Understanding the Differences Between Backdraft and Relief Dampers

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.

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

Dampers

Application

What is the difference between static and dynamic fire-rated dampers? Static fire-rated dampers are installed in HVAC systems intended to shut down during a fire. Triggered by a smoke detector, an alarm, or a localized increase in temperature, they are designed to close automatically, prohibiting the passage of air through ducts, when fire is detected. Dynamic fire-rated dampers are used in HVAC systems intended to remain operational during a fire. They are designed to operate under airflow and static pressure and to close when the temperature reaches a certain threshold.

When do I use a 1.5-hour fire damper, and when do I use a 3-hour fire damper? NFPA (National Fire Protection Association) 90A, Standard for the Installation of Air-Conditioning and Ventilating Systems, states penetrations in barriers rated for less than 3 hours must be protected with fire or combination fire/smoke dampers rated for 1.5 hours. Penetrations in barriers rated for 3 hours or more must be protected with fire or combination fire/smoke dampers rated for 3 hours.

Does the penetration for a fire damper in a fire-rated wall need to be framed? Yes. All metal- and wood-framed fire-rated walls have framing requirements for openings in which a fire damper or a combination fire/smoke damper is to be installed (Photo A). Manufacturer installation instructions shipped with dampers and usually available online show or have a link to standard framing details

Which type of heat-responsive device should I use, a fusible link or a thermostat? 
Fusible links typically are found on curtain and multiblade fire dampers, while thermostats mostly are found on combination fire/smoke dampers.

Single-use devices, fusible links consist of two strips of metal soldered together (Photo B). When the solder reaches a specific temperature (typically, 165°F [74°C] or 212°F [100°C]), it melts, and the two strips of metal separate, enabling a damper to close.

A thermostat (Photo C) is a bimetal thermal disc designed to cut power to an electric damper actuator when a specific air temperature (typically, 165°F [74°C], 212°F [100°C], 250°F [121°C], or 350°F [177°C]) is reached. Many thermostats can be reset in the field once a damper has been inspected and found to be free of damage.

Do building codes explain how to install fire, smoke, and combination fire/smoke dampers?  Model codes refer to manufacturer installation instructions and damper listings. Though many manufacturers’ installation instructions are similar, the differences, however slight, may be important. Therefore, be sure to review the instructions for the specific damper to be installed.

How do I know whether I need a smoke damper with a leakage rating of Class I or one with a leakage rating of Class II? Consult your local codes for leakage requirements. If your local codes do not specify a minimum leakage for your application, then refer to the national codes.

For most applications, a minimum leakage rating of Class II is required. Class II leakage means a damper was tested and is approved to leak less than 20 cfm per square foot at 4.0 in. wg (102 l/s/m2 at 1.0 kPa).

For applications such as hospitals, schools, and stairwells, a leakage rating of Class I is required. Class I leakage means a damper was tested and is approved to leak less than 8 cfm per square foot at 4.0 in. wg (40 l/s/m2 at 1.0 kPa).

If a leakage rating is not specified, the designer will need to determine how tight a penetration should be sealed during a fire event.

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Design/Installation/Controls

How do you replace an old pneumatic actuator with a new electric model? Locate the damper’s control shaft. If the pneumatic actuator is mounted externally (Photo D), the simplest solution would be a direct-coupled, modern fire/smoke-rated electric actuator. Remove the existing pneumatic actuator and all of the linkages connecting it to the control shaft. Next, place a properly sized direct-drive fire/smoke-rated electric actuator over the drive shaft. If the existing mounting bracket is not long enough to support the anti-rotation strap of the electric actuator, a new mounting bracket will be needed. Make sure the electric actuator is oriented so that it will correctly power-open and, upon the loss of power, spring-close the damper.

If an old pneumatic actuator is internally mounted or connected via a system of linkages, a new linkage and/or mounting plate is/are likely to be needed. Any negator springs used to close a damper likely will need to be removed and/or replaced. Where a swivel, crank arm, and rod are used, a new electric actuator supplied with a built-in crank arm and mounting bracket will be required.

With the new electric actuator (and, if required, linkages) in place, open the damper by hand and power open the actuator. When both the damper and actuator are fully open, connect the actuator to the drive shaft. Next, disconnect power going to the damper and inspect that the damper has fully closed. Lastly, cycle the damper electrically—open and then closed—to ensure proper operation.

Typically, replacing a pneumatic actuator with an electric one is easy. In about 10 percent of cases, however, some investigation and modification is needed. Communication with the damper manufacturer is important to ensuring torque requirements for the damper model and size, airflow conditions, and actuator location are met.

Electro-pneumatic (E/P) switches typically need to be replaced with a relay. Air valves (Figure 1) require new electric thermostat-type heat-responsive devices. Fusible links and shaft springs generally can be retained, with no changes needed.

Does changing out an actuator affect a damper’s UL 555S, Smoke Dampers, certification? Actuator field replacement is outside the scope of UL 555S certification. UL suggests it be done in accordance with the damper manufacturer’s normal field-servicing program. Ultimately, then, it is up to the local authority having jurisdiction (AHJ) to determine if an actuator replacement is sufficient. At the building owner’s expense, a representative of UL can be hired to visit a job site and inspect.

