicates a fault. Table 2.2
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.


One of the rules in the set for Mode 4 is Rule 18, which first checks whether there is enough of a
difference, 5.6 °C (10 °F), between the return and outdoor air temperatures in order to proceed.
In the case illustrated by figures 4.5 and 4.6, the difference becomes sufficient from
approximately 250 minutes after the beginning of the occupied period until the end of the
occupied period. If the difference is sufficient, the next step in the evaluation of Rule 18 is to
calculate the fraction of outdoor air by dividing the difference between mixed air and return air
temperature by the difference between outdoor air and return air temperature. If the calculated
outdoor air fraction varies by more than a specified threshold, 0.30, from the minimum amount
of outdoor air needed for ventilation (in this case, 0.15), Rule 18 is satisfied, indicating that this
fault has been detected. The fault status data collected from the AHU-A controller indicates that
this fault was detected on this particular day of operation.

4.1.4 Economizer Control Logic Fault

When this fault is introduced, the logic used to select economizer or minimum ventilation
operation is reversed. The AHU-B controller compares the outdoor air temperature to a fixed
changeover temperature, 22.2 °C (72 °F), to determine whether to enable or disable economizer
operation. Normally, economizer operation is disabled if the outdoor air temperature is greater
than the changeover temperature and enabled if the outdoor air temperature is less than the
changeover temperature, subject to a deadband of 1.1 °C (2.0 °F). When the economizer fault is
implemented, this relationship is reversed: economizer operation is enabled if the outdoor air
temperature is greater than the changeover temperature and disabled if the outdoor air
temperature is less than the changeover temperature, still subject to the specified deadband.

Figures 4.7 and 4.8 show data from AHU-B from the occupied portion of one day during the
emulation of this fault. On this particular day, the outdoor air temperature (green, figure 4.8)
ranges from 26 °C (79 °F) to 33 °C (91 °F), which is greater than the changeover temperature
(orange, figure 4.8) by more than the deadband as is typically the case for cooling season. If
AHU-B was operating without any faults, these conditions would cause the AHU controller to
disable the economizer and align the mixing box dampers to bring in the minimum amount of
outdoor air needed for ventilation. Due to the control logic fault, the mixing box dampers
(green, figure 4.7) are actually aligned to bring in 100 % outdoor air. Qualitatively, this can be
seen by comparing the mixed air temperature (brown, figure 4.8) to the return (red, figure 4.8)
and outdoor air temperatures (green, figure 4.8). If the dampers were aligned for minimum
outdoor air, the mixed air temperature would be very close to the return air temperature, but
actually the mixed air temperature is nearly identical to the outdoor air temperature, because the
fault in the AHU control logic has positioned the dampers to bring in 100 % outdoor air. The
AHU controller modulates the cooling coil valve (blue, figure 4.7) to maintain the supply air
temperature (light blue, figure 4.8) at its setpoint (dark blue, figure 4.8). The heating coil valve
(red, figure 4.7) is closed.

Based on this combination of control signals, APAR determines the system to be operating in
Mode 3 (mechanical cooling with 100 % outdoor air) and evaluates the applicable rule set. One
of the rules in the set for Mode 3 is Rule 9, which, as modified to match the absolute
temperature-based economizer control strategy employed for AHU-B, states that if the outdoor
air temperature (green, figure 4.8) is greater than the changeover temperature (orange, figure

31


120 ——- — — —
Economizer Control Signal
100 a
soie0)
5S
x
& 60
z
a i
é |
aa 4 Cooling Coil Control Signat
20
Heating Coil Control Signal
7 Cet
o 100 200 300 400 500 600
Time (min from beginning of occupancy)
Figure 4.1 VCBT AHU-A Outdoor Air Temperature Sensor Failure — Control Signals
ies oe aT 1
Mixed Air Temperature |
25
‘SRetum Air Temperature
20
~ 5 |
2
; Lee |
= 10
2
E
5
Cae
°
5 — Outdoor Air Temperature
-10 i
0 100 200 300 400 500 600

‘Time (min from beginning of occupancy)

Figure 4.2 VCBT AHU-A Outdoor

ir Temperature Sensor Failure - Process Temperatures

28



1 Introduction

Building HVAC equipment routinely fails to satisfy performance expectations envisioned
at design. Such failures often go unnoticed for extended periods of time. Additionally,
higher expectations are being placed on a combination of different and often conflicting
performance measures, such as energy efficiency, indoor air quality, comfort, reliability,
limiting peak demand on utilities, etc. To meet these expectations, the processes, systems,
and equipment used in both commercial and residential buildings are becoming
increasingly sophisticated. 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 diagnostics (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 does not scale well because all of the data must be obtained at a single point. A
better solution is to embed FDD in the local controller for each piece of equipment, so
that the FDD algorithm is executed as a component of the control logic. NIST has
developed 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
sufficiently simple that they can be embedded in commercial building control systems
and only rely upon sensor data and control signals that are commonly available in
commercial building automation and control systems. AHU Performance Assessment
Rules (APAR) and VAV box Performance Assessment Control Charts (VPACC) have
been designed “from the ground up” to be embedded in commercial HVAC equipment
controllers.

