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Fire & Smoke Actuated Damper Control

The #1 question that both mechanical engineers and contractors ask

about fire and smoke dampers is, “How do the controls work with the actuators?”

Here are the essentials.

Damper types We must clearly distinguish between three types of dampers.

1. Fire dampers are rarely actuated. Their purpose is to close upon a rise

in temperature and stay shut to stop fire from passing through a barrier. About

90% are curtain type dampers that close when a mechanical fusible link melts

and releases them. The damper must be in the plane of the fire wall although

some jurisdictions make exceptions if the ducts are heavy duty.

Some large fire dampers are actuated because the UL5551 requirements

for dynamic2 closure rating cannot be met by curtain dampers so the

manufacturers provide multi-bladed dampers with actuators. In a few cases,

auxiliary contacts are needed to prove opening and closing in periodic testing.

Dampers with blade switches or actuators with internal auxiliary switches provide

proof of closure.

Fire dampers normally close when the temperature of the fusible link

reaches 165ºF (75ºC) although some local variations exist. For example, if

steam coils are present in ducts, use of a 210F (100C) high limit may be used.

The fire damper temperature may then rise a couple thousand degrees

but the damper will hold for the time rating for which it is designed.

1 UL555 is the test standard for fire dampers. UL555S is the standard for smoke dampers. Combination fire and smoke dampers must meet both. 2 Under the present UL555 dampers designed to close while fans operate must maintain their fire resistance under dynamic airflow pressure. Static rated dampers are used when fans shut down in event of a fire.

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Several million fire dampers are installed each year in barriers that are

designed to slow the spread of fire. See Figure 1.3

Figure 1 Curtain Fire Dampers

Fire dampers are rarely actuated inthe US.

2. Smoke dampers are actuated. They must open and close when

required to provide fresh air or to stop smoke passage. They do not have to

have high limit sensors or fusible links to close automatically. Smoke dampers

often provide signaling for indicator lights as discussed below.

Practices vary by geographical region and by type of smoke control

mandated by codes. Most commonly, a smoke detector (or two) inside the ducts

will shut down the fan(s) and close the damper(s) if smoke is detected. Area

smoke detectors are sometimes wired to a central fire alarm panel and a panel

contact or a remote relay initiates closing.

Smoke dampers need not be within the wall providing the smoke barrier,

merely within 24 inches. As a result, the actuators may be axle shaft mounted.

However most are jackshaft mounted with linkage to the damper blades. See

Figure 2.

3. Combination fire and smoke dampers are more common than simple

smoke dampers. Since these are fire rated, the damper blades must be in the

barrier wall. The actuator cannot be in the wall so a jackshaft and linkage are

3 Photographs courtesy of Pottorff.

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employed with the actuator connected to the jackshaft extension outside the

damper sleeve. 4 Almost all actuators today are direct coupled.

Figure 2 Smoke Damper

Smoke and combination dampers are actuated

Courtesy Pottorff-PCI

Basic Control

There are several accepted methods for temperature sensing and actuator

control.

A fusible link that restrains a shaft spring can perform the fire damper

function while an actuator provides the smoke control function. The fusible link

disconnects the damper blades from the actuator and ensures its closure. The

actuator can open and close for smoke management only until the fusible link

melts.

Alternately, an electric bimetal sensor with a reset button can be used for

the single primary sensor. See Figure 3.

Combination fire and smoke dampers in more sophisticated smoke control

systems will have two sensors – primary and secondary. The primary can be

overridden by firefighters’ control; the secondary is manual reset only.

When a dual sensor is used, the first sensor is always electric. It opens,

removes power, and lets the actuator spring the damper closed. The second

4 There are other methods; however this is standard in the US today.

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sensor may be a fusible link or a higher temperature electric sensor. Typically,

the temperatures are 165ºF (74ºC) for the primary and 250ºF (121ºC) for the

secondary. In about 20% of cases, 350ºF (171ºC) is used for the secondary

sensor.

The system in Figure 3 is found in about 80% of the combination fire and

smoke dampers. A smoke detector with a local thermal sensor is used as

shown. This is a containment damper application.

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Reopenable Dampers

Less common is the engineered smoke control system with reopenable

dampers. These have provisions for the fire fighter to control the dampers more

precisely.

See Figure 4 for the wiring for a dual sensor combination fire and smoke

damper. If a fire were to break out and the duct temperature increase to 165ºF,

the damper springs closed to keep fire from spreading. But if the fire fighters

choose, they can reopen the damper for smoke control purposes. Then if the

temperature continues to rise as the fire gets closer or bigger and it reaches the

250ºF (sometimes 350°F) limit, the power is again removed from the actuator

and it springs closed without being able to be opened again without pressing a

reset button. The fire fighters can also close an open damper to prevent oxygen

from feeding the fire or air pressure from pushing smoke into other areas.