What does the “8-13 psi” on my pneumatic actuator mean? That is the air pressure in pounds per square inch (psi) required for the actuator’s shaft to move fully without a load. In other words, 8 psi is required for the shaft to begin to move, and 13 psi is required for it to fully extend without being connected to the damper. In the same manor, an 8-13-psi spring produces the force required to close the damper. A spring with a different psi range can be used to produce more or less torque.

Can I change an actuator from 24 V to 120 V in the field? Yes. Any modification of an actuator in the field should be performed in accordance with the damper manufacturer’s guidelines and reviewed by the local AHJ or the damper manufacturer. Keep in mind this is outside the scope of UL’s certification approval.

What if the actuator on my fire, smoke, or combination fire/smoke damper no longer operates? NFPA 80, Standard for Fire Doors and Other Opening Protectives, and NFPA 105, Standard for Smoke Door Assemblies and Other Opening Protectives, require any nonfunctioning damper to be repaired as soon as possible. Nonfunctioning actuators need to be replaced per the damper manufacturer’s guidelines. If a direct replacement for a nonfunctioning actuator is not available, contact the damper manufacturer or the actuator manufacturer.

Can a fire, smoke, or combination fire/smoke damper be shipped without an actuator? UL 555, Fire Dampers, and UL 555S require smoke and combination fire/smoke dampers to be shipped with factory-supplied actuators to ensure correct installation and proper operation. If a fire damper has a heat-responsive device requiring an actuator, it, too, must be shipped with an actuator factory-installed.

How far can a fire damper protrude from a wall or floor? A standard 1.5-hour or 3-hour fire or combination fire/smoke damper must be installed with the centerline of the blades in the closed position within the plane of a wall or floor. Some manufacturers, however, have approved dampers that may be installed with the blades in the closed position outside the plane of a wall or floor. The maximum distance varies by manufacturer. Dampers for out-of-wall/floor applications have to be ordered from the factory and may require special accessories for installation.

How much larger does the opening in a wall or floor for a fire or combination fire/smoke damper need to be? The standard opening is larger by 1/8 in. (3.2 mm) per linear foot of the height and width of a damper. Damper manufacturers have optional instructions allowing larger openings.

Can mounting angles be field-supplied? Mounting angles (Photo E) can be field- or factory-supplied. Consult the damper manufacturer’s installation instructions for details.

What is the minimum size and thickness of mounting angles? Minimum size and thickness can vary by manufacturer. Typically, the minimum size is 1.5 in. by 1.5 in. (38 mm by 38 mm). For dampers up to 36 in. (914 mm) wide and 24 in. (610 mm) high, the minimum thickness is 16 gauge. For larger dampers, the minimum thickness is 14 gauge.

When do you use one-sided mounting angles in lieu of two-sided mounting angles? You use one-sided mounting angles when one side of a rated wall is difficult to access or inaccessible.

Is a one-sided mounting angle better than a two-sided mounting angle? Both one-sided and two-sided mounting angles are tested and approved by UL. One-sided mounting angles can be more economical to purchase and install, but typically have more restrictions in terms of size and hourly ratings, the limitations varying by manufacturer.

One-sided mounting angles are attached to a wall/floor as well as to a damper, while two-sided mounting angles are attached to only a damper.

How flush against a wall or floor should mounting angles be? Mounting angles should be flush against a wall or floor. If an opening does not allow for flush mounting, consult the AHJ. Gaps of up to 1/8 in. (3.2 mm) generally are acceptable.

What do I do if I have an old brick wall and mounting angles do not fit flush? Sealants or firestop material may be used to seal the top of a mounting angle to a wall. See the manufacturer’s installation instructions for approved sealants or firestop material.

Do I have to seal/caulk mounting angles? The sealing/caulking of mounting angles to a wall or floor is required neither by national codes nor for fire-damper approval. See the damper manufacturer’s installation instructions for approved sealants or firestop material, should local codes require sealing.

How much space between a mounting angle and a damper is allowed? A mounting angle should sit flush against a damper sleeve/frame. Normally, a gap of no more than 1/8 in. (3.2 mm) is acceptable.

Standards and Listings

How do I know if my damper’s ratings are current and correct per the submittal?

Per UL requirements, the damper should bear a label indicating the maximum static pressure and airflow the damper is approved to open and close under. Additionally, you can search UL’s UL Product iQ website. The minimum airflow and static pressure for a UL 555S listing is 2,000 fpm (10.2 m/s) and 4.0 in. wg (1 kPa) at 250°F (121°C).

Inspection and Testing

How do you test auxiliary switches on fire dampers (actuator-driven only), smoke dampers, combination fire/smoke dampers, and actuators? Make sure the switch package is not connected to anything. With the damper closed, check for continuity across the switches to determine the closed wiring. Next, power the damper open and check for continuity across the other set of wires. The most common containment-damper wiring is shown in Figure 2.

How do you remotely test actuator-controlled fire, smoke, and combination fire/smoke dampers? Manufacturers provide a number of different configurations of panels, with options such as keyed switches, momentary switches, and selector switches. All use the same concept shown in Figure 3.

When a damper is open, a green light or light-emitting diode (LED) on the test panel will be on. Activating the test switch will cut power to the actuator and, thus, close the damper. Once the damper is closed, the green light or LED should be off, and a red light or LED should be on. Deactivating the test switch will restore the power, and the green light or LED should be on again. The test then is finished. Following installation, damper-blade position should be visually inspected to confirm the wiring and switches are correct and operational.

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