NISTIR 7216

A Simulation Study of a Hierarchical, 

Rule-Based Method for System-Level 

Fault Detection and Diagnostics in HVAC 

Systems 

Jeffrey Schein

Steven Bushby

U.S DEPARTMENT OF COMMERCE

National Institute of Standard and Technology

Building Environment Division

Building and Fire Research Laboratory

Gaithersburg, MD 20899-8631

April 2005

A Simulation Study of a Hierarchical, RuleBased Method for System-Level Fault 

Detection and Diagnostics in HVAC Systems

Jeffrey Schein

Steven Bushby

U.S DEPARTMENT OF COMMERCE 

National Institute of Standard and Technology 

Building Environment Division 

Building and Fire Research Laboratory 

Gaithersburg, MD 20899-8631 

NISTIR 7216

U.S. Department of Commerce 

Carlos M. Gutierrez, Secretary 

National Institute of Standards and Technology 

Hratch G. Semerjian., Acting Director 

1 

A Simulation Study of a Hierarchical, Rule-based Method for 

System-level Fault Detection and Diagnostics in HVAC Systems 

Abstract 

A system-level fault detection and diagnostic (FDD) method for heating, ventilation, and air conditioning 

(HVAC) systems was developed. It functions as an interface between multiple, equipment-specific FDD 

tools and a human operator. The method resolves conflicting fault reports from equipment-specific FDD 

tools, performs FDD at the system level, and presents an integrated view of an HVAC system’s fault 

status to an operator. A simulation study to test and evaluate the method was conducted. 

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 which inadvertently causes more problems. 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). 

A number of FDD tools that are emerging from research are described in [1] and [2]. Most of these tools 

are equipment-specific, meaning they are meant to detect faults in a chiller, air handling unit (AHU), etc. 

Results from a study to field-test equipment-specific FDD tools for AHUs and terminal units [3] show 

the need for another tool to act as an interface between equipment-specific FDD tools and the building 

operator in order to resolve conflicting fault reports from the various equipment-specific FDD tools and 

to present an integrated view of the HVAC system’s fault status to the operator. In one case, a fault 

detected in a terminal unit at one of the field sites, in which the zone air temperature increased well 

above the setpoint, was traced to a problem with the supply air temperature provided by the AHU serving 

the terminal unit. The results from another site include several cases of terminal unit faults caused by 

problems with the supply air temperature and pressure provided by the associated AHU. There are also 

numerous terminal unit and AHU faults documented, which were traced to scheduling inconsistencies 

between terminal units, AHUs, and the chilled water plant. 

The common thread in the previous examples is the set of hierarchical relationships between the different 

pieces of equipment comprising the HVAC system. Each AHU delivers supply air to the terminal units it 

serves, the chilled water plant supplies chilled water to the AHUs, and the boiler plant provides hot water 

to the AHUs and terminal units. These relationships are each characterized by a single “source” 

supplying multiple “loads”. In each case, there is an air or water fluid interface between the “source” and 

the “loads”. The temperature and pressure of the fluid are maintained at setpoint values by the “source” 

equipment in order to enable the “load” equipment to operate as intended. Normally, each AHU and the 

terminal units it serves form a separate, distinct hierarchy. However, the output of each of the chillers is 

aggregated to form a single chilled water plant, which then supplies chilled water to all the loads in the 

system. Similarly, the output of the boilers is aggregated to form a single boiler plant supplying all the 

hot water loads in the system. 
2 

In order to present an integrated view of an HVAC system’s fault status to the operator, the system-level 

FDD method developed in this project uses these hierarchical relationships to act as a filter, resolving 

conflicting fault reports from equipment-specific FDD tools. Knowledge of these relationships, 

combined with access to some selected raw data, also allows the method to perform FDD at the system 

level. 

Methodology 

A number of different approaches to solve the problem of FDD integration and conflict resolution were 

considered. One possible approach is described in [4], in which a high level structural model is used as a 

topological basis to link together models of each of the components of the mechanical system. Any of a 

number of different types of models could be used for the individual components of the system. 

However, the computational demands of a model-based solution require that it run on a general-purpose 

computer. In general, it is not possible for any of the various embedded devices that make up the balance 

of the building automation system to perform model-based FDD analysis. 

Another alternative under consideration was a case-based solution to conflict resolution as described in 

[5]. Briefly, a template is defined containing sufficient fields to describe the state of the HVAC system. 

Current data are entered into the template, and a database of historical cases is searched for the closest 

match to the current state. Several different algorithms can be used to determine which historical case is 

the best match. This approach, while less computationally demanding than the model-based approach, 

requires far more memory than is available from an embedded device in a typical building automation 

system and therefore, as with the model-based solution, requires a general-purpose computer. 

A rule-based approach has a number of advantages, including the transparency, flexibility, and 

adaptability obtained by expressing the knowledge base as a set of rules, explicitly separated from the 

decision-making framework [6]. Furthermore, this approach is suitable to run on an embedded device 

with respect to both computational and memory requirements. The rule-based approach was selected 

because it offers the most flexibility for implementation in a variety of control system architectures. 

