VAV box types and selection

A VAV terminal unit, usually called a VAV box, is the device that controls airflow to each zone in a variable air volume system. It contains a damper, an airflow sensor, a controller and often a reheat coil or fan. The box responds to the zone thermostat, opening to deliver more cool air when the space is warm and closing to a minimum airflow when it is cool.

VAV boxes look simple, but their selection affects comfort, noise, energy use and ventilation. Oversized boxes cannot measure low airflows accurately; undersized boxes are noisy and waste fan energy. This guide explains the main types and how to select them well.

Main VAV box types

Single duct, cooling only

The simplest type: a damper and airflow sensor in a single inlet. It serves interior zones that need only cooling. When the load drops, airflow drops to the minimum.

Single duct with reheat

The same box with a hot water or electric heating coil on the discharge. It serves perimeter zones or any zone that may need heat. When the zone needs heat, airflow is held at a minimum or modestly increased while the coil heats the air. This is the most common VAV box in Canadian buildings.

Fan-powered series

A small fan in the box runs continuously whenever the zone is occupied, drawing return air from the ceiling plenum and mixing it with primary air from the AHU. The fan delivers a constant airflow to the space, while the primary air damper varies.

• Constant airflow to the space improves air distribution and reduces dumping at low loads.
• Allows lower primary air temperatures in some designs.
• The fan runs continuously, adding fan energy and noise at the box.

Fan-powered parallel

A fan in the box runs only when the zone needs heat. In cooling, primary air flows through the damper alone. In heating, the fan draws warm plenum air (heated by lights and other gains) to provide heat before or along with a reheat coil.

• Uses recovered plenum heat, reducing reheat energy.
• Fan runs only part of the time, saving energy compared with series boxes.
• Space airflow varies, so diffuser selection must consider low airflows.

Dual duct

A box with separate hot and cold inlets, each with its own damper. It mixes the streams to meet the zone load. Found mainly in older buildings and some hospitals. Energy use is high when both decks are active at once.

Induction units

Induction terminals use primary air jets to draw plenum or room air through the unit without a fan. They are less common today but appear in some older buildings and in chilled beam systems, which work on a similar principle.

Comparing box types

Pressure-independent control

Modern VAV boxes are pressure independent: the controller measures airflow and adjusts the damper to maintain the airflow setpoint calculated from the zone temperature, regardless of changes in duct pressure. This requires a reliable airflow sensor, usually a multi-point velocity pressure cross or ring at the inlet.

Accuracy falls at low velocity because velocity pressure becomes very small. This is the main limit on how low a box can control. In laboratories and other critical applications where accuracy at low flow is essential, designers often use venturi air valves instead; see venturi valves vs VAV boxes with airflow stations.

Selecting box size

Box size is chosen by inlet diameter. A practical procedure:

1. Calculate design (maximum) cooling airflow for the zone from the load calculation.
2. Determine the minimum airflow: the larger of the ventilation requirement for the zone and the lowest airflow the controller can reliably measure.
3. Determine any heating airflow required.
4. Select an inlet size so that the design airflow falls within the manufacturer's recommended inlet velocity range, without being so large that the minimum airflow falls below the controllable limit.
5. Check radiated and discharge sound at design airflow and pressure against the room noise criteria.
6. Check the box pressure drop at design airflow and the minimum inlet static pressure needed.
7. Select the reheat coil for the heating load at the heating airflow and the design hot water temperature.

Larger boxes reduce pressure drop and noise but sacrifice low-flow controllability. Smaller boxes control well at low flow but can be noisy and increase required duct pressure. A good selection balances both.

Minimum airflow settings

Minimum airflow is one of the most important and misunderstood settings. High minimums waste fan and reheat energy and can overcool zones. ASHRAE Guideline 36 sets the minimum as the larger of the zone ventilation requirement and the lowest controllable airflow. The controllable airflow depends on the sensor, transducer and controller, and manufacturers publish minimum velocity pressure or minimum airflow data.

For reheat, ASHRAE Standard 62.1 lowers the zone air distribution effectiveness for ceiling supply of warm air, especially when supply air is more than about 8 K (15 °F) above space temperature. Limiting discharge air temperature in heating keeps ventilation effective and reduces stratification. See ASHRAE Guideline 36 high performance sequences.

Noise

VAV boxes generate noise from the damper and, in fan-powered boxes, the fan. Noise ratings are published per AHRI Standard 880, and AHRI 885 provides methods to estimate resulting room sound levels. Selection should target the room criteria for the space, often expressed as NC or RC values. Common noise problems come from boxes located over quiet rooms, excessive inlet static pressure and inadequate discharge duct lengths. See HVAC noise and vibration control.

Installation and commissioning

• Provide a straight inlet duct, commonly several duct diameters, to ensure accurate airflow measurement. Elbows and flex duct right at the inlet cause errors.
• Allow access to controls, dampers and coils.
• Verify flow sensor calibration during balancing; the balancer adjusts the flow coefficient so the box reading matches measured flow.
• Functionally test each box through cooling, minimum, heating and unoccupied modes. Control system verification confirms that boxes respond correctly to their sequences.

Frequently asked questions

What is the most common VAV box problem?

Inaccurate airflow measurement, often caused by poor inlet conditions, dirty sensors or incorrect flow coefficients. This leads to wrong airflows, comfort complaints and ventilation shortfalls.

Should I use series or parallel fan-powered boxes?

Series boxes provide constant space airflow and suit low-temperature air or spaces sensitive to air movement. Parallel boxes use less fan energy and recover plenum heat. The choice depends on the zone, climate and system design.

Can VAV minimums be lowered in an existing building?

Often, yes. After confirming ventilation requirements and controller capabilities, lowering minimums usually reduces reheat and fan energy significantly.

Key takeaways

• Single duct with reheat is the most common VAV box; fan-powered boxes suit specific needs.
• Pressure-independent control relies on accurate airflow sensing, which limits low-flow control.
• Size boxes to balance controllability at minimum airflow with noise and pressure drop.
• Set minimum airflows to the larger of ventilation needs and controllable flow.
• Good inlet conditions, calibration and functional testing are essential.