HVAC Zone Damper Control usually comes in two types and they are typically used for controlling the temperature a single room or a small area. The first control type is known as “Pressure Dependent” and the second type is “Pressure Independent”.

A “Pressure Dependent” Damper Control can be just a controllable damper in the ductwork which can either be 2-position (open/closed) damper or modulating (varies the amount it opens) damper. A thermostat or space temperature sensor along with a controller would send either an open/close signal or modulating signal to the damper actuator based upon the sensed zone temperature going away from the zone set point. This type of control is called pressure dependent because the zone control does not know how much airflow is being supplied to the zone. As more dampers on the air handler open the airflow is reduced because some of the air is flowing into the other zones. The opposite occurs when other zone dampers close, the open damper will receive more air so the grill can at times be loud for a lot of airflow and other times seem like there is no air flowing at all. If the air handler or AC unit can also supply heated air then the zone control must compare the temperature of the air in the duct against the zone (space) temperature and verify it is useable for what the zone requires. IE: If the space is warm and the supply air is cold then the supply air is useable to cool the space so the damper would be allowed to open to cool the space. The opposite would occur if the space is cold and the supply air is warm the damper would be allowed to open however if the space is cold and the supply air is cold the damper would remain at its minimum position.

A “Pressure Independent” Damper Control will have a modulating controlled damper and an air flow measuring device. Usually these are part of a Variable Air Volume Terminal Unit (VAV Box) but they can just be a damper and airflow measuring device installed in ductwork. This type of control is more sophisticated than a pressure dependent control because a controller reads the airflow and modulates the damper to control the airflow to a setpoint. The setpoint is varied from a minimum to maximum based upon the zone temperature deviation from the zone temperature setpoint. With this type of control the airflow is much more regulated and only varies based upon the zone temperature changing from the zone temperature setpoint. If the air handler or AC unit can also supply heated air then the zone controller must compare the temperature of the air in the duct against the zone (space) temperature and verify it is useable for what the zone requires. IE: If the space is warm and the supply air is cold then the supply air is useable to cool the space so the CFM setpoint can increase to cool the space. If the supply air is warm, then the CFM setpoint would stay at the minimum CFM setpoint. The opposite would occur if the space is cold and the supply air is warm the CFM setpoint would be allowed to increase to warm the space however if the space is cold and the supply air is cold the CFM setpoint would remain at its minimum setting.

The “Pressure Independent” Damper Control can have different CFM setpoints for heating mode and cooling mode.