A previous study [1], describes the theoretical basis of APAR and VPACC and evaluates
the tools’ performance using data generated by simulation, emulation, and laboratory
testing. The study examined the breadth of faults that can be detected and the conditions
under which they can be detected. This report describes the results of a research study to
evaluate the feasibility of embedding APAR and VPACC in HVAC equipment
controllers in a laboratory setting.


NistT

National Institute of Standards and Technology
Technology Administration, U.S. Department of Commerce

Table of Contents

1 Introduction... 5
2 Methodology ..
2.1 FDD for Air Handling Units .
2.1.1 System Description
2.1.2 AHU Performance Assessment Rules (APAR)
2.1.3 Instrumentation Accuracy Requirements

2.2 FDD for VAV BoXs.........sscsesesseceeseneeseeees
2.2.1 VAV box Performance Assessment Control Charts - VPACC..
2.2.2 Control Charts .....
2.2.3 System Description
2.2.4 CUSUM Applied to VAV Box Diagnostics .
2.2.5 Point requirement:
2.2.6 Parameters...
2.2.7 Special Cases

3 Testing Environment
3.1 VCBT .........00000
3.1.1 AHU Control Strategies
3.1.2 Embedded APAR....
3.1.3 VCBT Fault Descriptions.

3.2 ERS. eesesesseseeesseseeeeeeeneneeee
3.2.1 Embedded VPACC
3.2.2 ERS Fault Descriptions .

4 Results...
4.1 VCBT
4.1.1 Outdoor Air Temperature Sensor Failure.
4.1.2 Mixed Air Temperature Sensor Drift.
4.13 Recirculation Damper Stuck Closed
4.1.4 Economizer Control Logic Fault.

4.2 ERS....eesesseseeeeeetene

5 Summary and Future Work
6 References



The CUSUMs of this error, Sg and Tg, are effective for detecting damper faults and differential
pressure sensor faults associated with airflow measurement.

The absolute value of the airflow rate error, IQerrorl, is defined as
\Qerrorl = \Qactuat - setpoint! (4)

Only one CUSUM value, Sig), is defined for this error since the error is never negative. Sig, is
effective for detecting unstable damper control faults.

The temperature error, Terror, is defined as

Terror = Tzone- CSP: If Tzone > CSP (5a)
Terror = O : If HSP < Tone < CSP (5b)
Terror = Tone -HSP — : If Tzone < HSP (5c)

where

Tone = Zone temperature
CSP =cooling set point
HSP = heating set point.

The CUSUMs of the temperature error, Sz and 77, are effective for detecting damper faults, valve
faults, 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 temperature error, AT error, is defined as

AT error = Taischarge ~ Tentering : If unc =0 (6a)
ATerror = 0 : Hf aye #0 (6b)

where

Tuischarge = discharge air temperature (the temperature of the air leaving the reheat coil)
Tentering = entering air temperature (the temperature of the air entering the reheat coil)
Unc = control signal to the reheat coil valve.

The positive CUSUM of the reheat coil differential temperature error, S4r, is effective for
detecting a leaking reheat coil valve fault. The negative CUSUM, Tr, is effective for detecting
temperature sensor faults. The leaking valve fault highlights the advantages of automated FDD.
Without VPACC, the local controller may be capable of masking this fault by increasing the
airflow rate into the space. In this scenario there will be no “too hot” or “too cold” complaints, so
a significant energy penalty may be accrued.

17


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 2.2.
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.

In the heating mode (Mode 1 in Figure 2.2), 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 mixing box dampers are positioned to allow the minimum outdoor air
necessary to satisfy ventilation requirements.

A Mode 1 Mode2 Mode3 Mode 4
I< >< >

@ 100 ee
zg /——
q Heatin; Cooling
E) Coil Vaie Mixing Box Coil Valve
a 4 Dampers ,
g ’
8 .
"0 +4 + >

Figure 2.2: Typical operating modes of an air-handling unit

As cooling loads increase, the AHU transitions from heating to cooling with outside air
(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 over to mechanical cooling. When the AHU is
operating in one of the mechanical cooling modes (Modes 3 and 4), 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, generally 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.

2.1.2 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


3.1.3.5 Outdoor Air Damper Fault

The outdoor air damper faults are introduced by overriding the normal control signal to the
damper with a control signal to force the motor-driven actuator to the specified position, causing
the damper to stay at that position throughout the emulation period. While emulating the outdoor
air damper fault, the recirculation air and the exhaust air dampers follow normal operation. The
economizer may not operate correctly because of the fault, depending on the outdoor and indoor
conditions.

3.1.3.6 Recirculation Air Damper Fault

The recirculation air damper faults are introduced by overriding the normal control signal to the
damper with a control signal to force the motor-driven actuator to the specified position, causing
the damper to stay at that position throughout the emulation period. While emulating the
recirculation air damper fault, the outdoor air and the exhaust air dampers follow normal
operation. The economizer may not operate correctly because of the fault, depending on the
outdoor and indoor conditions.

