Rebholtz Mechanical, Inc.

What Is
Variable Air Volume?

	Rooftop HVAC Replacement Program
	Components of a VAV System
	What Is Variable Air Volume?


However, because this type of system can respond to the demands of only one thermostat, it can serve only those building spaces with similar cooling requirements. If a building has many spaces with diverse cooling needs, each must be served by its own system.		Other constant-volume system designs can serve the cooling requirements of more than one space with a central fan and cooling coil. However, to do so, the cool primary air must be either reheated or mixed with warm air to produce the supply temperatures needed to balance the various space cooling loads.

TeRebholtz Mechanicalnal Reheat System
The teRebholtz Mechanicalnal reheat system uses a central air handler and cooling coil to deliver cool primary air to all the spaces. Each space has its own heating coil to temper the air to satisfy the space load. Of course, any heat added to meet the part-load requirements of a space becomes a cooling load that the refrigeration system must overcome. This can result in a nearly constant refrigeration load, even when the building is at part-load conditions.

Therefore, reheating cooled air to achieve part-load supply air temperature control is not very energy efficient and is used only in special constant-volume applications, or when there is a "free" source of heat (i.e., heat recovery).

Primary air (PA)	is the air delivered by a central supply fan to a teRebholtz Mechanicalnal unit.
Supply air (SA)	is the air delivered to a space.

System Comparison
Contrasting the three systems discussed in this example, the simple, constantvolume, variable-temperature system consumes constant fan energy while providing refrigeration energy savings at part load. However, it can only deliver comfort to spaces with similar loads.
The teRebholtz Mechanicalnal reheat system can serve many spaces with dissimilar load requirements, but consumes constant fan energy and nearly constant refrigeration energy at part load. It also uses an increasing amount of reheat energy at part load.
On the other hand, the variable-air-volume system is capable of providing both fan and refrigeration energy savings at part load, making it the most energy efficient of these three systems. In addition, the VAV system can efficiently serve many spaces with dissimilar cooling and heating load requirements.

constant volume single zone
	Constant fan energy
	Refrigeration energy savings
	Delivers comfort to only one thermal zone

constant volume *teRebholtz Mechanicalnal reheat*
	Constant fan energy
	Nearly constant refrigeration energy
	Delivers comfort to many spaces inefficiently
	Reheat energy increases as part load

VAV
	Fan energy savings
	Refrigeration energy savings
	Delivers comfort to many spaces efficiently


Another feature of the VAV system is the ability to efficiently satisfy the comfort requirements of many different spaces within the building. As we saw from the comparison of the VAV and teRebholtz Mechanicalnal reheat systems, the VAV system is the most efficient at perfoRebholtz Mechanicalng this task.		Because the VA V system uses one central cooling coil and fan to serve many spaces, the need for a common path to allow the air to return from these spaces is significant. A return air plenum is commonly used. If return ductwork is required to connect each conditioned space to the central air handling unit, the system becomes more difficult to balance and control.