supply or return duct smoke detector

[yvette]

I have a unit that is greater than 2000 cfm. The IMC says a detector is required only on the return side. There has to be more to this, right? I mean, that makes no sense to me. But, i am far from an expert. it just seems that a smoke detector would make more sense on the supply side. help!

 

[klarenbeek]

That's what the requirement is, installed upstream of any fresh air connections or filters. I think the idea to have it in the return instead of the supply is to detect the smoke before it can get diluted with any outside air. If the detector was in the supply, the smoke will potentially be diluted with fresh air coming in and may not shut down the unit as quick. In some cases depending on the time of year it can be diluted by 50% or more.
Most people get the wrong impression that the detector is to shut down the unit in case there is a fire in the unit, but it's not. It is to stop smoke from a fire in room A being blown into room B.

If you are the designer and think one is needed in the supply also, you can do that. You are going above and beyond the code minimum at that point. I see that done quite regularly on larger air handlers.

 

[mtlogcabin]

it just seems that a smoke detector would make more sense on the supply side.

Why do you think the supply side makes more sense?

 

[mtlogcabin]

Dang you beat me to it.

NFPA 90A requires the first duct smoke detector to be installed on the supply side of air handler units of more than 2,000 cubic feet per minute (cfm) and requires one on the return side of units of more than 15,000 cfm and serving more than one story.

The code when specific over rides the standard and unless you have adopted NFPA 101 NFPA 90A is not applicable since it is not referenced in the I-Codes.

 

[bill1952]

I've always looked at it simply a means to stop smoke from a fire in one room from being spread to many other rooms on the same supply.

 

[Gregg Harris]

The other factor that is involved is the return air velocity is slower than the supply and can be sensed by the detector more easily.

 

[Fred_F]

While having one on the supply side might make more sense to you, there could be reasons for this requirement related to airflow patterns and detection efficiency.