The proposed system-level FDD tool consists of two components: a hierarchical decision-making 

framework and a set of rules. The decision-making framework collects fault reports and raw data from 

the local equipment controllers and passes these data to the rule set for evaluation. The rules are used to 

determine whether to pass the fault reports on to a human operator. Figure 1 is a schematic illustration of 

the framework. 
VAV Box

Controller

VAV Box

Controller

VAV Box

Controller

VAV Box

Controller

AHU

Controller

AHU

Controller

Chilled Water

Plant

Controller

Boiler Plant

Controller

Hierarchical FDD Algorithm

Fault

Reports

and Raw

Data

Fault

Reports

and Raw

Data

Fault

Reports

and Raw

Data

Fault

Reports

and Raw

Data

Fault

Reports

and Raw

Data

Fault

Reports

and Raw

Data

Fault

Reports

and Raw

Data

Fault

Reports

and Raw

Data

Building

Operator

Integrated

System

Fault

Report

VAV Box

Controller

VAV Box

Controller

VAV Box

Controller

VAV Box

Controller

AHU

Controller

AHU

Controller

Chilled Water

Plant

Controller

Boiler Plant

Controller

Hierarchical FDD Algorithm

Fault

Reports

and Raw

Data

Fault

Reports

and Raw

Data

Fault

Reports

and Raw

Data

Fault

Reports

and Raw

Data

Fault

Reports

and Raw

Data

Fault

Reports

and Raw

Data

Fault

Reports

and Raw

Data

Fault

Reports

and Raw

Data

Building

Operator

Integrated

System

Fault

Report

Figure 1. The Hierarchical-Decision Making Framework 

The fundamental methodology consists of four general principles for integrating equipment-specific fault 

reports and resolving conflicts given the source-load hierarchical relationships between the different 

pieces of equipment and subsystems in an HVAC system: 

1. Fault reports from a load are suppressed if there are any fault reports from a source serving that 

load. 

2. A scheduling conflict between a source and any of the loads it serves is considered a fault 

associated with the source. Any fault reports from any of the loads served by the faulty source 

are suppressed. 

3. A fault in the air or water fluid interface between a source and the loads it serves is considered a 

fault associated with the source. Any fault reports from any of the loads served by the faulty 

source are suppressed. 

4. Multiple fault reports from the loads served by a source are considered a fault associated with the 

source. Any fault reports from any of the loads served by the faulty source are suppressed. 

The rule set was generated by applying these general principles to the hierarchical subsystems in an 

HVAC system. See Table 1. A software implementation combining the hierarchical decision-making 

framework and the rule set was developed using the algorithm illustrated in Figure 2. 

3 
4 

Table 1. Rule set for integrating multiple fault reports. 

Principle Subsystem Rule 

Chiller plant 1. If there are any equipment-level chiller plant faults, pass them on to the 

operator, and suppress any AHU or terminal unit faults. 

Boiler plant 2. If there are any equipment-level boiler plant faults, pass them on to the 

operator, and suppress any AHU or terminal unit faults. 

AHU 3. If there are no chiller plant or boiler plant faults and there are any 

equipment-level AHU faults, pass them on to the operator, and suppress any 

fault reports from the terminal units served by the faulty AHU. 

Fault reports from a load are 

suppressed if there are any fault 

reports from a source serving 

that load. 

Terminal 

unit 

4. If there are no chiller plant or boiler plant faults and there are no faults in the 

associated AHU, pass any equipment-level terminal unit faults on to the 

operator. 

Chiller plant 5. If the chiller plant is off and any AHU is on and requires chilled water, 

report a chiller plant scheduling fault to the operator, and suppress any AHU 

or terminal unit faults. 

Boiler plant 6. If the boiler plant is off and any AHU or terminal unit is on and requires hot 

water, report a boiler plant scheduling fault to the operator, and suppress any 

AHU or terminal unit faults. 

A scheduling conflict between 

a source and any of the loads it 

serves is considered a fault 

associated with the source. 

Any fault reports from any of 

the loads served by the faulty 

source are suppressed. AHU 7. If there are no chiller plant or boiler plant faults and any AHU is off while 

any of the terminal units it serves is in occupied mode, report an AHU 

scheduling fault to the operator, and suppress any fault reports from the 

terminal units served by the faulty AHU. 

Chiller plant 8. If the chilled water supply temperature is greater than its setpoint (subject to 

the given threshold), report a chiller plant supply temperature fault to the 

operator, and suppress any AHU or terminal unit faults. 

Chiller plant 9. If the chilled water supply pressure is less than its setpoint (subject to the 

given threshold), report a chiller plant supply pressure fault to the operator, 

and suppress any AHU or terminal unit faults. 

Boiler plant 10. If the hot water supply temperature is less than its setpoint (subject to the 

given threshold), report a boiler plant supply temperature fault to the 

operator, and suppress any AHU or terminal unit faults. 

Boiler plant 11. If the hot water supply pressure is less than its setpoint (subject to the given 

threshold), report a boiler plant supply pressure fault to the operator, and 

suppress any AHU or terminal unit faults. 

AHU 12. If there are no chiller plant or boiler plant faults and the AHU supply air 

temperature is greater than its setpoint (subject to the given threshold), 

report an AHU supply air temperature fault to the operator, and suppress any 

fault reports from the terminal units served by the faulty AHU. 

A fault in the interface between 

a source and the loads it serves 

is considered a fault associated 

with the source. Any fault 

reports from any of the loads 

served by the faulty source are 

suppressed. 

