Energy managers like to say that equipment uses the least amount of energy when it is turned off. But, for variable-air-volume boxes (VAVs) it would be more accurate to say that when VAVs are turned off, they allow other equipment to use less energy. VAVs are a type of heating, ventilating, and/or air conditioning system which vary the airflow at a constant temperature.
Shutting a VAV’s damper allows the air handling unit (AHU) that serves it to use less fan power, especially if the AHU has a variable speed drive. Closing the heating and cooling valves will decrease the load on central chillers and boilers.
Generally, if all of the spaces served by an AHU are unoccupied, the AHU and all of its VAVs can be shut off. Additional opportunities arise when only some of the spaces served by an AHU become unoccupied. In that case, there is usually some potential in cutting back on conditioning, and possibly airflow, in the unoccupied spaces without jeopardizing conditions in adjacent occupied spaces.
Turn off the VAV to reduce the load on the AHU
If a space served by a VAV is unoccupied, the VAV can usually be turned off completely. The damper can be closed, and the heating and cooling valves (where present) can be shut off. This reduces the load on the AHU serving the VAV, as it needs to circulate less air. If the VAV was heating or cooling, it will also reduce the load on the relevant central plant. In some cases, a VAV can never be shut off because of constant relative pressurization requirements in adjacent spaces. This is common in healthcare. However, it still may be possible to stop conditioning the room (i.e., shut the heating and cooling valves, and never open the damper beyond its minimum position for the purpose of cooling) when it becomes unoccupied.
Ramping and setbacks are ideal times to give VAVs a break
Even if all the VAVs for a single AHU have the same schedule, there are two cases where only some of the VAVs need to run. The first is ramping – where an AHU is running to pre-heat or cool a soon-to-be-occupied section of a building. VAVs that are already satisfied, or which don’t need to run for as long to be satisfied in time for occupancy, can be shut off until the space is actually occupied, because their spaces don’t have any air requirements during the ramping period.
A second case – setbacks – matters when the AHU is the only way to condition an unoccupied section of a building. If the unoccupied section still needs heating or cooling to meet temperature setpoints, the AHU must run. However, only the VAVs that serve the spaces needing conditioning must run. The others can remain off, allowing the AHU to run at greatly reduced flow, and avoid conditioning the spaces that are okay.
In both ramping and setbacks, it is important to avoid turning off so many VAVs that the pressure in the AHU rises to a dangerous level. AHUs can be configured to never dial down their fan speeds below a predetermined threshold, regardless of how high static pressure gets. If such logic is in place, it is important to keep enough VAVs dampers open, in order to get the AHU to stay at its static pressure setpoint while running above the minimum fan speed. Dropping an AHU’s fan speeds anywhere near its minimum will drop its power draw to essentially zero, so there is no need to take risks to get as close as possible.
In all of the above cases, further savings can be realized by ignoring VAVs serving unoccupied spaces in the AHU’s discharge air temperature setpoint logic. This will often have a beneficial effect. For example, if the now-unoccupied space was calling for more cooling than the rest of the spaces, and the VAVs in the system were only capable of reheating the AHU’s discharge air (i.e., they only had heating coils), then the discharge air temperature setpoint at the AHU could be raised. This would, in turn, require less cooling at the AHU, and less reheating at the other VAVs in the system that are still occupied.
If you don’t have VFDs, don’t panic – savings can still be achieved
What if you don’t have Variable Frequency Drives (VFDs) that control motor speed and torque by varying motor input frequency and voltage? There is still energy savings in closing VAV dampers. Reducing the air volume needed downstream from the fan reduces the real work done by the fan. While the kVA of the fan may not change much, the kW will reduce a little, resulting in less kWh of electrical consumption. Certainly there are much greater electrical savings to be had when a VFD is available to reduce the fan speed. But all of the savings not-related to the fan’s motor (i.e., less heating and cooling at the AHU and VAVs) are still possible without a VFD, and some electrical savings can still be achieved.