Importance of Fire & Smoke Dampers in HVAC

Stopping smoke from migrating through HVAC systems is important to save lives and to minimize property damage. Deaths due to smoke inhalation far outnumber deaths due to burns. Interestingly, smoke inhalation deaths have dropped with the advent of improved smoke mitigation methods designed into HVAC systems. Correct selection of fire/smoke dampers with proper installation, along with knowledge of current codes and standards can save lives. Proper installation and periodic performance testing are required to ensure these dampers function as intended in a fire emergency.

Fire and smoke dampers are a crucial element of your passive fire protection system. By working together with the other elements of your passive fire protection system, they restrict the spread of flame and smoke throughout a building in the face of a fire. Your passive fire protection system protects your facility’s egress routes, minimizes property damage and ultimately saves lives. Approximately 80% of all deaths resulting from fires can be attributed to the effects of toxic smoke on the human body.

How Do Fire Dampers Work?

In general, a fire damper works when the heat from the fire causes the normal temperature of a room to rise about 165 degrees Fahrenheit. The fusible link attached to the damper would then melt, causing the damper’s door to close. In rooms, where the normal temperature of the room is consistently higher, a higher degree fusible link would be attached to the damper. These particular fusible links would melt at a temperature about 212 Fahrenheit. There are two types of fire damper designs used to help prevent the spread of flames, a dynamic fire damper and a static fire damper.

Dynamic fire dampers are located in vertical barriers, where the HVAC system fan will stay on in the event of a fire. Because they are installed in vertical barriers, these particular fire dampers have a spring loaded design. The spring loaded design is where the doors of the damper have a spring like action when they are forced to shut. Since the HVAC system fan will remain on, the air pressure from the fan will help the doors of the damper to spring shut.

Static fire dampers are located in barriers where the HVAC system fan will shut off in the event of a fire. These particular fire dampers would be installed in horizontal barriers, and would have a curtain like design. Since the HVAC system fan will turn off, there would be no air pressure and the door of the damper will fall due to gravity. Understanding how a fire damper works can help you have a better understand of how important it is in HVAC system.

Fire Damper

This device is actually used inside air transfer outlets and air ducts of the smoke control or air distribution system. It is specifically designed to automatically close whenever it senses heat. Additionally, it interrupts the migratory airflow, stops flames from spreading by obstructing its flow from the place on fire to the unaffected places. Fire Dampers are operated by a fusible device, which is typically a melting link. They are designed and equipped with a fusible link which holds back the blades in open position till the link melts. When the melting point is reached, the blades then close and prevent the flame from moving from the affected location to the corresponding location. These devices are generally installed near the wall or the floor, right at the point where the duct penetrates the wall for retaining the integrity as well as the fire rating of the walls or the floor.

Smoke Dampers

A fire smoke damper can be defined as a device which is installed in ducts and the opening of a smoke control system or air distribution system. They can be opened as well as closed from a remote fire control and command station if and when needed, to block the passage of smoke and air. The chief functionality of this device is to prevent the smoke from entering the room through heating, ventilation or the ducts of the air conditioning system. Some dampers are operated by an electric or pneumatic actuator. These devices are manned by smoke detectors and fire alarms. They come up with specially designed smoke control systems which are supposed to control the smoke migration using the floors and the walls as obstacles or barriers for creating pressure differences.

Damper Selection

Compartmentation is an integral component of building design and construction. Compartments are formed by subdividing each floor with numerous fire restrictive partitions or walls. A common way for fire to spread from one compartment to another is through the HVAC ductwork; therefore, fire dampers are installed in the plane of the fire wall to protect these openings. Upon detection of heat, the fusible link (usually rated for 165oF or 212oF) melts closing the fire damper blades and blocking the flame from penetrating the partition into the adjoining compartment.