AHU 13. If there are no chiller plant or boiler plant faults and the AHU supply air 

pressure is less than its setpoint (subject to the given threshold), report an 

AHU supply air pressure fault to the operator, and suppress any fault reports 

from the terminal units served by the faulty AHU. 

Chiller plant 14. If the number of AHUs reporting faults possibly related to the chiller plant is 

greater than the threshold and the chiller plant is on, report a possible chiller 

plant fault to the operator, and suppress any AHU or terminal unit faults. 

Boiler plant 15. If the number of AHUs reporting faults possibly related to the boiler plant is 

greater than the threshold and the boiler plant is on, report a possible boiler 

plant fault to the operator, and suppress any AHU or terminal unit faults. 

Boiler plant 16. If the number of terminal units with heating coils reporting faults possibly 

related to the boiler plant is greater than the threshold and the boiler plant is 

on, report a possible boiler plant fault to the operator, and suppress any 

AHU or terminal unit faults. 

Multiple fault reports from the 

loads served by a source are 

considered a fault associated 

with the source. Any fault 

reports from any of the loads 

served by the faulty source are 

suppressed. 

AHU 17. If there are no chiller plant or boiler plant faults and the set of terminal units 

served by any particular AHU reports a number of faults greater than the 

threshold, report a possible AHU fault to the operator, and suppress any 

fault reports from the terminal units served by the faulty AHU. 
Parallel Operation

Synchronize Operation

Parallel Operation

Parallel Operation

Synchronize Operation

Synchronize Operation

Start

Read raw data and

equipment level fault

reports

Time filter the raw data

Any chilled

water plant fault

reports?

Chilled water

scheduling fault?

Chilled water supply

pressure fault?

Chilled water supply

temperature fault?

Too

many AHU

faults?

Any terminal unit

fault reports?

End

Pass chilled water plant

fault reports to operator

Report chilled water plant

scheduling fault to

operator

Report chilled water

supply pressure fault to

operator

Report chilled water

supply temperature fault

to operator

Report possible chilled

water plant fault to

operator

Y

N

N

N

N

N

Y

Y

Y

Y

Any chilled water

or boiler faults?

N

Y

Any

 boiler plant fault

reports?

Boiler plant

scheduling fault?

Hot water supply

pressure fault?

Hot water supply

temperature fault?

Too

many AHU

faults?

Too

many zone

faults?

Pass boiler plant fault

reports to operator

Report boiler plant

scheduling fault to

operator

Report hot water supply

pressure fault to operator

Report hot water supply

temperature fault to

operator

Report possible boiler

plant fault to operator

Report possible boiler

plant fault to operator

Y

N

N

N

N

N

N

Y

Y

Y

Y

Y

Any AHU fault

reports?

AHU scheduling

fault?

AHU supply air

pressure fault?

AHU supply air

temperature fault?

Too

many zone

faults?

Pass AHU fault reports to

operator

Report AHU scheduling

fault to operator

Report AHU supply air

pressure fault to operator

Report AHU supply air

temperature fault to

operator

Report possible AHU

fault to operator

Y

N

N

N

N

N

Y

Y

Y

Y

Pass terminal unit fault

reports to operator

N

Y

For each

terminal

unit

served

by the

AHU

For each

AHU

Figure 2. Flowchart of Hierarchical FDD algorithm. 

5 
6 

Testing Environment 

A simulation model of a three-story office building and its HVAC system was developed using the 

HVACSIM+ simulation tool [7]. The simulation consists of nine zones, three air handling units, a chiller, 

a cooling tower, and a boiler. The terminal units are pressure independent variable-air-volume (VAV) 

boxes with hydronic reheat. Each of the three single-duct VAV AHUs serves the terminal units for three 

zones. The building shell and air handling systems are represented by aggregating HVACSIM+

component models which were developed in previous research [8]. The chiller and cooling tower models 

[9] as well as the boiler model [10] were also adapted from previous work. Faults were selected based on 

their impact on multiple pieces of equipment, in order to provide a meaningful test of the hierarchical 

FDD algorithm. The faults, listed in Table 2, were simulated through modifications to the boundary 

conditions or component models. 

Table 2. List of HVAC system and equipment faults. 

