# 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.