What is a Variable Air Volume Box?

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  • Written By: Michael Anissimov
  • Edited By: Niki Foster
  • Last Modified Date: 05 December 2018
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A variable air volume box, more commonly known as a VAV box, is an essential part of the air conditioning in any large industrial or commercial building. Air conditioning is part of a larger entity called a HVAC system, which encompasses heating, ventilation, air conditioning, and control systems. The VAV box is a HVAC component alongside other pieces of hardware such as coils and drain pans, humidification and dehumidification equipment, air dampers, air filters, ducts, exhaust systems, return air plenums, and so on. This box is in contrast to a CAV, or constant air volume system.

This air conditioning component is just what it sounds like: a box that can hold a variable amount of air. It helps make air conditioning systems more efficient by regulating the amount of cooling targeted toward any specific room or area. This is referred to as the cooling/heating load. A room whose windows face the sun for most of the day will have a higher cooling load than a room in the shade. When a VAV box constricts a valve to let less air through, it decreases the amount of energy consumed by fans that direct the air around the building.


Modern VAV boxes come with advanced control equipment that supports automated changes to air flow as efficiency dictates. This control equipment is also linked to central computers for the entire HVAC system for a building. Small pressure sensors detect the pressure of air in the box and hinges open and close doors to manipulate airflow and volume.

Research continues to improve variable air volume boxes, millions of which are installed throughout the world. Areas for improvement include minimizing noise, reducing the need for maintenance, fine-tuning the control systems, and creating diagnostic response systems that alert maintenance teams and give them accurate information about malfunctions before they even need to look at the device physically.


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Post 20

What is the minimum amount of pipe that can go on a v.a.v. box?

Post 19

Where do all the pneumatics hook up to on a SDR cool only VAV box -- no fan or electric.

Post 18

Can we have a combined VAVs and CAVs system connected to a variable speed air handling unit?

Post 17

Can we have a combined VAVs and CAVs system, connected to a variable speed air handling unit?

Post 16

what is the function of vav box and in which application and condition we can use this?

Post 15

Let me try and clarify this discussion; this is very simplified and only applies to the air side of a HVAC system. A constant volume system delivers a constant flow of air to the zones served. The temperature in the zone is controlled by modulating the supply air temperature.

A variable air volume system delivers a constant temperature air to the zones served. The temperature in the zone is controlled by modulating the air flow or varying the air volume (Variable Air Volume VAV).

The VAV systems permits one air handler to conditions multiple zones with differing load conditions. For example, a commercial office building may have only one air handling unit per floor. The perimeters of the floor may

have either heating or cooling loads due to the outdoor conditions, however, interior zones will always need cooling due to the heat production of people and office equipment.

To meet the constant cooling requirements of the interior zone, the supply air temperature is always cold, 55 F plus or minus.

On a multi zone VAV system, each zone is controlled by a VAV Terminal (box). The terminal’s air valve opens when the zone temperature rises, increasing the air flow. When the zone temperature falls, the air valve closes, decreasing the air flow to the minimum flow.

Typically each terminal has an associated temperature sensor. One sensor could control multiple terminals, but why? The terminal is the expensive thing, not the temperature sensor.

So far, we have only addressed the the cooling controls. The perimeter heating loads are typically addressed through reheat coils mounted in the VAV terminal. These reheat coils can be either electric or water. Upon a further decrease in zone temperature the reheat coil shall be activated to heat the zone. Typically the VAV terminal is at its minimum air volume setting when the reheat coil is activated.

I have never seen a four-pipe VAV terminal installed. The terminal’s air valve controls cooling and the reheat coil controls heating. What the last two pipes do, I have no clue.

Post 13

VAV systems are not dead. VAV boxes don't heat or cool, but simply regulate the amount of air traveling into a room or office.

The heating and cooling of the air is done by the AC unit, and, if not part of the unit, heating pumps. I worked in construction for years, HVAC to be precise, and have built VAV systems. They are very useful for large office complexes, such as a medical office building or a hospital.

The thermostat in the offices basically tells the VAV unit when to open/close/adjust, while the building operator, hospital management, regulate the cooling or heating of the air by the unit.

What this does is it gives the offices their own sense of

comfort, but lets the hospital manage the efficiency of the heating/cooling. Anything can be used to cool and heat the air. Whether this is done with water, ice cooling, or R-22 (now to be used is R-410A), it does not matter. The VAV does not control how the room is conditioned, just how much.
Post 12

have you ever heard of filters on a VAV box? This might make sense in a return air plenum.

Post 10

How does VAV work in the supply air duct?

Post 9

There is no such thing as 2 0r 4 pipe VAV - VAV is an all air system. You can get 2 and 4 pipe fan coil units - 2 pipe is cooling only ie 1x cooling coil (1 flow pipe, one return pipe); 4 pipe is heating and cooling (1 x cooling coil, 1 x heating coil). VAV CAN have shut off but it's best to have some fresh air in even empty room to prevent these rooms going stale. You can connect multiple VAV boxes but why would you? This would mean 3 room controlled by one thermostat - in an open plan office you would use a master and slave system where one box controls the operation of the others. Having said that, VAV is a dead system - VAV is all air which means that al the heating and cooling of a space is done by the air. The specific heat capacity of air is 1.2, the specifit heat capacity of water is 4.2 - therefore water systems are neary four times as efficiant at heating and cooling a space. With Part L (UK building regs) getting more onerous year after year and energy saving becoming more important with all new builds requiring an EPC it is fair to say that VAV won't be with us much longer.

Post 8

RSVAV has many answers about VAV Connecting three vavs to one t-stat is best with wireless. The function is best applied to large areas, such as function rooms in the hotel.

Post 7

Can you connect three vav's to one t-stat?

Post 6

show operation of a series vav box

Post 5

Does your building have VAV or CAV? Describe your system in details.

Post 4

what is the function of vav box and in which application and condition we can use this?

Post 3

should a vav box have a shut off valve?

Post 2

Explain the difference between a two-pipe VAV and a four-pipe VAV.

Post 1

What's the difference between a two-pipe vav and a four-pipe vav?

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