Central VAV Air Handling Unit (AHU) - Sequence of Operation

Version: 1.3 Date: 2025-12-29 Author: Gemini Agent Related SSO: SSO Temp Sensor, SSO CO2 Sensor, SSO Vfd Control, SSO Flow Switch, SSO Occupancy Sensor, SSO Relay Output, SSO Valve Actuator, SSO Damper Actuator, SSO Humidity Sensor

1. Overview

This sequence describes a central VAV Air Handling Unit that supplies conditioned air to classrooms. The system features redundant supply/exhaust fans, outside/exhaust/recirculation dampers, an energy recovery wheel (ERW), a chilled water coil with a heat wheel, and a hot water coil. The unit operates in multiple modes, including System-OFF, Occupied (Heating/Cooling/Economizer), and Unoccupied cycles. Control is managed by a DDC controller integrated with a fire alarm system, VAV terminal units, and numerous safety devices. The system utilizes 30% propylene glycol in its hydronic loops.

2. Point List

Inputs (Physical)

Outside Air Humidity (OA-H): AI, 0-10VDC
Outside Air Temperature (OA-T): AI, 10k Type II
Return Air Humidity (RA-H): AI, 0-10VDC
Return Air Temperature (RA-T): AI, 10k Type II
Space Humidity (ZN-H): AI, 0-10VDC, Representative Zone
Space Temperature (ZN-T): AI, 10k Type II, Representative Zone
Mixed Air Temperature (MA-T): AI, 10k Type II
Heating Coil Temperature (HC-T): AI, 10k Type II
Cooling Coil Temperature (CC-T): AI, 10k Type II
Energy Wheel Entering Air Temp (EW-ENT-T): AI, 10k Type II
Energy Wheel Leaving Air Temp (EW-LVG-T): AI, 10k Type II
Supply Air Temperature (SA-T): AI, 10k Type II
Exhaust Air Flow (EA-F): AI, 0-10VDC
Outside Air Flow (OA-F): AI, 0-10VDC
Supply Air Flow (SA-F): AI, 0-10VDC
Discharge Air Static Pressure (DA1-P): AI, 0-5VDC
Return Air Filter DP Alarm (RA-FILT-A): AI, 4-20mA
Outside Air Filter DP Alarm (OA-FILT-A): AI, 4-20mA
Outside Air Damper 1 Feedback (OAD1-FB): AI, 0-10VDC
Outside Air Damper 2 Feedback (OAD2-FB): AI, 0-10VDC
Return Air Damper Feedback (RAD-FB): AI, 0-10VDC
Low Temperature Alarm (LT-A): BI, Dry Contact (Freezestat)
Fire Alarm (FIRE-A): BI, Dry Contact
Discharge Air Pressure High Alarm (DAPHI-A): BI, Dry Contact
Exhaust Air Pressure Low Alarm (EAPLO-A): BI, Dry Contact
Supply Fan Fault VFD Alarm (SF-FAULT): BI, Dry Contact
Exhaust Fan Fault VFD Alarm (EF-FAULT): BI, Dry Contact
Supply Fan 1 Status/Failure (SF1-S/FAIL): BI, Dry Contact
Supply Fan 2 Status/Failure (SF2-S/FAIL): BI, Dry Contact
Exhaust Fan 1 Status/Failure (EF1-S/FAIL): BI, Dry Contact
Exhaust Fan 2 Status/Failure (EF2-S/FAIL): BI, Dry Contact
Supply Fan VFD In Hand/Bypass (SF-VFD-HAND): BI, Dry Contact
Exhaust Fan VFD In Hand/Bypass (EF-VFD-HAND): BI, Dry Contact
Unit Mtd HOA Switch in Hand (HAND-MODE): BI, Dry Contact
Unit Mtd HOA Switch in Off (OFF-MODE): BI, Dry Contact
Heating Wheel Rotation Status (HW-ROT-S): BI, Pulse
Energy Wheel Rotation Status (EW-ROT-S): BI, Pulse

Outputs (Physical)