Fault # Fault Description Implementation 

0 Fault free Run the simulation without modification 

1 AHU scheduling fault Schedule zone-1 on during the unoccupied periods 

2 VAV box reheat coil valve stuck nearly 

closed Fix the zone-1 reheat coil valve position to 5 % open 

3 VAV box reheat coil valve stuck nearly 

open Fix the zone-1 reheat coil valve position to 95 % open 

4 VAV box discharge air temperature 

sensor fault Fix the zone-1 DAT sensor output to -10 °C 

5 Chiller plant scheduling fault Schedule AHU-1 and its associated VAV boxes 

(zones 1, 4, and 7) on during the unoccupied periods 

6 AHU supply air pressure setpoint fault Set the AHU-1 supply air pressure setpoint to 

0.01 kPa 

7 AHU supply air pressure sensor fault Fix the AHU-1 supply air pressure sensor output to 

 -5.00 kPa 

8 AHU supply fan failure Fix the AHU-1 supply fan speed to zero 

9 AHU supply air temperature setpoint 

too low 

Change the AHU-1 supply air temperature setpoint to 

5 °C 

10 AHU supply air temperature sensor 

drift 

Increase the AHU-1 supply air temperature sensor 

offset calibration parameter by +5 °C 

11 AHU return air temperature sensor 

failure 

Fix the AHU-1 return air temperature sensor output to 

-10 °C 

12 AHU mixed air temperature sensor 

failure 

Fix the AHU-1 mixed air temperature sensor output to 

-10 °C 

13 AHU outdoor air temperature sensor 

failure 

Fix the AHU-1 outdoor air temperature sensor output 

to -20 °C 

14 AHU heating coil valve stuck nearly 

open 

Fix the AHU-1 heating coil valve position to 95 % 

open 

15 AHU cooling coil valve stuck nearly 

open 

Fix the AHU-1 cooling coil valve position to 95 % 

open 

16 Chilled water supply temperature 

setpoint fault 

Change the chilled water supply temperature setpoint 

to 20 °C 

17 Chilled water supply temperature fault Fix the chilled water supply temperature to 20 °C 

18 Hot water supply temperature setpoint 

fault 

Change the hot water supply temperature setpoint to 

20 °C 

19 Hot water supply temperature fault Fix the hot water supply temperature to 20 °C 
7 

Results 

Overview 

The simulation study consisted of a one-week simulation run for twenty different fault conditions during 

each of three seasons: heating season (using February weather data), swing season (using October 

weather data), and cooling season (using July weather data). The simulation output data files were then 

used as input data for the software implementation of the hierarchical FDD tool. The software 

implementation also included equipment-specific AHU and VAV box FDD tools which are fully 

described in [11]. The results are summarized in Table 3. The data were stored and processed using 5 

min intervals, so the number of alarms represents the number of 5 min periods during which an alarm 

was reported. The alarms passed on to the operator by the tool as well as those suppressed by the tool are 

presented. The actual source of the fault is shaded. The piece of equipment for which the most alarms 

are passed on to the operator is the primary diagnosis for that simulation run. Any other pieces of 

equipment for which alarms are passed on to the operator are secondary diagnoses. 

For Fault 0 (normal operation) none of the equipment was faulty, nor were any faults reported by the 

tool. Therefore, the FDD tool gave correct results for all three simulation runs (using cooling, heating, 

and swing season weather data). 

During the Fault 1 simulation runs, Zone 1 was scheduled on during a period when the AHU serving the 

zone, AHU-1, was scheduled off. This scheduling conflict was detected by Rule 7 of the tool (AHU 

scheduling conflict) and correctly attributed to AHU-1. Equipment-specific FDD alarms from Zone 1 

were correctly suppressed. 

Faults 2, 3, and 4 simulated various VAV box mechanical and control faults in Zone 1 that were detected 

by the equipment-specific VAV box FDD tool and passed on to the operator in accordance with Rule 4 

(terminal unit equipment-specific alarm). 

During the Fault 5 simulation runs, AHU-1 and its associated VAV boxes (Zones 1, 4, and 7) were 

scheduled on during a period when the chilled water plant was scheduled off. This scheduling conflict 

was detected by Rule 5 of the tool (chiller plant scheduling conflict) in cooling and swing seasons. 

Equipment-specific FDD alarms from AHU-1 and Zones 1, 4, and 7 were correctly suppressed. Rule 5 

requires that, in addition to the scheduling inconsistency, there must be a demand for chilled water in 

order for this fault to exist. The determination of the demand for chilled water includes a delay timer, 

which is reset upon an AHU mode change. There were three occasions during the cooling season 

simulation run when a few false alarms from Zone 1 were passed on, because an AHU-1 mode change 

caused the delay timer to be reset, briefly interrupting the hierarchical FDD tool’s suppression of the 

terminal unit alarms. However, there were many more chilled water plant alarms than terminal unit 

alarms so the primary diagnosis was the correct one: that the chilled water plant was the source of the 

fault. During the heating season simulation run there was no demand for chilled water, so no Rule 5 

alarms were generated. 

In the case of Fault 6, the inappropriate AHU-1 supply air pressure setpoint cannot be detected by the 

equipment-specific AHU FDD tool. The only way this fault can be detected is to infer an AHU fault due 

to multiple fault reports from the zones served by the AHU (Rule 17). During cooling and swing 

seasons, sufficient VAV box faults were reported to the hierarchical FDD tool that the AHU fault was 

correctly detected in both seasons. The equipment-specific FDD alarms from Zones 1, 4, and 7 were 

correctly suppressed. However, at certain times during the simulation runs only one of the zones 

reported a fault to the hierarchical FDD tool, which then incorrectly passed on the VAV box fault, 
8 

causing some false alarms during cooling and swing seasons. For the cooling season, the AHU-1 fault 

generated more alarms than any of the VAV boxes, so the primary diagnosis was correct. This was not 

the case for the swing season simulation run. For swing season, the number of Zone 1 alarms was greater 

than the number of AHU-1 alarms, so the primary diagnosis was incorrect. However, some AHU-1 

alarms were reported so the actual cause of the fault did appear as a secondary diagnosis. During heating 

season, the cooling loads in the zones were low enough that the fault had no impact on system operation, 

so no VAV box faults were generated which could be used to infer an AHU fault by Rule 17. A more 

detailed discussion of the results from Fault 6 is presented in the next section. 