3.1.3.7 Economizer Control Logic Fault

This fault is introduced by reversing the logic used to decide whether the economizer or the
minimum ventilation operation should become active. The fan speed and temperature control
loops operate normally.

3.1.3.8 Temperature Sensor Failure

A supply air, return air, mixed air, or outdoor air temperature sensor fault is introduced by
disconnecting the leads to the appropriate sensor terminals on the AHU controller.

3.2 ERS

The ERS is a laboratory facility for HVAC research that has two test VAV air-handling systems,
each serving four test zones. The HVAC equipment and controllers are typical of that found in
commercial 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 data for this
study. The VAV boxes are well instrumented; many more points are monitored than would
commonly be available in a commercial building. Details of the facility are provided in [5].

3.2.1 Embedded VPACC

VPACC was embedded in four VAV box controllers by adding additional logic to their control
Programs to carry out the VPACC algorithm. It was necessary to limit the testing to a single
manufacturer’s controllers to enable communication between the VAV box controllers and the
balance of the control system.

The airflow error, absolute value of the airflow error, temperature error, and reheat coil AT error
are calculated continuously. A sixty minute timer is started when the occupancy status goes
from unoccupied to occupied. When the timer expires, the CUSUM S and T cumulative sums
(each sum is represented by an analog value) are evaluated once per minute for each of the

23


NISTIR 7036

Results from Laboratory Testing of
Embedded Air Handling Unit and Variable
Air Volume Box Fault Detection Tools

Jeffrey Schein
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
Towa Energy Center
Ames, IA

NUST

National Institute of Standards and Technology
Technology Administration, U.S. Department of Commerce



min

Qod/Qsa

(Qod/Qsamin =

Une

Uce

ug

maximum number of mode changes per hour

supply air temperature

mixed air temperature

return air temperature

outdoor air temperature

changeover air temperature for switching between Modes 3 and 4
supply air temperature set point

temperature rise across the supply fan

temperature rise across the return fan

threshold on the minimum temperature difference between the
return and outdoor air

outdoor air fraction = (Tyg - Tyq(T oq - Tra)

threshold on the minimum outdoor air fraction

normalized heating coil valve control signal [0,1] where u;,, = 0
indicates the valve is closed and u,, = 1 indicates it is 100 % open
normalized cooling coil valve control signal [0,1] where u,, = 0
indicates the valve is closed and u,, = 1 indicates it is 100 % open
normalized mixing box damper control signal [0,1] where ug = 0
indicates the outdoor air damper is closed and ug = 1 indicates it is
100 % open

threshold for errors in temperature measurements

threshold parameter accounting for errors related to airflows
(function of uncertainties in temperature measurements)

threshold parameter for the heating coil valve control signal
threshold parameter for the cooling coil valve control signal
threshold parameter for the mixing box damper control signal

2.1.2.1 Operational and Design Data Requirements

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

and control signals:
e Occupancy status; ¢ Mixing box damper control
e Supply air temperature set point; signal;
e Supply air temperature; e Return air relative humidity
e Return air temperature; (for enthalpy-based economizers
© Mixed air temperature; only)
© Outdoor air temperature; © Outdoor air relative humidity
¢ Cooling coil valve control signal; (for enthalpy-based economizers
e Heating coil valve control signal; only).

10


Table 2.1: APAR Rule Set

Mode Rule #__| Rule Expression (true implies existence of a fault)
1 Tsa<Tma + AT yf - &
Heating 2 For |Tra - Toal 2 AT min 'Qoa/Qsa ~ (Qoa/Qsa)min | > &
(Mode 1) 3 Wac- 1 Se and Tsq5—-Tsa > &
4 lune — 11 S Eh
Cooling with 5 Toa > Tsa,5- AT yg + &
Ono Air 6 Tsq > Tra - AT ¢ + &
(Mode 2) 7 ITyq- AT sp~ Tmal > &
8 ‘| Tea < Tsa,s _ AT sg 74
Mech 9 Toa > Teo + &
lechanical
Cooling with | 10 Toa Tmal > &
100% Outdoor | 11 Tsq > Tma + 4T gg + &
Air
(Mode 3) 12 T sq > Tra - AT g + &
13 Wce- I SEge and Tsq—-Tsa,5 2&
14 Wee - 11 S Ege
15 Toa < Teo - &
Mechanical 16 Tsq > Tma + 4T yg + &
0 none with 17 Tq > Tra a Ag +&
Outdoor Air [18 For Tra Toa! >AT min: 'Qoa/Osa - (Qoa/Osalmin | > &
(Mode 4) 19 Wco- NS Ege and Tsq—-Tsa,5 2 &
20 Wee - HN S Ege
Unknown 21 Uge > Ege aNd Upe> Eye and &y < ug<1-&y
Occupied 22 Unc > Enc ANd Uee > Ecc
Modes 23 Une > Exe and ug > E,
(Mode 5) he ‘he id > &d
24 &g < ug<1-& and ue > &¢
Tq -Tsa,5!
All Occupied [2 | Tsa=Tsasl > &
Modes 26 Tina < min(T jg, Tog) - &
(Mode 1, 2, 3,4, | 27 Tima > MAX(T rq, Tog) + &
or) 28 Number of mode transitions per hour > MT max