Five basic decisions are required to select a fire, smoke, or a combination fire smoke damper. The process involves defining requirements for:

Blade Style

Blade style decisions are based on application, velocity, and the pressure required for your system. The curtain blade style is appropriate for fire damper applications only and airflow velocities up to 20.3 m/s. The 3V blade style is appropriate for use in airflow velocities up to 10.2 m/s. Airfoil style blades are appropriate for use with airflow velocities up to 20.3 m/s. A round style blade is appropriate for use in airflow velocities up to 15.2 m/s where you need to connect to round ductwork or where minimum pressure drop is desired.

Fire and combination fire smoke dampers installed in walls, floors, or partitions are required by the applicable building code to have a fire resistance rating. Select a fire damper and combination fire smoke damper with a 1½ hour rating for use with a wall, floor, or partition with a less than 3 hour fire resistance rating. If the wall, floor, or partition has a fire resistance rating of 3 hours or more, the damper must have a rating of three hours.

The purpose of a building Smoke Control System is to keep certain areas of a building free from smoke during the early phases of a fire so building occupants can safely evacuate fire fighters can more easily locate and attack the fire. The UL 555S Elevated Temperature Rating is a 30 minute emergency rating to ensure the damper (and its installed actuator) can resist a short exposure to elevated temperature and still operate as required.

Operational ratings are dependent on the damper’s size and installed actuator. Both UL 555 and UL 555S require a combination fire smoke damper and its installed actuator be rated for a maximum airflow rate (velocity in ft/min) through the open damper and a maximum pressure (in. wg) across the closed damper. The installed actuator must operate the damper open and close against these rated velocities and pressures.

Smoke Damper Application

Smoke dampers have two general applications:

1. Part of a “Passive Smoke Control System” where they simply close upon detection of smoke preventing the circulation of air and smoke through a duct or a ventilation opening. 2. Part of an “Engineered Smoke Control System” designed to control smoke migration using walls and floors as barriers and fans to create pressure differences. Pressurizing the areas surrounding the fire prevents the spread of smoke into other areas. Smoke dampers are motorized with either an electric or pneumatic actuator. They are controlled by a smoke or heat detector signal, fire alarm or some other building control system.
• Leakage Rating Class 1 (lowest leakage), 2 or 3 (highest leakage).
• Elevated Temperature Rating 121°C or 177°C.
• Operational Rating.
• Blade Design.

Combination Fire/Smoke Damper Application & Selection

A combination fire/smoke damper functions as both a fire damper and a smoke damper in a single unit. HVAC system designers often combine smoke barriers and fire rated partitions requiring both a fire damper and a smoke damper to be installed in the same location. In this situation, a combination fire/smoke damper is recommended. This type of damper must meet the requirements of both UL Standard 555 as a fire damper and 555S as a smoke damper. The things to consider when selecting and applying a combination fire smoke damper are the same as those for fire dampers and smoke dampers. They are: 1) Hourly Fire Resistance Rating, 2) Leakage Rating, 3) Elevated Temperature Rating, 4) Operational Rating, 5) Blade Design

Ceiling Damper Application

Ceiling fire dampers are also known as “Radiation” dampers and that describes what makes them different from standard fire dampers. They are installed on grilles and diffusers. Should a fire occur they close the openings created by these devices to protect the structure above from excessive heat and subsequent collapse by providing a fire and heat barrier between the fire area and the structural floor. The test standard by which they are evaluated is UL555C. The process of selecting a ceiling fire damper involves one major consideration: • Floor/ceiling or roof/ceiling assembly design Ceiling fire dampers are not assigned hourly ratings themselves. They are listed for use as a component in assemblies tested with air inlet/outlet opening in the membrane. These assemblies have a specific hourly fire resistance rating, and ceiling fire dampers can normally be used in any assembly with a restrained or unrestrained rating of 3 hours or less.

Considerations of blade style, fire resistance rating, and leakage rating lead directly to the selection of an appropriate fire, combination fire smoke or smoke damper based on your HVAC system requirement.