Duct smoke detectors, when installed in HVAC systems, automatically stop the system 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? Let’s look at the requirements in two of the prominent codes and standards.
Code Requirements
2012 International Mechanical Code (IMC)
The 2012 IMC requires duct smoke detectors to 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?
How to Decide
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.
For topics on future blog posts, make a suggestion.
I am having an issue with a single sales floor that has (5) AHUs in an open space, each are 1850cfm and have their own supply ducts. Isn’t there an exception that the detectors are not needed since the units are all supplying the same room?
I believe you are referring to an exception provided in paragraph 606.2 of the IMC (2015 ed.) that says, “Exception: Smoke detectors shall not be required where air distribution systems are incapable of spreading smoke beyond the enclosing walls, floors, and ceilings of the room or space in which the smoke is generated.
If the units are only serving one room and cannot spread smoke to other areas, this exception would apply and smoke detectors should not be required. Check your local codes to confirm this.
What about NFPA 90A: A.6.4.2 The summation of capacities of individual supply-air fans should be made where such fans are connected to a common supply air duct system (i.e., all fans connected to a common air duct supply system should be considered as constituting a single system with respect to the applicability of the Chapter 6 provisions that dependent on system capacity)
Our space has individual units that are not tied together. So would we still be required to add their values together according to the NFPA? We are waiting on the AHJ response, just trying to do our research too. The requirements of DDs span across multiple codes and some of them contradict one another.
My interpretation of A.6.4.2 is that it only applies to common duct systems. In your case, you have individual units and individual duct systems, they are under 2000 cfm, and they are only serving one room. Smoke detectors should not be required, but the code officials are always right.
I agree that the requirements in the building codes and the fire codes contradict each other when it comes to smoke detectors. One code wants them in the supply ducts and the other wants them in the return ducts.
I believe you have the sample tubes incorrectly shown. Two tubes of different length are required to allow the air to flow through the sample enclosure. Air will not circulate into the enclosure with out an in and an out. The different lengths create a difference in pressure and promotes the air circulation.
This may seem like a dumb question but who’s responsibility is it to locate duct detectors the Mechanical engineer or the fire alarm engineer? I would think the mechanical engineer is responsible for where and when the detection required, the fire alarm engineer is responsible for how the detection is provided.
SmokieJoe, I agree with your assessment. HVAC designers have the responsibility to determine when smoke detectors are required and the location. Both of these requirements are covered in the mechanical codes. It is good practice to locate the smoke detectors on your plans.
Typically, the mechanical contractor installs the duct smoke detector and the electrical or fire alarm contractor wires them up, which also aligns with this practice.
Greetings,
Why is 2,000 CFM the specific number requiring a smoke detector? Other than that is the number provided in various codes?
The International Mechanical Code (IMC) Code Commentary states, “Return air systems with design capacities equal to or less than 2,000 cfm are exempt from this requirement because their small size limits their capacity for spreading smoke to parts of the building not already involved with fire. The area that could be served by a 2,000 cfm system (approximately 5 tons of cooling capacity) is comparatively small. Therefore, the distribution of smoke in a system of that size would be minimal.
Even with that explanation, the 2,000 cfm limit still appears to be somewhat arbitrary. I have always assumed the fact that 5-ton units are the transition point between residential and commercial units may be a factor in the selection of that value.
i am thankful I found this. This was very helpful.
I am glad to hear that the post was helpful to you. Hopefully, I can provide more content in the future that will be helpful.
sir, so as per NFPA we can exclude smoke detector in return if the building protected with smoke detector??
Yes, according to NFPA 90A-2012, paragraph 6.4.2.2 states, “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.” Note that the “entire space served by the air distribution system” must be protected by a system of area smoke detectors.
Of course, also verify that the fire marshal/plans reviewer is in agreement. In addition, remember that the building/mechanical code may require duct smoke detectors in the return system, as is noted in this blog post.
good explanation
Thank you. I hope it was helpful.
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Hotels and nursing homes are commercial buildings that fall under the building code. They have a Residential Occupancy classification.