100 7
“ |
Cooling Coil Control Signal |
80
70
=
3 60
oe)
®@ 4
é |
30 |
|
20 ace Eeonomizer Control Signal
olf
= Heating Col Control Signal
0 —
° 100 200 300 400 500 600
Time (min from beginning of occupancy)
Figure 4.3 VCBT AHU-C Mixed Air Temperature Sensor Drift - Control Signals
40 ——
35
joe Outdoor Air Temperature
30
&
2
5
eg Return Air Te tt
3 turn Air Temperature
e ae
Fe
20 ew
+ mixed air Temperature |
15
‘Supply Air Temperature
Supply Air Temperature Setpoint
10
0 100 200 300 400 500 600

Time (min from beginning of occupancy)

Figure 4.4 VCBT AHU-C Mixed Air Temperature Sensor Drift — Process Temperatures

30



Figures 4.3 and 4.4 show AHU-C data from the occupied portion of one day during the
emulation of this fault. On this particular day, the outdoor air temperature (green, figure 4.4) and
humidity (not shown) are high enough to prohibit economizing, as they typically are during
cooling season, so the mixing box dampers (green, figure 4.3) are aligned to bring in the
minimum amount of outdoor air needed for ventilation. The AHU controller modulates the
cooling coil valve (blue, figure 4.3) to maintain the supply air temperature (light blue, figure 4.4)
at its setpoint (dark blue, figure 4.4). The heating coil valve (red, figure 4.3) is closed.

Based on this combination of control signals, APAR determines the system to be operating in
Mode 4 (mechanical cooling with minimum outdoor air) and evaluates the applicable rule set.
One of the rules in the set for Mode 4 is Rule 26, which states that the mixed air temperature
(brown, figure 4.4) should be between the return air temperature (red, figure 4.4) and outdoor air
temperature (green, figure 4.4). If the mixed air temperature is below the lesser of the return air
or outdoor air temperature, or above the greater of the return air or outdoor air temperature, by a
certain threshold (1.7 °C (3 °F), then a fault has been detected. The actual mixed air
temperature is, in fact, between the return air and outdoor air temperature, since it is the result of
blending the outdoor air and return air streams. However, due to the sensor drift, the measured
mixed air temperature is below the return air temperature (the lesser of the return air and outdoor
air temperature) by approximately 3 °C (5.4 °F). Rule 26 is satisfied, indicating that this fault
has been successfully detected. The fault status data collected from the AHU-C controller
indicates that this fault was detected on this particular day of operation.

4.1.3 Recirculation Damper Stuck Closed

When this fault is introduced, the AHU controller calculates the desired position of the
recirculation damper and sets the damper control signal normally. Within the emulation, the
damper position is set to the fully closed position, corresponding to 100 % outdoor air
throughout the emulation period. During emulation of this fault, the outdoor air and exhaust air
dampers follow normal operation.

Figures 4.5 and 4.6 show data from AHU-A from the occupied portion of one day during the
emulation of this fault. On this particular day, the outdoor air temperature (green, figure 4.6) and
humidity (not shown) are high enough to prohibit economizing, as they typically are during
cooling season, so the mixing box dampers (green, figure 4.5) are commanded to bring in the
minimum amount of outdoor air needed for ventilation. However, the recirculation damper is
stuck closed, so the actual outdoor air flow rate is greater than the minimum required. The
qualitative effect of the stuck recirculation damper can be seen by comparing the mixed air
temperature (brown, figure 4.6) to the return (red, figure 4.6) and outdoor air temperatures
(green, figure 4.6). If the dampers were aligned correctly, the mixed air temperature should be
very close to the return air temperature, but it is actually much closer to the outdoor air
temperature, due to the excessive amount of outdoor air being drawn into the AHU. The AHU
controller modulates the cooling coil valve (blue, figure 4.5) to maintain the supply air
temperature (light blue, figure 4.6) at its setpoint (dark blue, figure 4.6). The heating coil valve
(red, figure 4.5) is closed.

Based on this combination of control signals, APAR determines the system to be operating in
Mode 4 (mechanical cooling with minimum outdoor air) and evaluates the applicable rule set.

29


2 Methodology
2.1 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 and 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
contro] strategies directly related to temperature control. Figure 2.1 is a schematic
diagram of a typical single duct air handling unit (AHU).