Supply Fan Command (SF-C): BO, 12VDC (VFD Enable)
Exhaust Fan Command (EF-C): BO, 12VDC (VFD Enable)
Energy Wheel VFD Start Command (EW-C): BO, 12VDC (VFD Enable)
Heat Wheel VFD Start Command (HW-C): BO, 12VDC (VFD Enable)
Energy Wheel Bypass Dampers Command (EW-BYPD1/2-C): BO, 12VDC (Open/Close)
Exhaust Fan Output (EF-O): AO, 0-10VDC (VFD Speed)
Supply Fan Output (SF-O): AO, 0-10VDC (VFD Speed)
Heating Coil Valve Output (HTG-O): AO, 0-10VDC
Cooling Coil Valve Output (CLG-O): AO, 0-10VDC
Outside Air Damper 1 Output (OAD1-O): AO, 0-10VDC
Outside Air Damper 2 Output (OAD2-O): AO, 0-10VDC
Return Air Damper Output (RAD-O): AO, 0-10VDC
Energy Wheel VFD Speed Output (EW-O): AO, 0-10VDC
Heat Wheel VFD Speed Output (HW-O): AO, 0-10VDC

Software Points & Setpoints

Static Pressure Setpoint (DAP-SP): Software Output
Occupied Heating Setpoint (OCC_HTG_SP): Software Setpoint
Unoccupied Heating Setpoint (UNOCC_HTG_SP): Software Setpoint
Occupied Cooling Setpoint (OCC_CLG_SP): Software Setpoint
Unoccupied Cooling Setpoint (UNOCC_CLG_SP): Software Setpoint
Discharge Air Setpoint (DAT-SP): Software Output

3. Safety Devices & Interlocks

Supply Duct Overpressure (DAPHI-A): Shuts down supply fan. Manual reset required. Generates alarm.
Return/Exhaust Duct Suction Pressure (EAPLO-A): Shuts down exhaust fan. Manual reset required. Generates alarm.
Clogged Filter (RA-FILT-A, OA-FILT-A): Generates alarm when DP exceeds manufacturer recommendation.
ERW Alarms: Wheel stoppage (EW-ROT-S) or other internal faults generate an alarm.
Freeze Protection (LT-A): Freezestat on cooling coil inlet shuts down all fans at 39°F. Trips >30 seconds require manual reset. Generates alarm.
Fan Failure (SF1-S/FAIL, SF2-S/FAIL, EF1-S/FAIL, EF2-S/FAIL, SF-FAULT, EF-FAULT): Failure of any one fan or VFD will shut down the entire AHU and generate an alarm.
Fire Alarm Shut Down (FIRE-A): FACP signal shuts down supply and exhaust fans and closes associated dampers. This does not apply to manual pull station alarms or units under 2,000 CFM that are 100% OA.

4. Modes of Operation

Mode Selection (H-O-A)

Description: Allows manual override of the unit’s operating mode or automatic control via the DDC scheduler.
Conditions: Unit is powered. Input from HAND-MODE or OFF-MODE overrides scheduler.
Output Logic:
- SF-C: Varies by selected mode.
- EF-C: Varies by selected mode.
- EW-C: Varies by selected mode.
- HW-C: Varies by selected mode.
- EW-BYPD1/2-C: Last position.
- EF-O: Varies by selected mode.
- SF-O: Varies by selected mode.
- HTG-O: Varies by selected mode.
- CLG-O: Varies by selected mode.
- OAD1-O: Varies by selected mode.
- OAD2-O: Varies by selected mode.
- RAD-O: Varies by selected mode.
- EW-O: Varies by selected mode.
- HW-O: Varies by selected mode.