Faults 7 (AHU-1 supply air pressure sensor failure) and 8 (AHU-1 supply fan failure) were both detected 

in all three seasons by Rule 13 of the tool, which compares the AHU supply air pressure with the 

setpoint. All equipment-specific terminal unit alarms were correctly suppressed. 

Fault 9 (AHU-1 supply air temperature setpoint too low) was detected in all three seasons by the AHU 

equipment-specific FDD tool and passed on in accordance with Rule 3. The equipment-specific VAV 

box FDD tool generated some false alarms for Zone 4, which were suppressed, also in accordance with 

Rule 3. A more detailed discussion of the results from Fault 9 is presented in the next section. 

Fault 10 (AHU-1 supply air temperature sensor drift) was detected in all three seasons by inferring an 

AHU fault due to multiple fault reports from the zones served by the AHU (Rule 17). False alarms from 

the VAV box equipment-specific FDD tool were suppressed. Fault 10 was also detected by the 

equipment-specific AHU FDD tool in swing season. 

Faults 11 (AHU-1 return air temperature sensor failure), 12 (AHU-1 mixed air temperature sensor 

failure), and 13 (AHU-1 outdoor air temperature sensor failure) were detected by the AHU equipmentspecific FDD tool and passed on in accordance with Rule 3. Faults 11 and 12 were detected in all three 

seasons, while Fault 13 was detected only in cooling and swing seasons. In heating season, Fault 13 had 

no impact on system operation, therefore no alarms were generated. 

Fault 14 (AHU-1 heating coil valve stuck nearly open) was detected by the AHU equipment-specific 

FDD tool (Rule 3), from a discrepancy between the supply air temperature and the setpoint (Rule 12), 

and by inferring an AHU fault from multiple zone faults (Rule 17). False alarms from the VAV box 

equipment-specific FDD tool were suppressed in all three seasons, however, a small number of false 

alarms were passed on in heating season. The primary diagnosis was correct for all three simulation 

runs. 

Fault 15 (AHU-1 cooling coil valve stuck nearly open) was detected by the AHU equipment-specific 

FDD tool (Rule 3) and by inferring an AHU fault from multiple zone faults (Rule 17). False alarms from 

the VAV box equipment-specific FDD tool were suppressed in swing season, however, a small number 

of false alarms were passed on in heating season. The primary diagnosis was correct for all three 

simulation runs. 

Since there was no central plant FDD tool in this study, the high chilled water supply temperature 

setpoint fault (Fault 16) was not detected through Rule 1. The fault impacted the operation of the entire 

system so that the fault was detected by Rule 14 (chiller plant fault inferred due to multiple AHU faults). 

Most, although not all, of the false alarms from the AHU and VAV box equipment-specific FDD tool 

were suppressed. For the cooling and swing season simulation runs, the primary diagnosis indicated a 

chiller plant fault. For the heating season simulation run, the fault did not have as great of an impact on 

system operation. Although there were faults reported from the equipment-specific FDD tools for AHU-
9 

1 and AHU-2, they did not occur simultaneously, so there were no alarms generated for the chiller plant 

by Rule 14. Instead, the false alarms were passed on. Since AHU-2 had the most alarms, it was the 

primary diagnosis. For the cooling and swing season simulation runs, the primary diagnosis, indicating a 

chiller plant fault, was correct. For the heating season simulation run, neither the primary nor the 

secondary diagnosis was correct. 

Fault 17, the chilled water supply temperature fault, was correctly diagnosed in all three seasons by Rule 

8, which compares the chilled water supply temperature to its setpoint. False alarms from the AHUs and 

VAV boxes were suppressed, except for one alarm from AHU-3 during cooling season. However, the 

primary diagnosis was still correct, since there were far more chilled water plant alarms than AHU-3 

alarms. A more detailed discussion of the results from Fault 17 is presented in the next section. 

Fault 18, the hot water supply temperature setpoint fault can only be detected indirectly through Rules 15 

(boiler plant fault inferred due to multiple AHU faults) and 16 (boiler plant fault inferred due to multiple 

terminal unit faults). Fault 18 does not impact system operation during swing or cooling season so no 

alarms are generated for those simulation runs. During heating season, equipment-specific faults are 

reported for AHU-1 and Zone 4 only (because of its exposure, Zone 4 has a higher heating load than the 

other zones). Since an insufficient number of AHUs or VAV boxes report alarms, neither Rule 15 nor 16 

applies, and the equipment-specific FDD alarms are incorrectly passed on to the operator. For the 

heating season simulation run, neither the primary nor the secondary diagnosis was correct. 

Fault 19, the hot water supply temperature fault, was correctly detected by Rule 10, which compares the 

hot water supply temperature to its setpoint. False alarms from AHU-1 were suppressed. 

Of a total of 60 simulation runs, 49 indicated the correct source of the fault as the primary diagnosis. In 

addition to the three fault-free cases, there were five simulation runs in which the fault had no impact on 

system operation – in all eight cases the hierarchical FDD tool correctly reported no alarms. In one case, 

the primary diagnosis was incorrect, but the actual source of the fault was a secondary diagnosis. There 

were two cases in which the actual source of the fault was neither a primary nor a secondary diagnosis. 

Combining the 49 correct primary diagnoses with the eight correct non-fault cases, the hierarchical FDD 

tool provided a correct primary response 95 % of the time.
Table 3. Results from Simulation.

