Variable Air Volume


United Trades Exclusive
Variable Air volume

By Dale Yelnich

In its most basic iteration, a variable air volume, or VAV system, is comprised of a central supply duct that feeds cooled air into ducted branches, by way of a pressurized box. It is an air conditioning HVAC supply design, generally located at or above the ceiling structure, that incorporates a variable speed fan.

The heart of this set up is the VAV box, which is essentially a pressure sensitive air damper flow device that incorporates an automatically controlled actuator. VAV systems are used in commercial or industrial applications, and it is a cost effective way to reduce energy consumption by using less fan energy and less refrigerant energy when supplying cooled air throughout a building.

Variable air volume arrangements have their beginnings with traditional constant volume temperature layouts. Much like an HVAC system for a home, in a constant volume configuration, a single thermostat controls the cooling temperature for an entire space, regardless of how large it is. This may work fine if the space is predominantly open, but once walls, partitions or cubicles go up, the dynamics of the room change.

These individual spaces, or zones, cannot possibly be equally temperature controlled by one single unit. The limitations from a design like this means that some spaces around the room will become too hot and some spaces will become too cold. The answer to this quandary is called a constant air volume multiple space system, or CAV for short.

A basic CAV system means that both the supply fan and the air conditioning refrigeration unit supply a constant volume of cooled air at all times. This system still uses one thermostat for overall cooling control, but ducted branches extend from the main duct, much like VAV ducting, and supply cooled air to different parts of the space that they serve.

The difference is that each individual space has its own control to limit the amount of cooled air that comes out of the supply duct vent. Nothing fancy here, the control opens or closes a register which limits the amount of air flowing through it. The main supply fan is still running at a constant speed to supply all of the open registers, but by restricting the airflow to each individual space, there is some degree of temperature control.

Taking the basic CAV system a bit further, a terminal reheating coil system can be added into the ductwork to heat up the air that supplies a particular zone. Instead of restricting the airflow to effect the temperature of any given area, small heaters are added that heat the incoming air. If the air is too cold, a room thermostat is engaged, sending a signal to a reheating unit, that slightly heats up the air before it enters the zone. If the air is too warm, the heater is shut down and the cooled air, once again, flows directly into the room.

Although a CAV system like this gives good individual control of the temperature that supplies individual spaces or zones, it is not a very energy efficient solution. All of the air that has been warmed up, by passing through a reheating coil, must once again be cooled off as it moves to the air return duct and passes through the air conditioning refrigeration coils. This is almost like having an air conditioner do twice the work for the same cooling results.

The solution to all of this is the variable air volume system. This combines all of the attributes of a constant volume air system with the controllability of a CAV multiple space system, that both reduces energy consumption while allowing individual temperature control for any given zone.

A VAV system is dependent on two different parts. The first part begins with the thermostatically controlled VAV boxes that supply each room, space or zone. The second part is that fan speed becomes variable depending upon how much cooled air is being called for.

When a thermostat in any given supplied zone is dialed to a specific temperature, an automated damper in the VAV box regulates the airflow into that particular space. It automatically adjusts to allow either more or less cooled air to flow past it, and in this way it keeps the room at the required temperature. As the static pressure in the VAV boxes increase throughout an entire building, a pressure sensors tell the fan that cooling has been stabilized and the fan speed drops.

This is significant in two ways. Firstly, the fan is no longer working at full capacity and is saving energy by doing so. Secondly, with less air flowing through the refrigeration coil, less energy is needed to cool off the air. That also leads to energy savings, making a VAV design the most efficient HVAC system for a commercial setting.

Consider also that VAV units are typically as cost effective to install as a regular constant air volume system, with the only major difference being in the variably actuated VAV box dampers and the automatic speed control for the supply fan. Modern units are also capable of supplying controlled heat to different zones, and the automation required to make all of these adjustments are both pneumatic or digital control friendly.

VAV systems are the latest and greatest air conditioning system in a commercial HVAC setting, which will provide unsurpassed cooling (and heating) comfort in any commercial work space, while ensuring the most energy efficient, and energy effective, means for every climate control situation.

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