# VAV Box (Cooling Only) with Electric Reheat - Sequence of Operation **Version:** 1.1 **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-humidity-sensor]], [[training-structure/sso/sso-damper-actuator]], [[training-structure/sso/sso-valve-actuator]] --- ## 1. Overview This sequence describes a non-fan-powered, cooling-only Variable Air Volume (VAV) box with electric reheat controlled by a BACnet DDC controller. The VAV box works in conjunction with a Multi-Zone VAV (MZVAV) rooftop heat pump for cooling and a local electric baseboard for heating. The system utilizes space temperature and CO2 sensors for demand-controlled ventilation (DCV) and thermal comfort, participating in pre-occupied pull-down/purge cycles and unoccupied nighttime purge cycles. The VAV box reports its damper position for supply fan static pressure reset logic and is designed for 24/7 ventilation, with DCV being disabled during special ventilation modes. Electric baseboard heating is enabled only during Winter mode. --- ## 2. Point List ### Inputs (Physical) - **Space Temperature (ZN-T)**: Sensed value from the room temperature sensor. - **Space CO2 (ZN-Q)**: Sensed value from the room CO2 sensor. - **Outdoor Air CO2 (OA-Q)**: Networked value used for DCV calculation. - **Building Schedule (SCHEDULE-C)**: Network Input (Occupied, Unoccupied, Special Ventilation, Pre-Occupied Pull-Down, Pre-Occupied Purge, Unoccupied Purge). - **MZVAV Representative Temperature**: AI (Network Input) for system-wide unoccupied/warm-up modes. - **Seasonal Mode (Seasonal_Mode)**: Indicates Winter or Summer mode, affecting heating enable. ### Outputs (Physical) - **VAV Damper Position (DMP-O)**: 0-100% command to the VAV damper actuator. - **Electric Baseboard Command (HTG-O)**: On/Off command to the perimeter electric baseboard. ### Software Points & Setpoints - **Damper Position Feedback (DMP-FB)**: Provides current damper position to the network for static pressure reset. - **Unoccupied Cooling Setpoint**: 85°F `Adjustable` - **Unoccupied Cooling Cutout**: 83°F `Adjustable` - **Unoccupied Heating Setpoint**: 55°F `Adjustable` (-2°F differential) - **Occupied Cooling Setpoint**: 78°F `Nominal Adjustable` (-2°F differential) - **Occupied Heating Setpoint**: 72°F `Nominal Adjustable` (-2°F differential) - **Warm-up Cutout Setpoint**: 72°F `Adjustable` - **Warm-up Discharge Air Temp**: 85°F `Nominal Adjustable` - **Pre-Occupied Pull-Down Max Temp**: 85°F `Nominal Adjustable` (-2°F differential) - **Pre-Occupied Pull-Down Target Temp**: 78°F `Nominal Adjustable` (-2°F differential) - **CO2 Full Occupancy Threshold**: 1,000 ppm `Fixed` - **Vacant Standby Ventilation Rate**: From Drawing Schedules `Configurable` - **Fully Occupied Ventilation Rate**: From Drawing Schedules `Configurable` - **VAV Damper Position Polling Request**: Software Point (Internal) --- ## 3. Operational Notes & Interlocks - **Special Ventilation Mode**: When active, Demand Controlled Ventilation (DCV) is disabled. - **Static Pressure Reset**: VAV damper position is polled every 10 minutes and reported to the central unit to be used in the supply fan static pressure reset algorithm. --- ## 4. Modes of Operation ### Unoccupied Mode - **Conditions**: Entered when the building schedule is "Unoccupied" (excluding purge cycles). Ventilation is provided 24/7. - **Heating Logic**: - VAV damper is commanded **Closed**. - If `Seasonal_Mode` is `Winter`: Electric baseboard is cycled On/Off to maintain the **Unoccupied Heating Setpoint (55°F - 2°F differential)**. - If `Seasonal_Mode` is `Summer`: Electric baseboard is disabled. - **Cooling Logic**: - If the representative room temperature is ≤ 85°F, VAV damper is commanded **Closed**. - If the representative room temperature is > 85°F, VAV damper opens to the **Fully Occupied Ventilation Rate** position. The central MZVAV unit stages to cool the space until the representative temperature reaches the **Unoccupied Cooling Cutout (83°F)**. ### Warm-up Cycle (Optimum Start) - **Conditions**: Active before scheduled occupancy if the representative room temperature is below the **Warm-up Cutout Setpoint (72°F)**. Start time is optimized