Figure 1. A curtain-type fire damper.
Fire, smoke, and combination fire-smoke dampers are a vital part of any new or existing construction design. In a fire emergency, these UL-classified dampers help contain fire and the resulting smoke to the compartment of origin, thus maximizing life safety and minimizing property loss while helping the firefighters extinguish the blaze. The more stringent UL test methods approved in 2002 required manufacturers to retest all dampers used in duct systems where airflow is present. Here, we'll look at what makes effective selection and installation when using any of these three damper types.

Figure 2. Examples of multiple-blade fire dampers.

Fire Dampers

Fire dampers are installed in a wall or floor at the point of duct penetration, to stop fire from migrating through HVAC openings and are designed and tested to maintain the integrity of the fire-rated separation. Fire dampers are equipped with a heat-sensitive device called a fusible link. A fusible link holds the damper blades open until it reaches its melting temperature of typically 165°F or 212°. When the fusible link melts, the blades release to close and stop the flame from moving into an adjoining compartment.

Fire Damper Selection

Here are several important factors to consider when applying fire dampers.

Hourly fire resistance rating. The hourly rating comes from the UL555 Fire Endurance Test and indicates how long a damper will block a fire. There are two ratings for fire dampers in the United States: 11/2-hr and 3-hr. Any fire-resistant partition with less than a 3-hr rating requires a 11/2-hr rated fire damper and any fire-resistant partition with a 3-hr or more rating requires a 3-hr rated fire damper. These requirements are based on recommendations made by the National Fire Protection Association (NFPA).

Dynamic rating for airflow and pressure. Fire dampers have two distinct rating classifications: dynamic and static. Static fire dampers have not been tested for closure under airflow, and therefore, they can only be applied in HVAC systems that are designed to shut down in the event of a fire. Dynamic fire dampers, on the other hand, have been tested for closure under airflow and differential pressure. Based on the new 2002 UL test methods, the minimum rating for all dynamic fire dampers is 2,000 fpm velocity and a pressure rating of 4 in. w.g. Higher ratings are established in increments of 1,000 fpm and in 2 in. w.g. for elevated HVAC duct requirements.

Damper construction. Fire dampers are made with two designs, the curtain-blade type and the multiple-blade type. Curtain-type dampers (Figure 1) are the most common and consist of a row of blades stacked together and held up by a fusible link. Multiple-blade dampers (Figure 2) are also held open by a fusible link, but differ from curtain type in that blades are connected by linkage and are located fully in the airstream. While multiple-blade fire dampers generally offer greater restriction to airflow than a curtain-type fire damper for the same size duct, they can be applied in ducts where the system air velocities exceed the curtain-type fire damper closure ratings. Dynamic-rated, multiple-blade fire dampers have been UL tested for closure at 4,000 fpm and 8 in. w.g. An added benefit to the multiple blade design is that these fire dampers can be significantly easier to "reopen" or "reset" when job requirements call for the installer to cycle test (open to close to open again) all the fire dampers before the installation is accepted by the authorities.

Building codes. Building codes establish the minimum requirements for fire dampers and fire resistive construction. Generally speaking, fire dampers are required where ducts penetrate walls and floors constructed to resist fire. Requirements are based upon occupancy classification, application, and hourly rating of the wall or floor.

Figure 3. A typical smoke fire damper installation.

Fire Damper Installation

The installation of UL-classified dampers is an important aspect to consider when ensuring the performance of the damper in the wall or floor. Building codes require the installation of all UL-classified dampers to conform to the manufacturer's UL-approved installation instructions. Local or governmental authorities having jurisdiction (AHJ) may permit or approve variations to these installations, but variations are generally not recommended because of possible liability in the event of a failure.

Here are some important installation factors to consider.

Expansion gap or installation clearance. The wall or floor must be a minimum of 1/4 in. larger than the width and height of the damper sleeve assembly. The gap must increase 1/8 in. for each foot the damper exceeds 2 ft in either width or height. For example, a 24- by 24-in. damper requires a minimum opening size of 241/4- by 241/4-in., while a 36- by 36-in. damper requires a minimum opening size of 363/8- by 363/8-in. Maximum opening sizes are different for each manufacturer but are typically 1 to 2 in. greater than the minimum opening sizes.