# Alarms Sent to Operator (# Alarms Suppressed) Note: Actual source of fault is shaded. 

Fault Central Plant Air Handling System 1 Air Handling System 2 Air Handling Syste

ID # Season Chiller 

plant 

Boiler 

Plant AHU-1 Zone 1 Zone 4 Zone 7 AHU-2 Zone 2 Zone 5 Zone 8 AHU-3 Zone 3 Zone 

0 Cooling 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (00 Heating 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (00 Swing 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (01 Cooling 0 (0) 0 (0) 1475 (0) 0 (1326) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (01 Heating 0 (0) 0 (0) 1475 (0) 0 (1112) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (01 Swing 0 (0) 0 (0) 1475 (0) 0 (1112) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (02 Cooling 0 (0) 0 (0) 0 (0) 158 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (02 Heating 0 (0) 0 (0) 0 (0) 228 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (02 Swing 0 (0) 0 (0) 0 (0) 314 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (03 Cooling 0 (0) 0 (0) 0 (0) 581 (36) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (03 Heating 0 (0) 0 (0) 0 (0) 581 (28) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (03 Swing 0 (0) 0 (0) 0 (0) 581 (33) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (04 Cooling 0 (0) 0 (0) 0 (0) 581 (70) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (04 Heating 0 (0) 0 (0) 0 (0) 578 (60) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (04 Swing 0 (0) 0 (0) 0 (0) 581 (69) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (05 Cooling 1450 (0) 0 (0) 0 (1260) 8 (1120) 0 (140) 0 (119) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (05 Heating 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (05 Swing 807 (0) 0 (0) 0 (587) 0 (140) 0 (114) 0 (100) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (06 Cooling 0 (0) 0 (0) 264 (0) 0 (130) 75 (264) 51 (263) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (06 Heating 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (06 Swing 0 (0) 0 (0) 32 (0) 40 (28) 0 (8) 1 (32) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (07 Cooling 0 (0) 0 (0) 581 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (07 Heating 0 (0) 0 (0) 581 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (07 Swing 0 (0) 0 (0) 581 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (08 Cooling 0 (0) 0 (0) 581 (0) 0 (145) 0 (362) 0 (345) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (08 Heating 0 (0) 0 (0) 581 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (08 Swing 0 (0) 0 (0) 581 (0) 0 (54) 0 (25) 0 (94) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (09 Cooling 0 (0) 0 (0) 451 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (09 Heating 0 (0) 0 (0) 506 (0) 0 (0) 0 (44) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (09 Swing 0 (0) 0 (0) 383 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0

10 
11 

Table 3. (continued)

# Alarms Sent to Operator (# Alarms Suppressed) Note: Actual source of fault is shaded. 

Fault Central Plant Air Handling System 1 Air Handling System 2 Air Handling Syste

ID # Season Chiller 

plant 

Boiler 

Plant AHU-1 Zone 1 Zone 4 Zone 7 AHU-2 Zone 2 Zone 5 Zone 8 AHU-3 Zone 3 Zone 

10 Cooling 0 (0) 0 (0) 497 (0) 0 (497) 0 (497) 0 (497) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (010 Heating 0 (0) 0 (0) 510 (0) 0 (497) 0 (497) 0 (495) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (010 Swing 0 (0) 0 (0) 507 (0) 0 (497) 0 (497) 0 (497) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (011 Cooling 0 (0) 0 (0) 438 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (011 Heating 0 (0) 0 (0) 507 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (011 Swing 0 (0) 0 (0) 234 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (012 Cooling 0 (0) 0 (0) 479 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (012 Heating 0 (0) 0 (0) 485 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (012 Swing 0 (0) 0 (0) 407 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (013 Cooling 0 (0) 0 (0) 447 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (013 Heating 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (013 Swing 0 (0) 0 (0) 81 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (014 Cooling 0 (0) 0 (0) 654 (0) 0 (654) 0 (654) 0 (655) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (014 Heating 0 (0) 0 (0) 563 (0) 2 (399) 0 (358) 0 (394) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (014 Swing 0 (0) 0 (0) 484 (0) 0 (318) 0 (319) 0 (318) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (015 Cooling 0 (0) 0 (0) 1 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (015 Heating 0 (0) 0 (0) 55 (0) 2 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (015 Swing 0 (0) 0 (0) 14 (0) 0 (2) 0 (0) 0 (2) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (016 Cooling 495 (0) 0 (0) 36 (462) 0 (220) 0 (231) 2 (227) 56 (495) 0 (448) 0 (500) 8 (502) 69 (492) 0 (481) 3 (5316 Heating 0 (0) 0 (0) 1 (0) 0 (0) 0 (0) 0 (0) 25 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (016 Swing 301 (0) 0 (0) 4 (285) 0 (0) 0 (0) 0 (0) 28 (300) 0 (9) 0 (28) 0 (16) 59 (301) 0 (63) 0 (1217 Cooling 581 (0) 0 (0) 0 (477) 0 (71) 0 (208) 0 (148) 0 (493) 0 (467) 0 (514) 0 (523) 1 (489) 0 (494) 0 (5417 Heating 581 (0) 0 (0) 0 (1) 0 (0) 0 (0) 0 (0) 0 (8) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (017 Swing 581 (0) 0 (0) 0 (358) 0 (0) 0 (0) 0 (0) 0 (423) 0 (81) 0 (85) 0 (61) 0 (427) 0 (120) 0 (1718 Cooling 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (018 Heating 0 (0) 0 (0) 18 (0) 0 (0) 337 (18) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (018 Swing 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (019 Cooling 0 (0) 581 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (019 Heating 0 (0) 581 (0) 0 (18) 0 (0) 0 (355) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (019 Swing 0 (0) 581 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0) 0 (0
Air Handling Unit Supply Air Temperature Setpoint Too Low 

The heating season simulation of Fault 9 illustrates the principle that fault reports from a 

load are suppressed if there are any fault reports from a source serving that load. 

