The AHU anomaly function detects common hard and soft faults related to a building's air handling unit(s).

Key Performance Indicators (KPIs)

The AHU anomaly function identifies three common HARD faults and three common SOFT faults and calculates an AHU health index; this is done separately for each AHU. The AHU health index is the ratio of non-faults over the number of total faults that can be detected (six in total). If no faults were detected, the AHU health index would be 100%, and if all six faults were detected, the AHU health index would be 0%. Table 2 below is an example output of the detected faults.

The three possible HARD faults are:

• STUCK COOLING COIL VALVE - This fault is flagged if no or minimal use of the heating coil was observed in the data. This may be symptomatic of a faulty sensor, a stuck or uncalibrated valve, or a conflict in the operational logic.
• STUCK HEATING COIL VALVE - This fault is flagged if no or minimal use of the cooling coil was observed in the data. This may be symptomatic of a faulty sensor, a stuck or uncalibrated valve, or a conflict in the operational logic
• STUCK OUTDOOR AIR DAMPER - This fault is flagged if an inadequate or excessive amount of outdoor air was observed. This may be symptomatic of a stuck, sloppy, or uncalibrated outdoor air damper or faulty damper sensor/actuator.

The three possible SOFT faults are:

• CHECK ECONOMIZER LOGIC - This fault is flagged if the outdoor air damper does not exceed 70% open in the economizer with cooling state, or if the transition to mechanical cooling is premature (i.e., outdoor air temperature is less than 15°C). In either case, the economizer state settings may not be programmed correctly and is not taking full advantage of cooler outdoor air temperatures to cool the building.
• CHECK MODE OF OPERATIONS LOGIC - This fault is flagged if the AHU's weekly operational time (i.e. when the AHU fans are operational) exceeds 100 hours a week. This is typically considered excessive operations, and can simply indicate an inoperable or non-existent weekly schedule, or can be the result of frequent night-cycling caused by a few zones overheating beyond unoccupied indoor temperature setpoints. Consider checking the operational logic for any conflicts which may result in unintended operation of the AHUs, or implementing a weekly schedule if one does not already exist.
• CHECK SUPPLY AIR TEMPERATURE RESET LOGIC - This fault is flagged if a excessive use of the perimeter heating devices is detected in the AHU's economizer mode, which would indicate an ineffective or inoperative supply air temperature reset strategy. Ensure that an appropriate supply air temperature setpoint reset strategy is present and operating as intended. If one does exist, select rooms may be overheating in the heating season. If this is the case, consider increasing the maximum terminal airflow setpoints in these overheating rooms.

Figures

The AHU anomaly function generates three sets of visuals to help put some context to the identified faults. The first generated visual plots supply air temperature as a function of outdoor air temperature; this is generated for each analyzed AHU. Figure 1 below is an example of this visual.

The coolest, warmest, and average return air temperatures (zone air temperature) are also depicted. This visual is intended to inform a supply air temperature setpoint reset strategy. A setpoint reset strategy allows for higher supply air temperatures as outdoor air temperature decreases. Suboptimal supply air temperatures can result in excessive energy consumption from excessive perimeter heating use, economizing, or fan use. To guide supply air temperature setpoint adjustments, the typical 'ideal' supply air temperature range is highlighted in green. If the supply air temperature is lower than this range in the heating season, excessive use of perimeter heating and economizing may result; only a few overheating rooms may trigger this behaviour. If this is the case, consider increasing the maximum terminal airflow setpoints in these overheating rooms. If the supply air temperature is higher than this range in the cooling season, excessive fan power to deliver required necessary cooling may result; a few overcooling rooms may trigger this behaviour. If this is the case, consider decreasing the minimum terminal airflow setpoints in these overcooling rooms within reason. In either case, ensure that the airflow and temperature sensors work as intended in these rooms.

The second generated visual is a split-range controller diagram; this is generated for each analyzed AHU. This diagram depicts outdoor air damper (OA), heating coil valve (HC), and cooling coil valve (CC) position with respect to outdoor air temperature. The average fraction of active perimeter heating devices (RAD) is also shown. The four underlying color zones represent the four distinct AHU operating modes as per ASHRAE Guidelines 36: heating (red zone), economizer (yellow zone), economizer with cooling (grey zone), and cooling (blue zone). Figure 2 below is an example of the split-range controller diagram.

For healthy AHU operation, the following items should be especially considered:

• Heating coil valve is open only in the heating mode (red zone).
• Cooling coil valve is open only in the economizer with cooling (grey zone) and cooling (blue zone) mode.
• Heating and cooling should not operate simultaneously.
• Perimeter heating should be minimal in economizer mode. This can be achieved by increasing the supply air temperature setpoint in the heating season, while monitoring any occurrence of overheating.

The last set of generated visuals is a collection of simplified AHU diagrams. A set of four to six visuals per identified AHU are generated which depict average damper and valve positions and temperatures at characteristic operating periods. The fraction of time of operation is the percentage of the total time of the AHU's operation which exhibit the displayed damper and valve positions and temperatures. Figure 3 below is an example of the simplified AHU diagrams.