VAV Box Control Sequences
This guide covers control sequences for Variable Air Volume (VAV) terminal units, from basic cooling-only to advanced configurations per ASHRAE Guideline 36.
VAV Box Types
| Type | Description | Application |
|---|
| Cooling Only | Single duct, damper modulation | Interior zones |
| With Reheat | Heating coil for zone heating | Perimeter zones |
| Fan-Powered (Series) | Constant fan, variable primary | High load areas |
| Fan-Powered (Parallel) | Intermittent fan for heating | Perimeter zones |
| Dual Duct | Hot and cold deck mixing | Variable load zones |
Basic Cooling-Only Sequence
Control Logic
- Compare Zone_Temp to Cooling_Setpoint
- If Zone_Temp > Setpoint + Deadband: Increase airflow toward maximum
- If Zone_Temp < Setpoint - Deadband: Decrease airflow toward minimum
- Maintain minimum airflow for ventilation
PID Loop
| Parameter | Typical Value |
|---|
| Setpoint | 72-75°F (adjustable) |
| Deadband | ±1°F |
| Proportional Band | 2-4°F |
| Integral Time | 5-10 minutes |
Airflow Limits
| Limit | Purpose |
|---|
| Maximum CFM | Design cooling capacity |
| Minimum CFM | Ventilation requirement |
| Heating Minimum | Reduced during occupied heating |
VAV with Reheat Sequence
Dual Loop Control
Cooling Mode:
- Zone calls for cooling
- Damper modulates: Min → Max
- Heating valve: Closed
Deadband:
- Zone between setpoints
- Damper at minimum
- Heating valve: Closed
Heating Mode:
- Zone calls for heating
- Damper at minimum (or heating minimum)
- Heating valve modulates: 0% → 100%
Sequence Diagram
ASHRAE Guideline 36 Sequences
Zone Setpoint Reset
Occupied setpoints:
- Cooling: 75°F (adjustable 70-80°F)
- Heating: 70°F (adjustable 65-75°F)
- Deadband: Minimum 3°F
Unoccupied setpoints:
- Cooling: 90°F
- Heating: 55°F
Demand-Based Setpoint Adjustment
If Zone_CO2 > 1000 ppm:
Increase minimum airflow
(Demand Control Ventilation)
If Zone_Temp cannot be maintained:
Request more supply air from AHU
(Trim and Respond logic)
Occupancy Modes
| Mode | Behavior |
|---|
| Occupied | Normal control, ventilation minimum |
| Unoccupied | Setback setpoints, zero minimum |
| Warmup | Heating priority, no ventilation min |
| Cooldown | Cooling priority, no ventilation min |
| Setup | Prepare for occupancy |
Fan-Powered VAV Sequences
Series Fan-Powered
Fan runs continuously during occupied periods:
Occupied:
- Series fan: ON
- Primary damper modulates for cooling
- Heating coil modulates for heating
- Constant discharge airflow
Unoccupied:
- Series fan: OFF
- Primary damper: Closed
- Heating coil: Off (or cycle for setback)
Parallel Fan-Powered
Fan cycles for heating boost:
Cooling Mode:
- Parallel fan: OFF
- Primary damper modulates: Min→Max
Heating Mode:
- Primary damper: Closed or Min
- Parallel fan: ON
- Heating coil: Modulates
- Draws plenum air for recirculation
Minimum Airflow Setpoints
Ventilation Requirements
Per ASHRAE 62.1:
Zone Outdoor Air = (People × Rp) + (Area × Ra)
Where:
- Rp = Per-person outdoor air rate (CFM/person)
- Ra = Per-area outdoor air rate (CFM/ft²)
- Office: Rp = 5 CFM/person, Ra = 0.06 CFM/ft²
Variable Minimum Based on Occupancy
Occupied-Standby (no occupants detected):
- Reduce minimum to Ra × Area only
- Eliminate per-person component
- Requires occupancy sensor
Occupied (occupants detected):
- Full ventilation minimum
- Per-person + per-area
Discharge Air Temperature Control
Cooling Mode
Supply air from AHU typically 55°F. VAV damper modulates to meet load.
Heating Mode with Reheat
If Zone_Temp < Heating_Setpoint:
- DAT Setpoint = 90-95°F (max)
- Modulate hot water valve
- Maintain airflow at minimum
Discharge Temperature Limits
| Limit | Value | Reason |
|---|
| Max DAT (heating) | 90-95°F | Prevent stratification |
| Min DAT (cooling) | 55°F | From AHU |
Pressure-Independent Operation
What is Pressure Independence?
VAV maintains airflow setpoint regardless of duct pressure variations.
Control Method
- Calculate airflow from velocity sensor
- Compare to airflow setpoint
- Modulate damper to maintain setpoint
- Faster inner loop compensates for pressure
Benefits
- Stable zone temperatures
- Better system balancing
- Reduced commissioning time
Alarms and Monitoring
Critical Alarms
| Alarm | Condition |
|---|
| Airflow below minimum | Potential ventilation issue |
| Damper hunting | Control problem |
| Zone temp out of range | Comfort complaint likely |
| Reheat valve stuck | Energy waste or comfort issue |
Trending Points
- Zone temperature
- Discharge air temperature
- Airflow (CFM)
- Damper position
- Heating valve position
Reference
- ASHRAE Guideline 36-2021 - High-Performance Sequences
- ASHRAE Standard 62.1 - Ventilation Requirements
- ASHRAE Handbook - HVAC Applications - Chapter 47