In the fault free simulation, the supply air temperature setpoint is reset based on the zone 

temperatures of the spaces served by the AHU. The fault was implemented by replacing 

the reset schedule for AHU-1 with a fixed value of 5 °C, which is less than the 10 °C 

minimum value in the reset schedule. 

Figure 3 shows the results from the occupied period of one day of the simulation. On this 

day, the supply air temperature could only be maintained at the new setpoint by keeping 

the cooling coil valve fully open for the entire day. This condition was detected as a fault 

by the equipment-specific AHU FDD, which generated an alarm and sent it to the 

hierarchical FDD tool. 

Figure 4 shows the effect of Fault 9 on Zone 4 for the same time period as in Figure 3. 

Since there was a heating load on the zone, the VAV box damper closed as far as possible 

given its minimum airflow requirement, while the reheat coil valve opened fully. The air 

supplied to the VAV box by AHU-1 was so cold that even with the minimum airflow and 

maximum reheat, the zone temperature remained below the heating setpoint. Due to the 

low zone temperature, the equipment-specific VAV box FDD generated an alarm and 

sent it to the hierarchical FDD tool. 

The hierarchical FDD tool received the equipment-specific FDD fault reports from both 

AHU-1 and Zone 4. Rule 3 states that if there are any equipment-level AHU faults, they 

are passed on to the operator, and any fault reports from the terminal units served by the 

faulty AHU are suppressed. In accordance with Rule 3, the AHU-1 fault report was 

correctly passed on to the operator, while the Zone 4 fault report was correctly 

suppressed.

12 
 

0

5

10

15

20

25

8:00 9:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00

Time (h)

Temperature (oC)

0

0.2

0.4

0.6

0.8

1

1.2

Control Signal (0 to 1)

Supply Air Temperature

Supply Air Temperature Setpoint

Cooling Coil Valve Control Signal

Figure 3. Fault 9, AHU supply air temperature setpoint too low, heating season. AHU-1 

temperature and control signal data.

16

18

20

22

24

26

28

8:00 9:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00

Time (h)

Temperature (oC)

0

0.2

0.4

0.6

0.8

1

1.2

Control Signal (0 to 1)

VAV Box Reheat Coil Valve Control Signal

Cooling Setpoint Temperature

Heating Setpoint Temperature

Zone Temperature

VAV Box Damper Control Signal

Figure 4. Fault 9, AHU supply air temperature setpoint too low, heating season. Zone 4 

temperature and control signal data.

13 
Chilled Water Supply Temperature Fault 

Fault 17, the chilled water supply temperature fault, provides a good example of the 

principle that a fault in the air or water interface between a source and the loads it serves 

is considered a fault associated with the source. The fault was implemented by fixing the 

chilled water supply temperature in the simulation model to a constant value of 20 °C 

(the setpoint is 6 °C). There was no equipment-specific chilled water plant FDD tool in 

this study, so no fault report was sent to the hierarchical FDD tool. 

Chilled water is supplied from the central plant to AHU-1, AHU-2, and AHU-3. Figure 5 

shows the effect of this fault on AHU-3 from the occupied period of one day of the 

simulation; the results for AHU-1 and AHU-2 are similar. The AHU control logic 

modulates the cooling coil control valve to maintain the supply air temperature at its 

setpoint. Because the chilled water supplied to the AHU cooling coil was too warm, the 

cooling coil valve saturated at 100 % open, while the supply air temperature remained 

well above its setpoint. The equipment-specific AHU FDD tool detected the presence of 

a fault and sent a fault report to the hierarchical FDD tool. 

The cascading effects of the fault can also be seen in the VAV boxes that are supplied 

with conditioned air from the AHUs. Results for Zone 3 are shown in Figure 6 (same 

time period as Figure 5); the results are typical for six of the nine zones in the simulation. 

The supply air temperature was too warm to maintain the zone air temperature at the 

cooling setpoint. The equipment-specific VAV box FDD generated a fault report and 

sent it to the hierarchical FDD tool. 

Rule 8 implements the principle that a fault in the air or water interface between a source 

and the loads it serves is considered a fault associated with the source, specifically for the 

chiller plant – AHU relationship. The rule states that if the chilled water supply 

temperature is greater than its setpoint by more than a threshold amount, then a chilled 

water plant fault is reported and any AHU or terminal unit fault reports are suppressed. 

In this case, the difference between the chilled water supply temperature and the setpoint 

was greater than the threshold value (10 °C), so a chilled water plant fault was correctly 

reported, while at the same time the AHU and VAV box fault reports were correctly 

suppressed. The relatively severe faults selected for this study were chosen to maximize 

their impact on multiple pieces of equipment, in order to have a more meaningful test of 

the hierarchical FDD tool. 

