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

1. Compare Zone_Temp to Cooling_Setpoint
2. If Zone_Temp > Setpoint + Deadband: Increase airflow toward maximum
3. If Zone_Temp < Setpoint - Deadband: Decrease airflow toward minimum
4. 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:

1. Zone calls for cooling
2. Damper modulates: Min → Max
3. Heating valve: Closed

Deadband:

1. Zone between setpoints
2. Damper at minimum
3. Heating valve: Closed

Heating Mode:

1. Zone calls for heating
2. Damper at minimum (or heating minimum)
3. 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:

1. Series fan: ON
2. Primary damper modulates for cooling
3. Heating coil modulates for heating
4. Constant discharge airflow

Unoccupied:

1. Series fan: OFF
2. Primary damper: Closed
3. Heating coil: Off (or cycle for setback)

Parallel Fan-Powered

Fan cycles for heating boost:

Cooling Mode:

1. Parallel fan: OFF
2. Primary damper modulates: Min→Max

Heating Mode:

1. Primary damper: Closed or Min
2. Parallel fan: ON
3. Heating coil: Modulates
4. 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:

1. DAT Setpoint = 90-95°F (max)
2. Modulate hot water valve
3. 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

1. Calculate airflow from velocity sensor
2. Compare to airflow setpoint
3. Modulate damper to maintain setpoint
4. 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