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Do 100% Outside Air Units require Duct Detectors?

Do 100% Outside Air Units require Duct Detectors?

Do 100% Outside Air Units require Duct Detectors?

Are duct (smoke) detectors  or building fire alarm interlock (for disabling the unit)required on 100% make-up air systems with 2000 or more CFM?

I have a commercial kitchen in our nursing home that has a 100% 3600 CFM make-up air unit as relief for our grease hood. Our suppression alarm company responsible for the hood ansul system says that the NFPA 90 only requires that the hood exhaust fan be interlocked with an over-ride such that when the ansul system is tripped the exhaust fan is kept on to remove smoke and heat, and that no interlock or lock-out from the ansul control to the make-up air unit is required.

Please advise.


RE: Do 100% Outside Air Units require Duct Detectors?

GeorgeGitt: The following is courtesy of the current rendition of NFPA-90A:


6.4* Smoke Detection for Automatic Control.

6.4.1 Testing. All automatic shutdown devices shall be tested at least annually.

6.4.2* Location. Smoke detectors listed for use in air distribution systems shall be located as follows:

(1)      Downstream of the air filters and ahead of any branch connections in air supply systems having a capacity greater than 944 L/sec (2000 ft3/min)

(2)      At each story prior to the connection to a common return and prior to any recirculation or fresh air inlet connection in air return systems having a capacity greater than 7080 L/sec (15,000 ft3/min) and serving more than one story Return system smoke detectors shall not be required where the entire space served by the air distribution system is protected by a system of area smoke detectors. Smoke detectors shall not be required for fan units whose sole function is to remove air from the inside of the building to the outside of the building.

6.4.3* Function. Smoke detectors provided as required by  6.4.2 shall automatically stop their respective fan(s) on detecting the presence of smoke. Where the return air fan is functioning as part of an engineered smoke-control system and a different mode is required, the smoke detectors shall not be required to automatically stop their respective fans.


Reading, I would interpret that the duct smoke is not required in your application.

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