Duct Detector in Supply (72) or Return (IMC) Duct?

11/25/2024

NFPA 72 (2016) Section 17.7.5.3 says a duct smoke detector is required in the supply air duct; on the other hand, the IMC (2020) Section 606.2.1 says a duct smoke detector is required in the return air duct.

Which should I follow?

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11 Comments

Glenn Berger

11/25/2024 08:26:02 am

All depends on what codes are applicable to the project. I have had many projects where detectors are provided in compliance with both documents.

Nic J

11/25/2024 08:28:43 am

This should come down to what code/standard is applicable for the installation of the HVAC equipment (NFPA 90A vs IMC).

The section you reference in NFPA 72 finishes by stating "..as required by NFPA 90A and 17.7.5.4.2.1." Of which section 17.7.5.4.2.1 just covers duct detection requirements that may be present in all other NFPA standards. So NFPA 72 does not necessarily require the installation of the duct detector but rather tells you how/where to put it, NFPA 90A or the IMC tells you when you need it.

NFPA 90A and the IMC have slightly different requirements and exceptions for duct detectors. Double check what is the enforceable code and follow the requirements from there, it could be both in some instances.

Dave

11/25/2024 03:03:39 pm

My in-house alarm expert agrees with ^^Nick^^

M. Newell

11/25/2024 08:28:57 am

In the 2025 edition of 72 it changed the language to say:
17.7.6.5.4.1

Detectors shall be mounted in accordance with the manufacturer’s published instructions.

I think that would be a good bet to default to the manufacture

Ricardo Gonzales Jr

11/25/2024 10:20:41 am

Code 1st meaning the IMC to be followed. But then also 72. Note when i.e. cfm volume. They are different and apply accordingly. Most times it's 2,000 cfm for the supply side and 15k cfm for the return. And then IF it's a combined return system then on each area the air comes from. If you have a copy of the IMC with comment and NFPA 72 with comments. They make it fairly clear when to add the duct detectors to the building.

Jack G

11/25/2024 10:43:29 am

Since both are connected to some sort of air handler, which in the event of a unit fire could propagate smoke, or in a building fire— distribute smoke I would lobby for both supply and return.
Smoke is a powerful killer.

Brett

11/25/2024 10:46:33 am

What codes are you following? Is it a DoD project?

Unfortunately, NFPA 90A and the IMC have opposing requirements for this. Check with your AHJ, but the IMC should prevail. A reasonable AHJ would understand that there's no sense in providing the detectors in both the supply and return ductwork.

chad

11/25/2024 02:53:31 pm

There are plenty of State and LAHJ's that have adopted and enforce both, there is no conflict, you simply enforce the more stringent parts of each one, ie like Pete said "both"

I can think of many use cases that both supply and return are best practice to minimize risk. Its cheap and easy to do in most cases as well.

Of course most duct smokes are not installed properly, often causing no or false alarms. Although its up to the AHJ, I think whether its a supervisory or general alarm should be based on the occupancy type and/or hazards associated with the use of the space associated with the AHU.

Smith Choi

11/26/2024 09:49:56 am

Usually, Code triumph the standard. IMC took the precedence when in conflict with NFPA.

KFITZ

12/2/2024 02:36:16 pm

Applicability of NFPA 90A (as referenced by NFPA 72) and IMC is dependent on the prevailing building code requiring a fire alarm system.

NFPA 101 refers to NFPA 90A for duct detector requirements while the IBC refers to IMC for duct detector requirements.

Note there are some jurisdictions which adopt both sets of codes, in that case I've seen duct detectors installed on both the supply and return side.

Ivan J. Humberson

12/26/2024 10:48:11 am

You should check with the AHJ. In the jurisdiction from which I recently retired, both NFPA and IBC/IMC need to be met, so we amended the IMC to require duct detection on the return side for systems between 2,000 and 15,000 cfm. Without such an amendment, for jurisdictions requiring compliance with both NFPA and ICC codes, one would have to install duct detection on both supply and return for all systems over 2,000 cfm. We felt that was not necessary, hence the amendment.

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Duct Detector in Supply (72) or Return (IMC) Duct?

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