Backdraft & Motorized Dampers
General
In this section, we provide a quick overview of backdraft and motorized dampers, along with their typical applications. The general purpose of either type of damper is to:
Allow or restrict the transfer of air through a wall opening or end of a duct. For example, if a bathroom exhaust system is in operation, the exhaust fan and system ductwork work together to exhaust the air from the bathrooms to a location outside the building through a ducted wall opening. In such a case, a damper is required to cover the duct/wall opening. When the exhaust system is off, the damper remains closed and opens when the system is in operation.
Prevent any unwanted transfer of air through building openings. For efficient heating and cooling of a space or building, your conditioned areas need to be tightly sealed. Any openings in the boundaries of the space, either communicating with an unconditioned space or the outside environment, can increase the heating or cooling loads on the system. For example, on a hot day, you do not want hot, unconditioned outside air entering your cool conditioned space. Similarly, during the heating season, you want to seal off any openings to prevent air from the heated, conditioned space to exit the space through a wall opening.
Wall openings are typically required to be sealed and protected by code. To build on the previous point, building codes, and energy codes specifically, require that openings are protected and sealed. This seal allows greater temperature control of the conditioned space and prevents increased heating or cooling loads on the equipment serving the space.
Backdraft dampers (BDD) and motorized dampers (MD) are essentially the same piece of hardware. Both assemblies consist of similar features such as the damper housing frame, damper blades, crankshaft (a mechanism that can open or close the blades in unison). The primary difference between a backdraft damper (BDD) and a motorized damper (MD) is that a motorized damper has an actuator, a small motor device that attaches to the damper’s crankshaft, rotating the shaft causing the blades to open or close. Some additional advantages & disadvantages of each are listed below.
Backdraft dampers (BDD)
Backdraft dampers, also referred to as gravity dampers, open and close based on air pressure or air velocity in the ducted system it serves. If the fan serving the ductwork system is off (de-energized), there is no airflow in the system, no air pressure/velocity produced, and therefore the backdraft damper remains closed. However, if a fan is turned on, the air pressure/velocity produced would force the damper to open. There are no actuators that control the opening and closing of the damper, as in the case of a motorized damper. Since the weight of the damper blades can be significant, manufacturers may provide a counterweight to assist in the opening of the damper blades so that the air pressure/velocity in the system can more easily open or close the damper.
BDDs are often constructed such that they open in one direction only, similar to a piping check valve. This ensures that airflow is in one direction only, either intake or exhaust, regardless of the environmental conditions outside the building or space. This would prevent the unintentional opening or closing of a damper on a windy day, for example. Backdraft dampers can be used for simple applications where a tight air seal is not required, such as in a bathroom exhaust system as mentioned previously. Since there is no motorized features on the damper, this also reduces the equipment cost as well as the construction cost.
Motorized dampers (MD)
However, not all applications can use a backdraft damper. Since BDDs open and close based on air movement, they don’t always provide a tight seal. Damper blades may open or close for a variety of reasons, such as external wind movement or stack effects inside the ductwork system. Where greater control is preferred, a motorized damper should be used. This would provide a tight seal and prevent any unwanted air transfer through the opening. A common application of motorized dampers is at the end of an outside air (OA) intake duct. When the HVAC equipment the OA duct serves is off, the damper would close and only open when the equipment is on to allow outside air to be brought into the system and delivered to the occupied space for occupant comfort.