Exhaust Air Damper Return Air
Normally Closed T&H
— ]
Pe-s «<— Return From
E Fan Zones
Ramper Recirculation
lotor a Air Damper .
Jp Heating
Outdoor Ais Temperature ~~ PETTY) Normally Open eatin
& Humidity Sensors [)—— oO Mixed - Cooling
Ta Hf|c
To
| > ~He TM Zones
| > cj//c a
‘Outdoor Air Damper T
| Normally Closed Filter Supply.
a Outputs Air
Air Handling Controller
Inputs

Figure 2.1: Schematic diagram of a single duct air-handling unit

2.1.1 System Description

The AHU controller typically controls the supply air 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. A single mixing box
damper control signal is mapped to the outdoor air damper, the exhaust air damper, and
the recirculation air damper. A control signal of 100 % means that the dampers are
positioned for 100 % outdoor air (outdoor and exhaust air dampers are fully open and
recirculation air damper is fully closed); a control signal of 0 % means that the dampers
are aligned for 0 % outdoor air (outdoor and exhaust air dampers are fully closed and
recirculation air damper is fully open). The mixed air passes over the heating and cooling
coils, where if necessary, it is conditioned prior to being supplied to the building. The
typical operating sequence for AHUs consists of four primary modes of operation during


3 Testing Environment

Two different testing environments were used to evaluate the embedded FDD tools: the NIST
Virtual Cybernetic Building Testbed (VCBT) and the Iowa Energy Center Energy Resource
Station (ERS).

3.1. VCBT

The VCBT is a simulation-emulation environment that combines simulations of a building and
its HVAC system with actual commercial HVAC equipment controllers. It provides a way to
conduct tests under a wide variety of carefully controlled conditions and to compare the results
of several different commercial products. 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. Details of the VCBT design and operation are documented in [4].

For this study, the VCBT was configured with one AHU for each of the three floors, designated
AHU-A, -B, and -C. AHU-A and -C are VAV systems, each with three VAV boxes. AHU-B is
a constant volume system, with three zone reheat coils.

3.1.1 AHU Control Strategies
The control strategies described below reflect the logic that is executed by the AHU controllers.

3.1.1.1 AHU-A Control Strategy

3.1.1.1.1 _Fan Control

The supply air fan speed is controlled to maintain the supply air pressure at a fixed set point.
The return air fan speed is controlled to maintain a constant difference between the supply and
return air flow rates.

3.1.1.1.2 Temperature Control

AHU-A uses a single PI control loop to determine a temperature control signal to maintain the
supply air temperature at a fixed set point. Depending on the magnitude of the signal, it is
mapped to one of three outputs which control the heating coil, cooling coil, and mixing-box
dampers. Additional logical sets the position of the other two outputs appropriately depending
on which one is active. For example, if the heating coil valve is active, the cooling coil valve
will be fully closed and the mixing box damper will be set to the minimum position (which
depends on the occupancy status). The outdoor air and the return air enthalpies are compared to
determine whether to enable or disable economizer operation.

3.1.1.2 AHU-B Control Strategy

3.1.1.2.1 Fan Control

AHU-B is a constant volume system, so the supply air fan and return air fan both operate at fixed
speeds.

20


5 Summary and Future Work

The purpose of this report is to present the results of an investigation into embedding APAR, a
tule based FDD tool for AHUs, and VPACC, a statistical quality control based FDD tool for
VAV boxes, into AHU and VAV box controllers, respectively. APAR consists of a set of expert
tules, derived from mass and energy balances. Control signals are used to determine the AHU’s
mode of operation, which identifies the subset of the rules to be evaluated. VPACC uses a small
set of process errors, valid for most VAV box control strategies, to measure VAV box
performance. CUSUM charts, a statistical quality control tool, are used to evaluate the process
errors. Thresholds are determined by statistical analysis of a database of “normal operation”
data.

APAR was evaluated in an emulation environment, while VPACC was evaluated in a laboratory
environment. Consistent results detecting a variety of common mechanical and control faults
show that the FDD tools are both effective at detecting these faults and are suitable for
embedding in commercial HVAC equipment controllers.

In a parallel study, the FDD tools were evaluated using trend data from a number of field sites.
Follow-on work will require partnering with control system manufacturers to conduct field tests
of APAR and VPACC, embedded in their own controller products. NIST’s vision of full
commercialization of automated fault detection and diagnostics is one in which APAR and
VPACC, along with appropriate parameters and thresholds, are packaged within HVAC control
products. In order for this vision to become reality, more work is needed in three main areas.
First, it is impractical to expect trend data to be evaluated to determine the necessary parameters
and thresholds for each site, as was done in this study. Ideally, sets of robust parameters and
thresholds that are effective across specified ranges of applications would be identified.
Additional field data from a wide variety of systems must be collected in order to determine
these robust parameters and thresholds. Also, the current embedded FDD tools are written using
generic mathematical functions available in the languages in which the controllers are
programmed. Although this approach is suitable for a technology demonstration, built-in APAR
and VPACC functions would greatly simplify the task of embedding FDD in a control program.