System-OFF Mode

Description: The default state of the unit when not required to run. All mechanical components are de-energized and dampers are in their fail-safe positions.
Conditions: Enter when commanded by Schedule (SYSTEM_OPR_MODE), OFF-MODE is active, or after a critical safety trip. Stays in OFF unless conditions for another mode are met.
Output Logic:
- SF-C: OFF
- EF-C: OFF
- EW-C: OFF
- HW-C: OFF
- EW-BYPD1/2-C: Closed
- EF-O: 0%
- SF-O: 0%
- HTG-O: 0% (Closed)
- CLG-O: 0% (Closed)
- OAD1-O: 0% (Closed)
- OAD2-O: 0% (Closed)
- RAD-O: 100% (Open)
- EW-O: 0%
- HW-O: 0%

Unoccupied Summer Cooling Mode

Description: Cycles the unit on to prevent the representative space from exceeding a high-limit setpoint during unoccupied summer hours.
Conditions: Enter when SYSTEM_OPR_MODE is Unoccupied Summer, Seasonal_Mode is Summer, and ZN-T > 85°F (UNOCC_CLG_SP). Exit when ZN-T < 83°F (UNOCC_CLG_SP - 2°F).
Output Logic:
- SF-C: ON
- EF-C: OFF
- EW-C: OFF
- HW-C: OFF
- EW-BYPD1/2-C: Last position.
- EF-O: 0%
- SF-O: Modulates to maintain DAP-SP.
- HTG-O: 0% (Closed)
- CLG-O: Modulates to maintain a 55°F DAT-SP.
- OAD1-O: 0% (Closed)
- OAD2-O: 0% (Closed)
- RAD-O: 100% (Open)
- EW-O: 0%
- HW-O: 0%

Pre-Occupancy Purge Cycle

Description: Purges the building with 100% outside air for a fixed duration before occupancy if conditions are favorable (OA enthalpy < RA enthalpy).
Conditions: Enter when SYSTEM_OPR_MODE is Pre-Occupancy Purge (Morning/Nighttime Purge Schedule) AND OA-ENTHALPY < RM-ENTHALPY. Exits after 15 minutes.
Output Logic:
- SF-C: ON
- EF-C: ON
- EW-C: OFF
- HW-C: OFF
- EW-BYPD1/2-C: Last position.
- EF-O: Modulates to maintain building pressure (Exhaust Airflow = 85-90% of Outside Airflow).
- SF-O: Modulates to maintain DAP-SP.
- HTG-O: 0% (Closed)
- CLG-O: 0% (Closed, mechanical cooling locked out).
- OAD1-O: 100% (Open)
- OAD2-O: 100% (Open)
- RAD-O: 0% (Closed)
- EW-O: 0%
- HW-O: 0%

Pre-Occupancy Pull-Down Cycle

Description: Cools the building using 100% recirculated air after a purge cycle or if the space is warm before occupancy.
Conditions: Enter after a Purge Cycle, or if RAT > 78°F before Occupied mode. Exits when RAT ⇐ 78°F or when SYSTEM_OPR_MODE enters Occupied mode.
Output Logic:
- SF-C: ON
- EF-C: OFF
- EW-C: OFF
- HW-C: OFF
- EW-BYPD1/2-C: Last position.
- EF-O: 0%
- SF-O: Modulates to maintain DAP-SP.
- HTG-O: 0% (Closed)
- CLG-O: Modulates to maintain a 55°F DAT-SP.
- OAD1-O: 0% (Closed)
- OAD2-O: 0% (Closed)
- RAD-O: 100% (Open)
- EW-O: 0%
- HW-O: 0%

Occupied Mode - Winter Heating

Description: Provides heating and minimum ventilation to the building during occupied winter hours.
Conditions: Enter when SYSTEM_OPR_MODE is Occupied Heating and Seasonal_Mode is Winter. Exits when conditions for another mode are met.
Output Logic:
- SF-C: ON
- EF-C: ON
- EW-C: ON
- HW-C: OFF
- EW-BYPD1/2-C: Last position.
- EF-O: Modulates to maintain building pressure.
- SF-O: Modulates to maintain DAP-SP, reset by AVG_VAV_DPR_POS.
- HTG-O: Modulates to maintain a 65°F DAT-SP (OCC_HTG_SP).
- CLG-O: 0% (Closed)
- OAD1-O: Modulates to provide minimum required outside air based on VAV demand.
- OAD2-O: Modulates to provide minimum required outside air based on VAV demand.
- RAD-O: Modulates to accommodate OA intake.
- EW-O: 100%
- HW-O: 0%

