Topic: Dampers
Understanding the Differences Between Backdraft and Relief Dampers – Application and Selection
by Julie Bernett |
A successful HVAC system installation requires the correct components. Selecting most components is pretty simple. However, some can create problems. Backdraft and relief dampers, because of subtle differences and uses, can fall into that category. (Figure 1)

 Figure 1 

Backdraft dampers, also known as gravity dampers, are used in ventilation systems to allow airflow in one direction and prevent airflow in the opposite direction. A relief damper has an elevated and adjustable start-open pressure while providing the backdraft function - think of it as controlled leakage in the direction of flow. The following discusses specific types of backdraft and relief dampers, including the operation and uses of each.

Commercial Backdraft Dampers

A commercial backdraft damper (Figure 2) is a gravity damper when non-motorized allowing airflow in one direction only. For example, when placed on a propeller fan, it will prevent the wind from causing the fan to run backward when the power is off. When a backdraft damper is motorized, it functions as a control damper.

 Figure 2 

Backdraft dampers may utilize springs, adjustable counterbalance weights, or a motor pack to assist with opening the damper blades.

  • Spring-assist means the spring is attached to the damper that helps open or close the damper blades. The spring is adjustable using a series of holes in the frame or blade assembly to increase or decrease the tension.
  • Adjustable counterbalance weights are a more precise means of reducing the pressure required to open the damper.
  • A motor pack is necessary when the damper needs to open and close without relying on air velocity or pressure.

Applications for commercial backdraft dampers are:

  • Exhaust
  • Air intake
  • Roof ventilation
  • Sidewall ventilation
  • In-duct ventilation

Heavy Duty/Industrial Backdraft Dampers

A heavy duty/industrial backdraft damper (Figure 3) prevents backflow at higher pressures and velocities. Counterbalance weights are mounted externally to allow for field adjustment and balancing. These dampers are typically flange mounted with size dimensions measured from the inside of the damper frame.

 Figure 3 

Applications for heavy-duty/industrial backdraft dampers are:

  • Blower outlets
  • Branch duct isolation
  • Industrial process isolation
  • Emergency generator radiator outlets

Relief Dampers

Relief dampers are backdraft dampers with an adjustable start-open pressure capable of maintaining a relatively constant pressure at various airflows, then closing upon a decrease in differential pressure. Uses can include pressure build-up in zoned duct systems that could potentially cause damage to HVAC equipment.

Barometric Relief Dampers

A barometric relief damper (Figure 4) is a backdraft damper with an adjustable start-open pressure used for gravity ventilation and low-velocity systems. Counterbalance weights provide the ability to fine-tune start-to-open and full-open operations. A common use for this damper is to relieve built-up pressure in zoned duct systems that could potentially cause damage to HVAC equipment. When introducing fresh air into a building, one method is to use a barometric relief damper to exhaust the present air.

 Figure 4 

Applications for barometric relief dampers are:

  • Gravity hood intake and exhaust
  • Stairwell pressurization
  • Room pressurization
  • Ductwork outlets

Heavy-Duty/Industrial Pressure Relief Dampers

A pressure relief damper (Figure 5) is a backdraft damper with an adjustable start-open pressure capable of maintaining a relatively constant pressure at various airflows and closes upon a decrease in differential pressure. Pressure relief dampers do not immediately open completely upon reaching their start-open pressure. The damper maintains tight closure until pressure reaches approximately 60% of the start to open pressure and then a relatively constant flow control beyond the start to open pressure. Counterbalance weights are mounted externally for field adjustment and balancing.

 Figure 5 

A pressure relief damper may be a safety device or a controlling device. It can mount in a duct section to relieve an unexpected overpressure or relieve duct vacuum downstream of a rapidly closing fire damper. These dampers can also be used as a control device, such as opening to allow additional air when used parallel to a direct fire gas burner or allow more air into fume exhaust to maintain 3000 feet per minute (fpm) exhaust velocity.

Applications for heavy-duty/industrial pressure relief dampers are:

  • Fume exhaust
  • Duct/plenum protection

Energy Code

The two common energy code standards (Figure 6) that pertain to backdraft dampers are:

  • ASHRAE Standard 90.1 – Energy standard for Building Except Low-Rise Residential Buildings
  • IECC - International Energy Conservation Code
 Figure 6 

Both IECC and ASHRAE codes state that gravity (non-motorized) dampers shall have an air leakage rating not greater than 20 cfm/ft² where not less than 24 inches in either dimension and 40 cfm/ft² where less than 24 inches in either direction. The rate of air leakage shall be determined at 1-inch water gauge when tested in accordance with AMCA 500-D for such purpose. IECC additionally requires backdraft dampers to be labeled by an approved agency, such as the Air Movement and Control Association, International.

Selection of a Backdraft Damper


When selecting the correct damper for your application, you will need to know the following:

  • Airflow direction
  • Damper operation (gravity or motorized)
  • System velocity and back pressure requirements
  • Mounting configuration (inserted into the duct/opening or flange mounted)
  • Mounting orientation (vertical, horizontal airflow up, horizontal airflow down)
  • Start-open pressure
  • System velocity and back pressure requirements

This information will help determine whether a backdraft or relief damper works best for the application.

Learn more about backdraft and heavy-duty industrial dampers.

References: 2021 IECC (International Energy Conservation Code)
                      2019 ASHRAE 90.1 Energy Standard for Buildings Except Low-Rise Residential Buildings



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Julie Bernett
Julie Bernett
Julie Bernett
Julie Bernett has served as a senior marketing specialist with Greenheck’s dampers business unit for 23 years. She is an active member of the National Fire Protection Association and is responsible for communication with technicians and representatives for all things dampers.
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