Retooled HVAC Will Affect Fire Alarms: Increased air flow may trigger duct smoke detectors

Published On
Apr 15, 2021

COVID-19 has changed how buildings are maintained and operated. Facility managers are following guidelines from the Centers for Disease Control and Prevention and American Society of Heating, Refrigerating and Air-Conditioning Engineers in retooling HVAC systems, according to “Forever Changed: The Pandemic’s True Influence on Buildings,” by Maryellen Lo Bosco in Building Operating Management magazine. Another source states that these changes will include doubling the amount of outside air entering the building, more frequent purging of inside air and the use of high-performance air filters changed on a more frequent schedule.

How will these changes affect our fire alarm systems? Although NFPA 72, the National Fire Alarm and Signaling Code, does not require detection devices be used to cause the operation of HVAC system smoke dampers, fire dampers and fan control or be connected to the fire alarm system, the International Building Code (IBC) does. The IBC requires duct smoke detectors to be located “in the main return air and exhaust air plenum of each air-conditioning system having a capacity greater than 2,000 cubic feet per minute.”

These detectors must be in a serviceable area downstream of the last duct inlet. NFPA 72 has provisions for the basic method that fire alarm systems interface with the HVAC systems.

For example, if the duct smoke detectors are connected to the fire alarm system serving the protected premises, all the devices used to cause the operation of HVAC systems, smoke dampers, fire dampers or fan control must be monitored for integrity. Prior to the pandemic, HVAC systems were not cleaned or maintained regularly, so duct smoke detectors also gave false alarms on a regular basis. Because the alarms were so prevalent, the technical committee decided—after much discussion with authorities having jurisdiction—to require that duct smoke detectors provide a supervisory signal when activated instead of an alarm condition.

There are two exceptions to this requirement. Smoke detectors mounted in the air ducts of HVAC systems in a fire alarm system without a constantly attended location or supervising station are permitted to initiate an alarm signal. And the second exception is that duct smoke detectors are permitted to initiate an alarm signal where required by other governing laws, codes or standards. The code also requires that when the fire alarm control unit actuates the HVAC system for the purpose of smoke control, the automatic alarm initiating zones be coordinated with the smoke control zones they actuate.

One new factor in all these changes is that air flow will increase in the HVAC systems, which may trigger duct smoke detectors to activate. Keep in mind that these smoke detectors, although not causing an alarm condition in the building, will shut down the ventilation system.

This activity will frustrate facility managers and increase occupants’ exposure to any virus because of a lack of ventilation. So, it is important to locate the duct smoke detectors properly while ensuring the detector is UL listed for the increased air flow and that this increase will not activate the duct smoke detectors. This last item can be determined by contacting the manufacturer or UL.

The changes we are seeing in buildings are expected to continue after the COVID-19 pandemic is over. Of course, that means the changes will be applied to new construction and retrofits, and the HVAC system changes will occur in almost every occupancy except for data centers, where extreme air filtering has been used for years to protect the computer equipment.

As these changes are being implemented to HVAC systems, take a proactive stance with customers by offering information on fire alarm systems best practices. The fire protection goal for duct smoke detection is to prevent the recirculation of smoke from the fire area to the entire building. The design, location and application of smoke detectors for use in HVAC system controls are given in NFPA 72. The code allows for area spot-type smoke detectors and active air sampling smoke detectors to be used for the control of HVAC system smoke dampers, fire dampers and fan control. The standard duct smoke detector that uses metallic sampling tubes is considered a passive air-sampling-type detector. It is important to remember that you are not only responsible for ensuring your detectors do not falsely alarm, but that they also detect a fire as soon as possible. The industry has already accepted that duct smoke detection is not to be considered early warning smoke detection. That said, you still want to ensure detection in ducts so the primary goal of preventing recirculation of smoke will be met.

I am sure the pandemic will cause even more changes to buildings. Be aware of the opportunities and use your background in fire detection to meet those challenges.

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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