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

Version: 1.3 Date: 2025-12-29 Author: Gemini Agent Related SSO: training-structure/sso/sso-temp-sensor, training-structure/sso/sso-co2-sensor, training-structure/sso/sso-vfd-control, training-structure/sso/sso-flow-switch, training-structure/sso/sso-occupancy-sensor, training-structure/sso/sso-relay-output, training-structure/sso/sso-valve-actuator, training-structure/sso/sso-damper-actuator, training-structure/sso/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.

6. Commissioning & Verification Checklist

  • Test H-O-A switch (HAND-MODE, OFF-MODE) and verify correct mode changes and SOC/HMI advisories.
  • For System-OFF Mode, verify all fans, wheels, and valves are de-energized and dampers are in their fail-safe positions.
  • For Unoccupied Summer Cooling, verify the unit cycles on/off at 85°F and 83°F respectively, with 100% recirculation.
  • For Pre-Occupancy Purge, verify the unit runs with 100% outside air for 15 minutes.
  • For Pre-Occupancy Pull-Down, verify the unit cools with 100% recirculated air.
  • For Occupied Winter Heating, verify HTG-O modulates and EW-O is at 100%.
  • For Occupied Summer Economizer, verify OAD1-O/OAD2-O move to 100% and CLG-O remains closed.
  • For Occupied Summer Mechanical Cooling, verify CLG-O modulates for a 55°F coil temp and HW-O modulates to control RAT.
  • Test all hard-wired safeties (DAPHI-A, EAPLO-A, LT-A, FIRE-A) and confirm fan shutdown and proper alarm generation.
  • Verify DAP-SP resets up and down based on AVG_VAV_DPR_POS.