Things to Consider when Sizing VAV Box Terminals:
- Don’t oversize or undersize them! Terminal units should be sized to perform at as close to maximum capacity as possible. Oversizing the terminal unit to reduce noise and/or pressure drop can cause problems in some systems; for example, higher minimum CFM’s waste energy. Improperly sizing a VAV terminal’s minimum airflow can result in significant extra fan power consumption and higher energy consumption. First cost is higher when boxes are oversized. Moreover, oversizing boxes can cause continuous hunting for its proper control set-point.
- As noted above, oversizing a VAV terminal affects the control of the unit. Airflow at the VAV inlet sensor creates a pressure differential ranging from zero to one inch of water column. Optimize the design by selecting CFM values which will take advantage of as much of the available range as possible to get the best control performance of the unit.
- Undersizing VAV terminals can increase pressure drops effecting fan performance, plus noise is increased due to higher system pressures, and energy consumption is increased
- When you add excessive system supply air pressure to an oversized unit, the terminal box damper only has to move just a few degrees for the unit to go from minimum airflow to maximum airflow. This is not a desired control situation, because the terminal box will never be satisfied and will always be hunting, thus creating large fluctuations of airflow in the space.
- The specified accuracy for the majority of pressure independent controllers is (+ or –) 5%. Most controllers will maintain airflow setpoint within that tolerance down to approximately 20% of maximum rated airflow. When operating the box below 20% of the maximum unit airflow, the control accuracy decreases. Most DDC controllers are not recommended to be used when CFM values are below the 0.03” delta Ps sensor differential pressure.
- For an acceptable air flow control range which guarantees that the unit will properly control at minimum airflow, the maximum CFM setting of a terminal unit should be no less than 60% of its maximum CFM capacity rating. Utilize this criteria when selecting single duct terminal units for constant volume airflow, as well.
- Over the course of time, the gaskets on 90 degree dampers (used by most VAV box manufacturers) will become deformed. When opened at minimum airflow and high inlet pressure situations, the damper gasket can fluctuate, creating a low frequency sound. Without replacing the box, the best solution to this problem is to change either the minimum airflow setting, or to slightly change the duct pressure control setting.
- Use caution if your manufacturer’s VAV terminal is listed with a minimum airflow below 0.03” delta Ps for each inlet size. Performance at these low flows cannot be guaranteed, no matter whose controls are affixed to the terminal box.
- Oversizing concerns are relevant to all terminal unit manufacturers. Similar controls are used by most VAV box manufacturers. Careful unit selections will avoid problems in the field.