Figure 4 Dual Sensor Reopenable Damper Wiring

N

ACTUATOR

165F 250F

HOA

FIRE FIGHTERS CONTROL PANEL

HOT

A

O

H

SMOKE DETECTOR OR OTHER ALARM SYSTEM

CONTACTS LOCATED HERE

Sequence of operationAuto: If duct temperature reaches 165°F or external alarm contact opens, power is cut to actuator and damper springs closed.

If the Hand-Off-Auto switch at the Fire Fighters’ Control Station is moved to hand, then the contact and thermal switch are bypassed and the actuator again receives power and the damper opens.

If the duct temperature reaches 250°F, then the actuator springs the damper closed. Manual reset is required.

If the HOA switch is moved to off at any time, the actuator springs closed.

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Original damper designs were released by external springs only and the

actuator did not have an internal spring. Later, a fusible link and dual springs

were standard. They used one fire spring that closed the damper until manually

reset and a spring to close the damper if the actuator lost power. The actuator

could reopen against this spring. Now most manufacturers use electronic

methods of sensing and control. This means the actuator spring is the only

spring in the most recent designs.

Indicator lights

The Fire Fighters’ Smoke Control System5 (FSCS) has status indication

lights in addition to the manual HOA switches. The lights allow the position of the

dampers to be determined and allow verification of damper positions and

override status.

The indicator lights can be initiated from auxiliary switches on the actuator

or damper blade switches. Proximity switches are sometimes used, but the cost

is usually prohibitive. With the spring in the actuator to operate the damper,

actuator switches are often preferred as they are as reliable as blade switch

indication. The linkage between the actuator-jackshaft and the damper is more

robust than the typical coat hanger connection from damper blade to switch

package.

Full engineered smoke control systems exist only in about 10% of

applications. With these the fire fighters have control of dampers and indicator

lights show status.

Figure 5 shows a simple method of wiring to indicator lights. This system

would be typical for a containment damper and the lights could be in a panel or a

local junction box, possibly in the ceiling. A FSCS panel would often have an

amber light indicating fault. If the damper is closed, the red light is on. If the

damper is open, the green light is on. A variety of wiring methods is possible.

5 Also referred to as Fire Fighters' Control Panel and Fire Fighters' Control Station in different sources. NPPA 92 uses Fire Fighters Smoke Control System.

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In some jurisdictions the red light means “FIRE” or “Problem” and the

green means OK. In others, green means on and red means off, particularly for

fans. Although rare, some dampers are normally open when not powered and

the red and green could be confused. This can confuse the fire fighter in the

confusion of a fire. Indication lights should be discussed with the fire department

to ensure correct operation.

Figure 5 Indication Light Wiring for a single damper

Changes over whendamper opens >10 degrees

degrees

Hot 24V or L1

Com24V or N

Green On=

Open

Red On=

Closed

NO Contact

damper opens >85 Changes over when

NC Contact

Contacts can be actuator auxiliary

contacts or damper blade switches.

Proportional Damper Control

Figure 6 shows two proportional applications. The pressure in a stairwell

or in a duct may need to be controlled at a certain setpoint. A smoke damper is

required if the wall is also part of a smoke barrier. Given typical space

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constraints, one damper and actuator are technically and economically superior

to two (or three if the wall is also a fire barrier).

Figure 6 Proportional Applications

Shaft to Above Ceiling or Underfloor

StairwellPressurization

Actuator

Shaft

Proportional

There are two ways to provide proportional damper control:

1. No proportional electric actuator meets the UBC (Uniform Building Code)

criteria for 15 second operation. They may not be installed in UBC

regions (California 6 and a few cities). In these cases, two dampers are

required. A two position fire & smoke damper sits in the rated wall. A

standard proportional damper and actuator are installed in series.

2. In IBC code regions, a proportional fire & smoke damper and actuator can

be installed since the code (based on UL555S) requires 75 second

6 In January of 2008, California will go over to the IBC (International Building Code) which uses the UL555S 75 second operation time. This is the standard in almost all of the US.

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maximum for operation. The Belimo FSAF24-SR meets the 75 second

requirement.

Conclusion

The different damper types are used in different applications. Actuation is

provided for smoke control dampers and combination fire and smoke.

Containment systems are the most common while reopenable dampers provide

the fire department with more options.

Indication lights can be applied in a number of ways.

By following each wiring diagram here and comparing with the sequence

of operation, one can gain an understanding of the operation requirements for

interfaces between the mechanical and control systems.

Codes and practices differ geographically and one should consult local

inspectors, contractors, and reps with specific questions. The methods shown

here are typical and practices may vary.