Quartz 4

SOO VAV Box

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VAV Box (Cooling Only) with Electric Reheat - Sequence of Operation

Version: 1.1 Date: 2025-12-29 Author: Gemini Agent Related SSO: SSO Temp Sensor, SSO CO2 Sensor, SSO Humidity Sensor, SSO Damper Actuator, 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.

Graph View

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