Duct-Detectors Req'd for RTUs Under 2,000 CFM?

5/17/2023

I have a Business occupancy building that is being remodeled and new RTUs units are being installed.

There was a mechanical inspection done recently and the inspector is asking for duct smoke detectors on the supply-side of units under 2,000 CFM.

The units supply air to common areas to a single floor and don’t physically share ducts or plenums.

There is no smoke detector coverage since it’s a sprinklered building. We do have smoke detectors above fire alarm equipment are required.

I tried searching for an answer in IBC, IFC, and NFPA standards, but couldn’t find a definite answer.

Thanks in advance.

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8 Comments

Dan Wilder

5/17/2023 07:58:35 am

Not a FA guy however this chart comes to mind...

https://www.meyerfire.com/uploads/1/6/0/7/16072416/duct_detector_requirements_-_nfpa_90a_v1.pdf

Dan Wilder

5/17/2023 08:01:17 am

Check the comments for some additional requirements from the original post as well, maybe there is some confusion on what is being asked...

https://www.meyerfire.com/blog/does-my-unit-require-a-duct-smoke-detector

Ira Matthews

5/17/2023 07:59:49 am

The requirements for duct smoke detection are found in section 606 of the 2018 International Mechanical Code. 606.2.1 has an exception that does not require duct detection in units that equal to or less than 2,000 CFM.

Jesse

5/17/2023 08:07:36 am

Did he provide a code reference?

Scott McCormick

5/17/2023 08:16:12 am

Duct detectors are the most common device installed wrong 98% of the time. I would ask the Enforcement Agency for a Code Section.

I’ve had engineers argue with me about a Big Box Store whose RTU was more than 2,000 CFM and I stated that the duct detectors are not required. There was no duct work and was totally impossible to spread smoke beyond the 4 walls. The answer was “what if the next inspector required it?” It has nothing to do with feeding oxygen to a fire.

SCHULMAN

5/17/2023 08:40:14 am

I would contact the design team (Mechanical Engineer and Architect) and get a read on the Life Safety approach to the design - they may be using an obscure exception in the code that could trigger the requirement. I've not gotten anywhere asking Inspectors to provide me with code references ...

Fire Inspector AHJ

5/17/2023 09:46:48 am

They are requiring it because it is a shared space with over 2,000 CFM. The smaller units combined, feeding one space, would trigger a global shut down of all of the units at once.

2012 IMC (sorry I only have an old Mechanical code for reference) Section 606.2.1- Return air systems. Smoke detectors shall be installed in return air systems with a design capacity greater than 2,000 cfm (0.9 m3/s), in the return air duct or plenum upstream of any filters, exhaust air connections, outdoor air connections, or decontamination equipment and appliances.

The argument is that it says "return air systems". The units combined are one system for that space. Just like smoke detectors are part of one fire alarm system, or individual sprinklers are part of one fire sprinkler system. Combined, they are required for that space to work together to provide enough CFM for that area. So, (x4) 600 CFM units serving one room move the same amount of air as one large unit of 2400 CFM in the same room. The same consequences result and therefore a global shutdown of all 4 units is required.

California is a bit clearer on this subject in their amendments, but the rule still applies industry wide in my opinion. I hope this helps.

Douglas Krantz link

5/17/2023 03:33:42 pm

From the NFPA 72 Code 2016 Edition, under the heading "3.2 NFPA Official Definitions":

"3.2.2* Authority Having Jurisdiction (AHJ).

An organization, office, or individual responsible for enforcing the requirements of a code or standard, or for approving equipment, materials, an installation, or a procedure."

Official Appendix in the NFPA 72 Code:

"A.3.2.2 Authority Having Jurisdiction (AHJ).

The phrase "authority having jurisdiction," or its acronym AHJ, is used in NFPA documents in a broad manner, since jurisdictions and approval agencies vary, as do their responsibilities. Where public safety is primary, the authority having jurisdiction may be a federal, state, local, or other regional department or individual such as a fire chief; fire marshal; chief of a fire prevention bureau, labor department, or health department; building official; electrical inspector; or others having statutory authority. For insurance purposes, an insurance inspection department, rating bureau, or other insurance company representative may be the authority having jurisdiction. In many circumstances, the property owner or his or her designated agent assumes the role of the authority having jurisdiction; at government installations, the commanding officer or departmental official may be the authority having jurisdiction."

The published commentary of the Handbook (found in the NFPA.ORG website, right next to the NFPA 72 Codebook) from the writers and editors of the NFPA Code, goes this way:

"Any given physical property may have multiple authorities having jurisdiction, who may be concerned with life safety, property protection, mission continuity, heritage preservation, and environmental protection. Some authorities having jurisdiction may impose additional requirements beyond those of the Code. If requirements for the installation of a specific fire alarm system conflict, the installer must follow the most stringent requirements."

In essence, the Inspector, having a more stringent requirement for duct detectors than listed in the code, has the last say in this.

Of course, pointing out exactly what the NFPA 72 Code says may help change the mind of the AHJ, and the AHJ may then decide to go with the NFPA 72 Code. However, in the final analysis and according to the NFPA, if the AHJ has a better idea, the AHJ can overrule the NFPA Code.

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Duct-Detectors Req'd for RTUs Under 2,000 CFM?

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