Sleeves. Most installations require a sleeve. Specific sleeve requirements depend on the damper size and type of duct to sleeve connection. Generally, the sleeve gauge must be equal to or greater than the duct connected to it. Also, the depth of the wall or floor determines the sleeve length. The sleeve should not, generally, extend more than 6 in. beyond the wall or floor, but in some instances the sleeve can extend up to 16 in. beyond the fire-rated barrier. Most sleeves have a breakaway connection where they connect to the ductwork, allowing the duct to separate from the sleeve if a fire causes some parts of the building to fall. This separation means the damper will remain in the penetration and continue to block the passage of flame. If a breakaway connection is not one listed in the manufacturer's installation instructions, then the sleeve gauge must be minimum 16-gauge on ducts up to 36 in. wide by 24 in. high, and 14-gauge on larger ducts.

Damper-to-sleeve connections. These can be done in the factory or in the field, and the installation instructions must show the method of attaching the damper to the sleeve and the spacing of attachments. Factory sleeves are generally preferred due to the attachment and spacing requirements.

Securing the damper/sleeve assembly to wall or floor. Fire dampers are secured to the wall or floor by the use of retaining devices such as angles or straps. Retaining angles, the most common device, are attached to the sleeve on each side, top, and bottom on both sides of the wall or floor. The angles must overlap the partition a minimum of 1 in. Some manufacturers have tested dampers utilizing retaining angles on only one side of the wall. These "one side" angles can be used in most applications and may offer designers and installers with significant labor-saving opportunities. Other retaining methods also are offered by manufacturers to minimize labor for installations.

Maximum damper size. Maximum size limitations are based solely on what a manufacturer wants to test to meet UL555 requirements. UL requires manufacturers to list the damper maximum sizes on the installation instructions. Occasionally, the opening in the wall or floor is larger than the largest UL-listed size of damper. In those cases, the opening should be subdivided to permit the installation of multiple dampers, or the AHJ should be consulted for a solution. In some cases, damper manufacturers can provide steel mullions that can be used to subdivide larger openings in concrete block or poured walls. These mullions, however, can only be used with static dampers.

Smoke Dampers

Smoke dampers differ from fire dampers in that they include factory installed electric or pneumatic actuators and are activated by smoke detectors, heat detectors, or fire alarms. Smoke dampers are qualified under UL Standard 555S, Smoke Dampers, and they are designed to resist the passage of air and smoke. Smoke dampers have two general applications:

  • Part of a passive smoke control system in which they close upon detection of smoke and prevent the circulation of air and smoke through a duct or ventilation opening; or
  • Part of an engineered smoke control system designed to control smoke migration using walls and floors as barriers to create pressure differences. Pressurizing the areas surrounding the fire prevents the spread of smoke into other areas.


Figure 4. An example of a combination fire smoke damper installation.

Smoke Damper Selection

UL Standard 555S identifies three leakage classes as shown in Table 1.

UL classified leakage ratings. HVAC system designers are advised to select the lowest leakage class damper to minimize smoke migration. Low-leak dampers may also be desirable for energy conservation or where the entrance of freezing outdoor air must be reduced. Smoke dampers are sometimes used in gas suppression systems to limit the migration of fire suppressant gas through the air system from the point of discharge. The International Building Code requires smoke damper leakage ratings to be no less than Class II.

Elevated temperature degradation rating. The temperature degradation test requires the damper and its factory installed actuator to be exposed to an elevated temperature for 30 minutes and then immediately cycled for operational acceptance. UL classifies smoke dampers by the maximum temperature they can withstand and still operate in their intended manner. The rating is a minimum 250° and can be increased in increments of 100°. An application for smoke dampers rated at 350° would be a dynamic smoke control system where the damper would be required to operate during a fire emergency. The International Building Code requires smoke dampers to carry at least a 250° rating.

Operational rating. Similar to fire dampers, smoke dampers are also rated for maximum velocity and pressure where minimum ratings are 2,000 fpm and 4 in. w.g. with the same incremental values as fire dampers.

Damper construction. Smoke dampers are available only as multiple blade dampers. These blades, however, are made of airfoil or triple V-groove shape. An airfoil blade is shaped like an airplane wing and provides excellent strength and control. Triple V-groove blades are flat-steel reinforced with V-shaped grooves for strength and are ideally suited for HVAC systems of 2,000 fpm and less. In HVAC systems exceeding 2,000 fpm duct velocity, airfoil blade dampers are better able to withstand the higher velocities and pressures due to the strength characteristics of the airfoil construction while providing better sound characteristics as well.