Finally, more work is needed to develop alternative ways to interpret FDD results and deliver
this information to the building operator. The most direct approach is to generate an alarm that
the operator must acknowledge whenever a rule is violated (APAR) or a cumulative sum exceeds
the alarm limit (VPACC). Refinements to the basic scheme are possible. For example, rather
than automatically sending the alarm to the operator, the building control system could highlight,
on demand, those devices having experienced the greatest number of alarms in a given period of
time. Or, if an automated maintenance management system is used, an alarm could automatically
generate an appropriate work order. However, many faults are the result of design or
commissioning issues that are beyond the scope of the building maintenance staff. Furthermore,
a fault in another piece of equipment, such as an air handling unit, boiler, or chiller, could result
in this approach generating a large number of alarms, perhaps overwhelming the operator. A
mechanism is needed to resolve multiple conflicting fault reports before reporting them to the
operator.

37


6 References

[1] Castro, N.S., Schein, J., Park, C., Galler, M.A., Bushby, S.T., and House, J.M., 2002,
“Results from Simulation and Laboratory Testing of Air Handling Unit and Variable Air Volume
Box Diagnostic Tools”, NISTIR 6964.

[2] 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.

[3] Ryan, Thomas P., 2000, Statistical Methods for Quality Improvement, 2nd Edition, Wiley and
Sons, New York.

[4] Bushby, S.T., Castro, N.S., Galler, M.A., Park, C., and House, J.M., 2001, “Using the Virtual
Cybernetic Building Testbed and FDD Test Shell for FDD Tool Development”, NISTIR 6818.

[5] Price, B.A. and T.F. Smith. 2000. Description of the Iowa Energy Center Energy Resource

Station: Facility Update I. Technical Report: ME-TFS-00-001, Department of Mechanical
Engineering, The University of Iowa, Iowa City.

38


4 Results

41 VCBT

The VCBT emulated an HVAC system operating during heating season (using February weather
data), swing season (using October weather data), and cooling season (using July weather data).
A variety of sensor, actuator, and control logic faults, along with fault free conditions, were
imposed. The results of the FDD calculations performed by the AHU controllers are shown in
Table 4.1. A false positive is a false alarm. A false negative is an undetected fault.

Table 4.1 VCBT Embedded FDD Result Summary

System Fault Season | Correct | False False
Positive | Negative
AHU-A__ | Supply Air Temperature Sensor Drift Heating x
Recirculation Air Damper Leakage Swing x
Outdoor Air Temperature Sensor Failure__| Swing 4
Mixed Air Temperature Sensor Drift Swing 4
Supply Air Temperature Sensor Failure Cooling x
Recirculation Air Damper Stuck Closed Cooling x
AHU-B Fault Free Heating x
Fault Free Swing x
Return Air Temperature Sensor Drift Swing x
Fault Free Swing x
Economizer Control Logic Fault Cooling x
Economizer Control Logic Fault Cooling x
AHU-C | Mixed Air Temperature Sensor Failure Heating x
Return Air Temperature Sensor Drift Swing x
Outdoor Air Damper Stuck at Minimum Swing x
Recirculation Air Damper Leakage Swing Xx
Return Air Temperature Sensor Drift Cooling x
Mixed Air Temperature Sensor Drift Cooling 4

The AHU controller determined the correct fault status in 15 of the 18 cases. There were zero
false positives and three false negatives. Two of the false negatives were return air temperature
sensor drift faults in AHU-B and AHU-C, both in swing season. The swing season weather
conditions resulted in the AHUs operating in Modes 2 (cooling with outdoor air) and 3
(mechanical cooling with 100 % outdoor air) most of the time. However, the return air
temperature sensor drift fault was also instigated in AHU-C in cooling season, when the AHUs
operated mostly in mode 4 (mechanical cooling with minimum outdoor air). This time, the
return air temperature sensor drift was detected. This shows that some faults can be detcted
under certain conditions, but not others. The other false negative was an outdoor air damper in
AHU-C stuck at the minimum position during swing season. In this case, the AHU controller

26





2.2. FDD for VAV Boxes

2.2.1 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. These constraints make the development of VAV box fault
detection tools difficult, but the quantity of VAV boxes makes the effort worthwhile, due to the
impact. 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.

2.2.2 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 the process output at
sampling time i denoted x;, the estimate of the expected value denoted %, and the estimate of the
Process standard deviation denoted by &, the normalized process output is given by:

13


2.2.3. System Description

Figure 2.4 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 2.4. 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.

VAV Box Controller [e—————

;
H ci "Reheat Coil Valve

: | Hot Water Supply) | Hot Water Discharge Air
Differential Return Temperature
Pressure Sensor
Transducer

Damper =
<a
Supply Air c \
| eevaeeeeree Hydronic
Reheat Coil

Room

Thermostat

Figure 2.4: Schematic diagram of a single duct pressure-independent VAV box with
hydronic reheat.