based on outdoor air temperature and space temperature. - **Logic**: - VAV damper opens to the **Fully Occupied Ventilation Rate**. - Central MZVAV unit provides **85°F discharge air** until the representative room reaches 72°F. - Electric baseboard setpoint is maintained at **55°F** (regardless of Seasonal Mode for warm-up period). ### Pre-Occupied & Purge Cycles - **Conditions**: Triggered by specific `Building_Schedule` modes prior to or during unoccupied periods. - **Pre-Occupied Morning Pull-Down Cycle**: - VAV boxes are commanded to their **Fully Occupied Ventilation Rate** positions. - Space temperatures are pulled down from **Pre-Occupied Pull-Down Max Temp (85°F - 2°F differential)** to **Pre-Occupied Pull-Down Target Temp (78°F - 2°F differential)**. - **Pre-Occupied Morning Purge Cycle**: - VAV boxes are commanded to their **Fully Occupied Ventilation Rate** positions. - **Unoccupied Nighttime Purge Cycle**: - VAV boxes are commanded to their **Fully Occupied Ventilation Rate** positions. ### Occupied Mode - **Conditions**: Entered when the building schedule is "Occupied." - **Ventilation Logic (DCV)**: - The VAV box minimum flow setpoint is actively reset based on space CO2 levels. - The required ventilation rate is calculated via linear interpolation between the **Vacant Standby** rate (at 0 ppm differential) and the **Fully Occupied** rate (at 1,000 ppm). - The VAV damper will **never throttle below the Fully Occupied Ventilation Rate**. - **Heating Logic**: - VAV damper maintains its active minimum airflow setting as determined by DCV. - If `Seasonal_Mode` is `Winter`: If Space Temperature drops below **Occupied Heating Setpoint (72°F - 2°F differential)**, the electric baseboard is commanded **ON**. If Space Temperature rises above 72°F, the baseboard is commanded **OFF**. - If `Seasonal_Mode` is `Summer`: Electric baseboard is disabled. - **Cooling Logic**: - If Space Temperature rises above **Occupied Cooling Setpoint (78°F)**, the VAV damper modulates from its current minimum position towards 100% open to meet the load. - If Space Temperature drops below 78°F, the damper modulates back towards its active minimum position. - **Economizer Logic**: - When the central system is in economizer mode, DCV is suspended. - The VAV damper modulates between the **Fully Occupied Ventilation Rate** and maximum flow to meet cooling demand. --- ## 5. Alarms & Fault Conditions - **Excessive Reset Driver Alarm (Static_Reset_Alarm)** - **Condition**: The control algorithm automatically detects that this zone is excessively driving the static pressure reset logic. - **Priority**: Medium - **Action**: Generate an alarm to the system operator, allowing for manual removal of the zone from the reset algorithm. - **Reset**: Manual. --- ## 6. Commissioning & Verification Checklist - [ ] Verify VAV damper closes during Unoccupied mode when temperatures are between 55°F and 85°F. - [ ] Verify electric baseboard maintains 55°F during Unoccupied Heating when `Seasonal_Mode` is `Winter`. - [ ] Verify electric baseboard remains OFF during Unoccupied Heating when `Seasonal_Mode` is `Summer`. - [ ] Verify VAV damper opens and MZVAV cools during Unoccupied Cooling when representative temperature exceeds 85°F. - [ ] Verify Warm-up cycle initiates correctly, opening the damper and commanding the baseboard setpoint to 55°F. - [ ] Verify Pre-Occupied Morning Pull-Down cycle commands VAV to fully occupied ventilation and space temperature pulls down from 85°F to 78°F. - [ ] Verify Pre-Occupied Morning Purge and Unoccupied Nighttime Purge cycles command VAV to fully occupied ventilation. - [ ] Verify in Occupied Mode that the damper modulates to maintain the 78°F cooling setpoint. - [ ] Verify in Occupied Mode that the electric baseboard turns on to maintain the 72°F heating setpoint when `Seasonal_Mode` is `Winter`. - [ ] Verify in Occupied Mode that the electric baseboard remains OFF when `Seasonal_Mode` is `Summer`. - [ ] With CO2 injection, verify that the VAV minimum flow position increases as CO2 levels rise from 0 ppm differential to 1,000 ppm. - [ ] Verify the "Static Pressure Reset Alarm" can be generated.