Occupied Mode - Summer Economizer Cooling

Description: Provides “free cooling” using 100% outside air when outdoor conditions are favorable.
Conditions: Enter when SYSTEM_OPR_MODE is Occupied Cooling, Seasonal_Mode is Summer, 55°F ⇐ OA-T < 65°F, and OA-ENTHALPY < RM-ENTHALPY. Exit if conditions are no longer met or if RAT > 78°F for 30 mins (then to Mechanical Cooling).
Output Logic:
- SF-C: ON
- EF-C: ON
- EW-C: OFF
- HW-C: OFF
- EW-BYPD1/2-C: Last position.
- EF-O: Modulates to maintain building pressure.
- SF-O: Modulates to maintain DAP-SP, reset by AVG_VAV_DPR_POS.
- HTG-O: 0% (Closed)
- CLG-O: 0% (Closed)
- OAD1-O: 100% (Open)
- OAD2-O: 100% (Open)
- RAD-O: 0% (Closed)
- EW-O: 0%
- HW-O: Modulates to maintain DAT-SP of 55°F (for reheat/dehumidification purposes) if DAT drops too low.

Occupied Mode - Summer Mechanical Cooling

Description: Provides primary cooling and dehumidification using the chilled water coil and heat wheel when economizer is not sufficient or unavailable.
Conditions: Enter when SYSTEM_OPR_MODE is Occupied Cooling, Seasonal_Mode is Summer, and Economizer mode is not active or cannot meet the load (RAT > 78°F for 30 mins).
Output Logic:
- SF-C: ON
- EF-C: ON
- EW-C: ON
- HW-C: ON
- EW-BYPD1/2-C: Last position.
- EF-O: Modulates to maintain building pressure.
- SF-O: Modulates to maintain DAP-SP, reset by AVG_VAV_DPR_POS.
- HTG-O: 0% (Closed)
- CLG-O: Modulates to maintain a 55°F CC-T (Cooling Coil Discharge Temp).
- OAD1-O: Modulates to provide minimum required outside air.
- OAD2-O: Modulates to provide minimum required outside air.
- RAD-O: Modulates to accommodate OA intake.
- EW-O: 100%
- HW-O: Modulates to maintain RAT between 76°F and 78°F for reheat/dehumidification.

Communication Failure Mode

Description: Defines the unit’s behavior upon loss of network communication to ensure continued operation.
Conditions: Enter when communication with the network is lost. Exits when communication is restored.
Output Logic:
- SF-C: Defaults to ON.
- EF-C: Defaults to ON.
- EW-C: Defaults to ON.
- HW-C: Defaults to ON.
- EW-BYPD1/2-C: Last position.
- EF-O: Defaults to maintaining building pressure.
- SF-O: Defaults to maintaining DAP-SP.
- HTG-O: Defaults to last known value, relevant for the last known seasonal occupied mode.
- CLG-O: Defaults to last known value, relevant for the last known seasonal occupied mode.
- OAD1-O: Defaults to last known value, relevant for the last known seasonal occupied mode.
- OAD2-O: Defaults to last known value, relevant for the last known seasonal occupied mode.
- RAD-O: Defaults to last known value, relevant for the last known seasonal occupied mode.
- EW-O: Defaults to 100%.
- HW-O: Defaults to last known value, relevant for the last known seasonal occupied mode.

5. Alarms & Fault Conditions

Duct Over/Under Pressure (DAPHI-A, EAPLO-A): Generates alarm; requires manual reset of safety switch.
Fan Failure (SF-FAULT, EF-FAULT, etc.): Generates alarm if fan status does not prove ON after a start command.
Freezestat Trip (LT-A): Generates alarm. Trips >30 seconds require manual reset.
Clogged Filter (RA-FILT-A, OA-FILT-A): Generates alarm when DP is high.
ERW Fault (EW-ROT-S): Generates alarm on wheel stoppage or failure.
Critical Zone Reset Fault
- Condition: A VAV zone is consistently driving the static pressure reset logic (e.g., VAV damper position remains at 100% open for an extended period, or the zone temperature cannot reach setpoint while the AHU is providing cooling/heating).
- Priority: Medium
- Action: Generate an alarm notification to the operator, potentially flagging the specific VAV zone.
- Reset: Manual reset by operator after investigation.

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