Figure 2.5 shows a typical control sequence for a pressure-independent VAV box. A heating set
point and a cooling set point are specified. As the zone temperature increases above the cooling
set point, the airflow rate to the zone increases proportionally. This is accomplished by resetting
the airflow rate setpoint and modulating the damper to achieve this airflow rate. As the zone
temperature decreases toward the cooling set point, the airflow rate setpoint is decreased and the
damper gradually closes 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 2.5, it is

15


NISTIR 7036

Results from Laboratory Testing of
Embedded Air Handling Unit and Variable
Air Volume Box Fault Detection Tools

Jeffrey Schein
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

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



0.3

>
a
&
$
5
Q
-0.3 + + >
0 60 120 180 240 300 360 420 480 540 600 660
Data Point (1-min. Intervals)
Figure 4.9 ERS data showing the effect of an unstable airflow fault.
1000
A fn
800 + / | ra |
| t
/ /
AY Pa | | a
soo, yoy ran vf
z / | Z ! Fa
am 4004 Y | / | J
=) / / | J
1S) J | i | vf
207 | / SgandTg | ¥
r Lf Lf
ok LLeemennenen Sel
-200 —— :

0 60 120 180 240 300 360 420 480 540 600 660

Data Point (1-min. Intervals)

Figure 4.10 VPACC output corresponding to the conditions in Figure 4.9.

36


errors, with the following exceptions: for the IQerror| the S cumulative sum only is calculated,
and the AT-;ro, S and T cumulative sums are set to zero if the reheat coil control valve is not
fully closed. There is an alarm status, represented by a binary value, for each cumulative sum. If
the sum is greater than the alarm limit (h), the corresponding alarm status is set to a value of
“on”. If the sum then falls below h, the alarm status is set to “off’. There is also an overall
VPACC alarm status, which is set to a value of “on” if any of the cumulative sum alarm statuses
are “on”. If the cumulative sum alarm statuses are all “off” then the VPACC alarm status is set
to “off”.

The operator interface software written by the manufacturer of the VAV box controllers was
installed on a workstation connected to the ERS control network and configured to trend the raw
data (temperatures, humidities, etc.) as well as intermediate values and results of the VPACC
algorithm (S and T cumulative sum values and alarm statuses). The raw data and VPACC values
were read once per minute. For every fault detected by VPACC, the raw data was examined to
verify that the FDD results matched the conditions.

3.2.2 ERS Fault Descriptions
3.2.2.1 Damper Stuck Partially Open

This fault is introduced by overriding the VAV box damper actuator to a fixed position that
produces a flow rate between the minimum and maximum specified for that box. If the zone
airflow is lower than necessary, the zone temperature will drift above the cooling set point. If the
zone airflow is higher than necessary, the controller will transition to the heating mode and the
reheat coil valve will modulate to maintain the zone temperature at the set point.

3.2.2.2 Hydronic Reheat Coil Valve Stuck Partially Open

This fault is introduced by overriding the VAV box hydronic reheat coil valve actuator position
to allow a hot water flow rate of approximately 2 % to 10 % of the maximum flow through the
coil. Depending on the zone conditions and the severity of the fault, the stuck reheat valve either
creates an additional cooling load that the AHU 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 cooling set point. In the
second case, the zone temperature will tend to gradually decrease below the zone heating set
point.

3.2.2.3 Failed Differential Pressure Sensor

This fault is introduced by disconnecting both tubing leads to the differential pressure sensor.
The fault causes the VAV box damper to go to the full open position because the flow sensor
indicates an airflow rate of zero and the control loop will attempt to correct for this condition.

24




Control Signal (0 % - 100 %)

Temperature (°C)

120

30

&

8

a

oie

Cooling Coil Control Signal
Economizer Control Signal
esters
i {
a—Heating Coil Control Signal |
0 100 200 300 400 500 600

Time (min from beginning of occupancy)

Figure 4.5 VCBT AHU-A Recirculation Damper Stuck Closed — Control Signals

Outdoor Air Temperature

Mixed Air Temperature

“\Retum Air Temperature

ge ‘Supply Air Temperature

W NG Air Temperature Setpoint

100 200 300 400 500
Time (min from beginning of occupancy)

Figure 4.6 VCBT AHU-A Recirculation Damper Stuck Closed — Process Temperatures

32


can still adjust the ratio of return to outdoor air by modulating the recirculation and exhaust
dampers. This is an example of a fault that is masked by the control system.

Several examples are presented to illustrate the details of the APAR algorithm.
4.1.1 Outdoor Air Temperature Sensor Failure

This fault was introduced by disconnecting the leads to the terminals on the AHU-A controller
on which the outdoor air temperature sensor was connected. This is a0 — 10 VDC analog input
calibrated to a range of -6 °C — 49 °C (20 °F ~ 120 °F), so with the leads disconnected, the
controller reads 0 VDC, which is scaled to an outdoor air temperature of -6 °C (20 °F).

