Duct smoke detectors, when installed in HVAC systems, automatically stop their respective fans upon the detection of smoke. For HVAC systems with airflows exceeding 2,000 cfm, duct smoke detectors are required. This includes where multiple air handling systems utilize common ducts with a combined design capacity greater than 2,000 cfm.
But when duct smoke detectors are required, where should they be located? Lets look at the requirements in two of the prominent codes and standards.
2012 International Mechanical Code (IMC)
The 2012 IMC requires duct smoke detectors be located in the return air duct or plenum upstream of any filters, exhaust air connections, outdoor air connections, or decontamination equipment and appliances. Return system smoke detectors are not required where the entire space served by the air distribution system is protected by a system of area smoke detectors connected to a fire alarm system in accordance with the International Fire Code.
NFPA 90A, Standard for the Installation of Air-Conditioning and Ventilating Systems, 2012
NFPA 90A requires duct smoke detectors to be located in the supply duct downstream of air filters and ahead of any branch connections. For air distribution systems serving more than one story and having a capacity greater than 15,000 cfm, smoke detectors are also required in the return system at each story prior to any recirculation or fresh air inlet connection. Return system smoke detectors are not required where the entire space served by the air distribution system is protected by a system of area smoke detectors. Smoke detectors are not required in exhaust ducts.
The conflicting requirements are obvious. The IMC requires smoke detectors in the return ducts. NFPA 90A requires smoke detectors in the supply ducts. Given this, how should a design engineer proceed?
First, determine what are the governing code(s) for your particular project. If the IMC is the adopted code, then require smoke detectors in the return ducts. That may be the only code requirement. NFPA 90A is not referenced in the IMC. But you must also check the requirements of the fire officials in the jurisdiction of the project. If they have adopted NFPA 101 or other NFPA standards that reference NFPA 90A, compliance with NFPA 90A requirements may be mandated by the fire officials. If so, locate smoke detectors in the supply and return ducts.
I understand this doesn’t make much sense from an engineering standpoint, but if the building inspector requires smoke detectors in the return ducts and the fire inspector requires the smoke detectors in the supply ducts and neither will accept other locations, there is no other way to solve the dilemma.
Hopefully, at some future time these codes and standards can find a location for the smoke detectors they can both agree on.