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Fire Alarm Code Questions and Answers

Published On
Sep 28, 2022

During the year, I get a lot of questions regarding the National Fire Alarm and Signaling Code that on the surface create confusion, or worse, misinterpretation of the requirements. So, I thought it would be beneficial to discuss a few of the most often asked questions. It is important to know which edition of the code has been adopted in your jurisdiction, but also what the current edition has for the same requirement. If the current edition is different from the one adopted in your jurisdiction, then you can often discuss the change with the authority having jurisdiction (AHJ) (assuming it helps you) because the most recent edition has the latest thinking of the Technical Committee and is therefore more definitive.

We will start with defining notification zones, from the 2019 edition of NFPA 72. Section 3.3.238 states:

“A defined area within a protected premises.

3.328.1 Notification Zone. A discrete area of a building, or defined area outside a building, in which people are intended to receive common notification. (SIG-FUN)  

10.17.3 Control circuits used for the purpose of controlling NAC extender panels shall comply with all of the following:        

        (1) The NAC extender panel(s) connected to the control circuit shall not serve more than one notification zone.”      


Can “discrete area of a building” mean the whole building? If the whole building evacuates on general alarm, the answer is yes.

What about all floors of a five-story building, for instance? Again, the answer is yes.

Do we then need a control circuit for every floor if we call each one a discrete area? Yes, if evacuation is by floor, then each floor is a discrete area.

If we don’t use notification appliance circuits (NAC) power supply extenders for this five-story building and use one NAC from the fire alarm control unit (FACU), we are not using control circuits, just NA circuits, we are fine. But, if based on just good design or on the loading of the FACU, we decide to use an FACU NAC as a control circuit for an extender power supply, do we have to provide five control circuits for five extender power supplies to comply with Section 10.17.3(1)? No. See 10.17.3 (above). You can use the FACU NAC as the control circuit to an extender power supply.

The following highlights another common misconception:

In 2010 I designed a fire alarm system for an 8-story building. The HVAC system had an AHU on each floor with open air return with holes in the wall. The building was not covered by area smoke detectors. The fire marshal at the time said I could use a smoke detector mounted on the ceiling of the room to shut down the AHU. Being inquisitive, I asked, what if the room was larger than 900 square feet? He said that you would have to cover the room per NFPA 72 smoke detector spacing requirements.

NFPA 90 A States in Section 606.2.1, Return air systems, that “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 word plenum is what the fire marshal said that the detection on the ceiling was allowed for this. What are your thoughts?

Let’s start with a definition: Plenum is an enclosed portion of the building structure, other than an occupiable space being conditioned, that is designed to allow air movement, and thereby serve as part of an air distribution system.

Next, look at the whole section that relates to the question:

Smoke detection systems control 

606.1 Controls required. Air distribution systems shall be equipped with smoke detectors listed and labeled for installation in air distribution systems, as required by this section. Duct smoke detectors shall comply with UL 268A. Other smoke detectors shall comply with UL 268. 

606.2 Where required. Smoke detectors shall be installed where indicated in sections 606.2.1 through 606.2.3. 

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.

This is an important concept to understand. Duct-type or area smoke detectors provided to shut down the AHU are there strictly to prevent the recirculation of smoke from the fire area to other parts of the building.

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. 

Exception: Smoke detectors are not required in the return air system where all portions of the building served by the air distribution system are protected by area smoke detectors connected to a fire alarm system in accordance with the International Fire Code. The area smoke detection system shall comply with Section 606.4.

This is another important point. 

606.2.2 Common supply and return air systems. Where multiple air-handling systems share common supply or return air ducts or plenums with a combined design capacity greater than 2,000 cfm (0.9 m3 /s), the return air system shall be provided with smoke detectors in accordance with Section 606.2.1. 

Exception: Individual smoke detectors shall not be required for each fan-powered terminal unit, provided that such units do not have an individual design capacity greater than 2,000 cfm (0.9 m3 /s) and will be shut down by activation of one of the following: 

  1. Smoke detectors required by Sections 606.2.1 and 606.2.3. 
  2. An approved area smoke detector system located in the return air plenum serving such units. 
  3. An area smoke detector system as prescribed in the exception to Section 606.2.1. In all cases, the smoke detectors shall comply with Sections 606.4 and 606.4.1. 

606.2.3 Return air risers. Where return air risers serve two or more stories and serve any portion of a return air system having a design capacity greater than 15,000 cfm (7.1 m3 /s), smoke detectors shall be installed at each story. Such smoke detectors shall be located upstream of the connection between the return air riser and any air ducts or plenums.

[F] 606.3 Installation. Smoke detectors required by this section shall be installed in accordance with NFPA 72. The required smoke detectors shall be installed to monitor the entire airflow conveyed by the system including return air and exhaust or relief air. Access shall be provided to smoke detectors for inspection and maintenance.

[F] 606.4 Controls operation. Upon activation, the smoke detectors shall shut down all operational capabilities of the air distribution system in accordance with the listing and labeling of appliances used in the system. Air distribution systems that are part of a smoke control system shall switch to the smoke control mode upon activation of a detector. 

[F] 606.4.1 Supervision. The duct smoke detectors shall be connected to a fire alarm system where a fire alarm system is required by Section 907.2 of the International Fire Code. The actuation of a duct smoke detector shall activate a visible and audible supervisory signal at a constantly attended location. In facilities that are required to be monitored by a supervising station, duct smoke detectors shall report only as a supervisory signal, not as a fire alarm. 

The concern here has always been false alarms due to poor maintenance of DSDs.


1. The supervisory signal at a constantly attended location is not required where the duct smoke detector activates the building’s alarm-indicating appliances. 

(This is never a good idea.)

2. In occupancies not required to be equipped with a fire alarm system, actuation of a smoke detector shall activate a visible and audible signal in an approved location. Duct smoke detector trouble conditions shall activate a visible or audible signal in an approved location and shall be identified as air duct detector trouble.

So, if all the above is read in context, and if the plenum as defined above is provided with a single return from the plenum space, then the area smoke detector in that area meets the IMC code requirements. In other words, the fire marshal was correct. But always remember we know little of the operating characteristics of the HVAC system, so always ask he HVAC techs for the information we need to design as above.

Finally, none of our codes can be looked at in a vacuum. So, look at NFPA 72-2010 sections 17.7.4 through as well to ensure all bases are covered.

If you have a question about NFPA 72, send it to me at wmoorefpe@gmail.com.

About the Author

Wayne D. Moore

Fire/Life Safety Columnist

Wayne D. Moore, a licensed fire protection engineer, frequent speaker, writer and expert in the life safety field, has been a principal member and chair of NFPA 72, Chapter 24, as well as a former principal member of NFPA 909 and NFPA 914. He is the...

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