Duct Detectors Shut Only Associated Damper or All Dampers? How Should they be Controlled?

7/8/2020

Should return air duct smoke detectors associated with a fan system close only the associated damper and shutdown the fan or close all dampers on the system (including supply return spill and outside air)?

How should outside air and spill air fire smoke dampers be controlled? Should these to be controlled by fire alarm?

Thanks in advance.

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7 Comments

Jonathan Sullivan

7/8/2020 10:22:54 am

It depends. I think we need more information to help you. Is this equipment part of an engineered smoke control system, what type of fire alarm system do you have, is there a fire command center with damper control, etc.

You also want to review IMC assuming that’s your local code, the duct systems chapter addresses the basics of where, when, and how.

Manny Rios

7/8/2020 10:25:35 am

In our jurisdiction and in compliance with applicable code requirements, (Mechanical and NFPA72) the hvac systems are designed with all devices connected to the fire alarm.

During an activation of the fire alarm system by the use of a pull station or a fire/smoke detector, duct detector, all fire/smoke dampers close as designed.

Scott McCormick

7/8/2020 10:50:09 am

Duct detectors are the MOST misused fire alarm device. I've seen more duct detectors installed that weren't required in the first place. People don't understand what the purpose of the detector. Perfect example, Lowes, Home Depot, Walmart, BJ's etc. large RTU with minimal duct work. Bam duct detector. Why? Where is the smoke going? I've had more Engineers and contractors say, "it's more than 2000 CFM." my answer is, So! Again I ask, Where is the smoke going to go? I have done a lot of research, and I understand the purpose. Read the documents about the MGM fire, La Vegas 11/21/1980 most of the 85 dead, died of smoke inhalation. When connected to a fire alarm system, these should be supervisory only.

Bruce Verhei

7/10/2020 08:01:57 pm

The essentially no duct HVAC units at ceiling-roof level have been at top of Tomes Van Rickley & Assoc., then TVA, then Telgian’s Pre-app meeting building department checklist for twenty-five years. I believe by now they’ve managed to have the requirement explicitly not required any more in the IMC.

For the record, I would eliminate the duct detector requirement in many single sprinklered buildings that only shut down the ventilation system. I’m not convinced it improves life safety. If you want to shut down the HVAC system the sprinkler heads and water flow switch are adequate.

Best.

Bruce Verhei

Chris D

7/8/2020 11:46:21 am

Duct detectors typically shut down the entire HVAC system they are associated to but not all systems in the building.

In healthcare, I have a fire alarm relay close the associated damper immediately signal via an AHU interface to BAS or AHU and include a sequence to close all dampers upon shutdown (avoids air pressure ductwork damage and problems with pressurization).

For commercial, an HVAC unit shutdown via fire alarm interface to the associated AHU and closing all dampers as part of that interface can be the least messy, I use a hardware interface to the AHU rather an a BAS integration that includes software (unless smoke control listed equipment / programming is used.) In many cases the dampers can be shutdown by shutting power to a circuit that powers all dampers on the system other than individual relays, these should be fail-secure to avoid having to monitor power, a delay may be programmed via the fire alarm panel that allows a signal to the AHU to shutdown and then kills powers to the damper circuit. Individual relays per damper are possible, but expensive with little benefit.

Outside air typically wouldn't have a fire smoke damper, when I was a contractor I would assume anywhere a designer of record included a fire/smoke damper both detection and control is required; it may not hurt to RFI and ask them to confirm as I can't think of a code requirement to shutdown outside air.

Glenn Berger

7/8/2020 12:21:13 pm

There is no one right answer. It all depends on the geometry of the facility being protected and the arrangement of the HVAC System.

MHK

7/21/2020 02:46:09 am

I have looked at 6.4.3.1 of NFPA 90A (2018 ed.) and found that duct smoke detectors are only to stop their respective fan(s). Am I geeting anything wrong here?

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