Actuator type. Actuators may be electric or pneumatic, and each actuator is rated specifically for a manufacturer's damper. Field installation of actuators on smoke and combination fire smoke dampers is no longer allowed by UL. Most actuators are for two-position operation (open and closed), but some manufacturers have successfully tested modulating actuators with their smoke dampers, which allow them to be utilized as control dampers as well. Actuators can be factory installed inside the airstream or outside the airstream, or both if the damper is of certain sizes. Actuators mounted inside the airstream will increase the pressure drop through the damper.

Maximum damper size. Prior to July 1, 2002, when the UL standards changed, smoke dampers were available in unlimited sizes. They now are available only as large as their maximum classified size as specified on the manufacturer's installation sheets.

Smoke Damper Installation

Location. Smoke dampers are for use in or adjacent to smoke barriers. They must be installed no more than 24 in. from the smoke barrier they are intended to protect (Figure 3). Openings or branch ducts are not allowed in the duct between the smoke barrier and the smoke damper.

Sleeves and attachment. Smoke dampers do not necessarily have to be installed in sleeves. They can be installed directly in the duct. The manufacturer's installation instructions will include the approved method for attachment and spacing of the attachment.

Sealing. The joints between the damper frame and the duct must be sealed to prevent unwanted air leakage. Smoke damper leakage ratings are based on leakage through the inside of the damper and not on additional leakage between the damper frame and duct or sleeve.

Combination Fire/Smoke Dampers

Combination fire smoke dampers (FSDs) meet the requirements of both the UL555 and UL555S smoke damper standards. They are used in HVAC penetrations where a wall or floor is required to have both a fire damper and smoke damper. They close upon the detection of heat or smoke (via a smoke detector) and seal the opening. Unlike regular fire dampers, however, FSDs are more commonly available with electric heat release devices instead of fusible links. The exception to this is pneumatically actuated FSDs, since they typically will have a fusible link mechanism of some design. With electric actuators, the electric heat release device is not directly connected to the blade, and this allows the damper to close in a controlled manner rather than instantaneously slamming closed and causing pressure problems with the HVAC system. System designers should insist upon these "electric fuse links" when selecting a combination fire smoke damper.

FSD Selection

All the important considerations for fire dampers and smoke dampers mentioned earlier in this article also pertain to FSDs. Additionally, FSDs have several unique characteristics.

Space envelope. Space envelope is the area directly surrounding the fire separation opening. FSDs are installed in the wall or floor like fire dampers, but because of the actuator and heat release devices, they need more "space" on the outside of the opening (Figure 4). In an effort to maximize the FSD installation options, some manufacturers have tested FSDs made with the blades running vertically. This allows the actuators and heat release devices to be located on the bottom or top of the sleeve rather than on the side.

Warranty. The changes to the UL test standards and conscientious product development have allowed some manufacturers to increase their manufacturing warranty up to five years. FSDs are a vital part of a buildings fire protection system and should be constructed in the best possible manner utilizing the most up to date methods. System designers should insist upon the use of only FSDs that carry a significant warranty.

Green buildings. More system designers are becoming interested and involved in building practices that promote conservation. FSDs designed with airfoil blades have less pressure drop than other non-airfoil blade types. Less pressure drop in a system could mean significant energy savings. In addition, the Air Movement and Control Association (AMCA) now certifies some manufacturers' FSDs for pressure drop performance. System designers should select only those FSDs with certified performance ratings through a third party like AMCA.

Table 1. Leakage ratings.

FSD Installation And Maintenance

FSDs are installed in the same way as a fire damper and are sealed in to the sleeve like smoke dampers. A bigger issue involving FSDs is maintenance.

Maintenance of these dampers includes periodic checks of the damper, actuator, and accessories for proper operation. The maintenance interval will vary depending on the duration of system operation, condition of fresh air, and quantity of dust in return air, among other factors. NFPA makes recommendations regarding the maintenance of fire dampers. Cycle-test all smoke and combination fire smoke dampers every six months, because of the actuator. Perform this cycling on an automatic schedule to validate the proper operation of the damper in the unlikely, yet real, event that it may be needed in an emergency.

In Conclusion

The new UL test standards for fire-life safety dampers have resulted in better damper designs. The application and installation of fire, smoke and combination fire smoke dampers does not have to be a cause for trepidation. On the contrary, by using these guidelines, the system designer can make confident decisions that ultimately provide the building owner with better choices for life safety. ES