Swing season is characterized by substantial variation in outdoor air temperature and humidity.
Modes 2, 3, and 4 (cooling with outdoor air, mechanical cooling with 100% outdoor air, and
mechanical cooling with minimum outdoor air, respectively) are all encountered during a fault
free three week swing season emulation. When the fault is introduced, the controller reads an
outdoor air temperature of -6 °C (20 °F). The logic in the controller calculates outdoor air
enthalpy based on outdoor air temperature and outdoor air humidity, so the erroneously low
reading of outdoor air temperature will result in an erroneously low calculated value of outdoor
air enthalpy. The AHU-A controller compares the calculated value of outdoor air enthalpy to
return air enthalpy (calculated in a similar fashion based on return air temperature and return air
humidity) and enables economizer operation if the outdoor air enthalpy is less than the return air
enthalpy. The erroneously low calculated value of outdoor air enthalpy will cause the controller
to enable economizer operation, even if such operation is inappropriate.

Figures 4.1 and 4.2 show data from AHU-A from the occupied portion of one day during the
emulation of this fault. Since the actual outdoor air temperature is greater than the supply air
temperature setpoint, the mixing box dampers (green, figure 4.1) will saturate at the 100 %
outdoor air position, and the AHU controller will modulate the cooling coil valve (blue, figure
4.1) to maintain the supply air temperature at its setpoint. The heating coil valve (red, figure 4.1)
is closed. Based on this combination of control signals, APAR determines the system to be
operating in Mode 3 (mechanical cooling with 100 % outdoor air) and evaluates the applicable
rule set. One of the rules in the set for Mode 3 is Rule 8, which states that if the outdoor air
temperature (green, figure 4.2) is less than the supply air temperature setpoint (not shown,
constant 12.8 °C (55 °F)) minus the temperature rise across the supply fan (1.1 °C (2 °F)) by
more than a certain threshold (1.7 °C (3 °F)), then a fault has been detected. Another rule in the
set for Mode 3 is Rule 10, which states that if the absolute value of the difference between
outdoor air temperature and mixed air temperature (purple, figure 4.2) is greater than a certain
threshold (1.7 °C (3 °F)), then a fault has been detected. On the day shown in figures 4.1 and
4.2, Rules 8 and 10 are satisfied, indicating that this fault has been successfully detected. The
fault status data collected from the AHU-A controller indicates that this fault was detected on
this particular day of operation.

4.1.2 Mixed Air Temperature Sensor Drift

This fault is introduced as a sensor offset beginning at 0 °C and increasing linearly over a three
week emulation period to +4 °C. The Positive sensor drift means that the measured mixed air
temperature is greater than the actual mixed air temperature by the amount of the offset.

27


Table 2.2: Summary of Rule Relationships

ae
Rule | Mode’ Sensors and Control Signals’ Relationship Between Grouped Rules
Tea | Tra | Tra] Ton [Tens] ATa] AT] Teo | ues | Ure | us

7 7

Z 2 Coll Subsystem: The relational sign (<, >, etc.)

rT 3 changes based on the mode of operation.

16 4

2 4 Mixing Box Subsystem: Rules are related

18 4 through calculation of outdoor air fraction. If

26 T2a4 Rule 26 or 27 is satisfied, the outdoor air

27 1234 fraction will be negative or greater than unity.

25 1,2,3,4

3 7 Comfort Requirements: The first four rules

7] 3 indicate comfort is sacrificed (with ules 3, 13,
and 19 indicating the system is out of contol)

19 4 whereas the latter three rules indicate comfort

4 4 could soon be sacrificed (system is out of

14 3 control).

20 4

3 2 ‘The relational sign (<, >, etc.) changes based

3 3 ‘on the mode of operation.

6 2

12 3 Zone Subsystem: Rules are identical.

7 4

° 3 Z Economizer. The relational sign (<, >, ete.)

5 4 changes based on the mode of operation.

10 2

21 - Controller Logic/Tuning: Rules are related and

2 5 identity periods of operation associated with

23 : ZL controller problems, such as simultaneous

24 7 heating and cooling, and excessive mode

28 i changes.

' The dash symbol indicates either an unknown mode or multiple modes of operation.
? The checked box symbol indicates which quantities are compared for the given rule.

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 2.1 (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.

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 enthalpy-based economizer);

© Description of sequencing/economizer cycle strategy (used to verify that the rules are
suitable to a particular AHU installation).

i


24-7 )

The normalized process output is used to compute two cumulative sums defined as follows:
S; =max[0,zj-k+Sj)] (2a)
T, =min[0,z,+k+T,,] (2b)

where k is a slack parameter that must be specified. Positive values of z greater than k cause the
sum S to move away from zero and the sum T to approach or remain at zero. Negative values of z
less than -k cause the sum T to move away from zero and the sum S to approach or remain at
zero. A process is said to be out of control when either S exceeds a threshold value defined by
the parameter h, or T falls below -h. Figure 2.3 [3] 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 above its threshold of -5.

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 +36. In terms of the 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 andh = 0.

3 25

2 20

15

1
2 Ss

10

x0 3 5 Upper Chart Limit
a n/m SY T
os /

2 5
Lower Chart Limit

3 -10

0 20 40 60 80 100 0) 20 40 60 80 100
Data Point Data Point

Figure 2.3: A simple CUSUM control chart indicating an “out of control” process.

14