14 
0

5

10

15

20

25

30

8:00 9:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00

Time (h)

Temperature (oC)

0

0.2

0.4

0.6

0.8

1

1.2

Control Signal (0 to 1)

Supply Air Temperature

Supply Air Temperature Setpoint

Cooling Coil Valve Control Signal

Figure 5. Fault 17, chilled water supply temperature fault, swing season. AHU-3 temperature 

and control signal data. 

20

20.5

21

21.5

22

22.5

23

23.5

24

24.5

25

8:00 9:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00

Time (h)

Temperature (oC)

Cooling Setpoint Temperature

Heating Setpoint Temperature

Zone Temperature

 

Figure 6. Fault 17, chilled water supply temperature fault, swing season. Zone 3 temperature 

data. 

15 
Air Handling Unit Supply Air Pressure Setpoint Too Low 

Fault 6, the AHU supply air pressure fault, provides a good example of the principle that 

multiple fault reports from the loads served by a source are a fault associated with the 

source. The fault was implemented in the simulation by changing the AHU-1 supply air 

pressure setpoint from 0.25 kPa to 0.01 kPa. The AHU control logic modulates the 

supply fan speed to maintain the supply air pressure at the setpoint, so no fault was 

detected by the equipment-specific AHU FDD tool. 

The effect of this fault on Zone 1 for the occupied period of one day of the simulation is 

shown in Figures 7 and 8. The effects on Zones 4 and 7 (the other zones served by AHU1) are similar. When occupancy began, the cooling load, zone temperature, and airflow 

setpoint all increased. Figure 7 shows that, because the supply air pressure was too low, 

the actual airflow fell well below the setpoint while the VAV box damper saturated fully 

open. Figure 8 shows that, as a result, the zone temperature rose above the cooling 

setpoint. The equipment-specific VAV box FDD tool generated fault reports due to the 

difference between the zone airflow rate and setpoint as well as the difference between 

the zone temperature and the cooling setpoint. 

In order for the hierarchical FDD tool to interpret multiple VAV box faults as an AHU 

fault, the VAV box faults must occur simultaneously. Specifically in the case of this 

study, at least two of the three zones must report faults simultaneously for an AHU fault 

report to be generated. In general, the equipment-specific FDD tool for each VAV box 

will not report faults at exactly the same time, so some false alarms occurred. For 

example, for the day shown in Figures 7 and 8, the Zone 4 VAV box FDD tool reported a 

fault from 15:10 until 17:00 (the end of occupancy) while Zone 4 reported a fault from 

15:25 until 17:00 and Zone 7 reported a fault from 15:30 until 17:00. From 15:10 until 

15:25, there was only one VAV box reporting a fault, so the incorrect VAV box fault 

report was passed on by the hierarchical FDD tool. From 15:25 until 17:00, there were at 

least two VAV boxes reporting faults, so the hierarchical FDD tool correctly suppressed 

the VAV box fault report and generated an AHU fault report. The primary diagnosis for 

this day of the simulation is the correct AHU fault. 

16 
0

0.2

0.4

0.6

0.8

1

1.2

1.4

8:00 9:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00

Time (h)

Air Flow Rate (m3/s)

0

0.2

0.4

0.6

0.8

1

1.2

Control Signal (0 to 1)

Airflow Setpoint

Airflow Rate

VAV Box Damper Control Signal

Figure 7. Fault 6, AHU supply air pressure setpoint fault, cooling season. Zone 1 airflow and 

control signal data. 

 

20

20.5

21

21.5

22

22.5

23

23.5

24

24.5

25

8:00 9:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00

Time (h)

Temperature (oC)

Cooling Setpoint Temperature

Heating Setpoint Temperature

Zone Temperature

Figure 8. Fault 6, AHU supply air pressure setpoint fault, cooling season. Zone 1 temperature 

data. 

17 
Conclusions and Future Work 

A system-level hierarchical FDD tool was developed and a simulation study to test and 

evaluate the tool was conducted. In general, the results were positive: the tool’s primary 

diagnosis was correct for 57 of 60 simulation runs (95 %). In one case, the primary 

diagnosis was incorrect, but the actual source of the fault was a secondary diagnosis. 

There were two cases in which the actual source of the fault was neither a primary nor a 

secondary diagnosis. The hierarchical FDD tool was shown to effectively filter 

equipment-level fault reports and detect and diagnose system-level faults. 

The results from the simulation study, while promising, suggest some refinements to the 

hierarchical FDD tool, particularly the ability to infer a higher level fault from multiple 

lower level fault reports that occur close to the same time, but are not simultaneous. 

Another enhancement might be the ability to handle multiple, unrelated faults which may 

cause the hierarchical FDD tool to suppress real fault reports from lower levels in the 

hierarchy while correctly reporting higher level faults. 

The next step will be to refine the hierarchical FDD tool and conduct another simulation 

study. Future work will include further testing and refinement by collecting trend data 

from building control systems at field sites. Implementations of the FDD tool will then 

be developed for several different control system manufacturers’ product architectures. 

Testing in an emulation or laboratory setting will validate the implementations. Finally, a 

larger scale field test of the embedded implementations of the tool will be conducted to 

serve as a demonstration project in order to transition the tool from research to the 

marketplace. 

18 
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19