Variable air volume units differ from constant volume units in several ways. A Variable Air Volume (VAV) unit incorporates many features to allow the unit to vary its cooling capacity and airflow (CFM). This includes different control software, a Variable Frequency Drive to control the indoor fan system, and compressor unloading features among other unique features. Typically, a VAV unit’s cooling capacity is controlled to a specific supply air temperature set point and the airflow is controlled to a supply air pressure value.

Conversely, constant volume units base the cooling capacity operation on the temperature/humidity in the occupied space and supply a fixed amount of variable temperature air. Simply installing a VFD on a Constant Volume rooftop unit does NOT allow the CV unit to operate as a VAV unit. It would allow the fan system (RPM and CFM) to modulate based on the input signal to the VFD controller, however the limited cooling capacity stages and control logic would not allow for accurate control of supply air temperature. As a result, a CV unit with a VFD in a VAV application will not function properly and the application will have continuous operational problems.

“Simply installing a VFD on a Constant Volume rooftop unit does NOT allow the CV unit to operate as a VAV unit.”

In the HVAC industry, VAV equipped rooftop units are typically available for unit cooling capacity sizes 20 tons and above. This 20 ton size distinction represents the historical cost efficiency trade-off between zone size and the initial cost of the equipment. The larger the zone size, the greater the possibility that a simultaneous heating and cooling call can exist as well as an increased potential for more zones of control. This is why a VAV unit starts to emerge as the better choice. Besides the fan savings on the rooftop unit, the VAV terminals can be individually fitted with heating coils so one zone can heat while another zone can cool with air from the rooftop unit. Large areas with many zones can be handled by a single VAV rooftop unit.

Below 20 tons another system called variable volume and temperature (VVT) is commonly used. VVT is capable of supplying multiple zones of control from a single constant volume rooftop. The standard constant volume rooftop may be used because the unused supply airflow is bypassed back to the unit return.

Due to the potential for significant energy savings in reducing airflow another application that utilizes a variable volume of supply air has become popular in recent years: the Single Zone VAV concept. The Single zone VAV concept uses a rooftop unit with variable air volume capability to condition a single space or zone, resulting in considerable fan energy savings. With a Single Zone VAV unit, the volume of airflow is reduced as the space temperature falls below the cooling set point. The airflow will reduce to a minimum value that would equal the desired heating mode airflow. While it is not necessary for the rooftop unit to be able to control supply air temperature in a Single Zone VAV application, advantages such as enhanced humidity control capability, more even space temperature and reduced energy consumption can be gained by using unit with this ability.

For a more complete discussion of VAV systems, variable frequency drives, fan modulation methods, and VVT applications, see the Carrier Training Development Program manuals for Variable Volume Systems and Constant Volume Rooftop Units at Carrier University.

Used On:

• Packaged rooftop units using Puron (R-410A)
• Packaged rooftop units using R-22
• 48PD Centurion® single-zone VAV unit
• 48PD Centurion® single-zone VAV unit