Clearing the Smoke: Eliminating confusion from the use of duct smoke detectors

By Wayne D. Moore | Jul 15, 2021
Shutterstock / ArtOptimum / Peter Sobolev

When are duct smoke detectors required and by which codes? Who must supply and install them, and how they should be connected into the fire alarm system?

The ICC Mechanical Code, Section 606, states, “Air distribution systems shall be equipped with smoke detectors listed and labeled for installation in air distribution systems.” The code requires that smoke detectors be installed in all supply and return air systems with a capacity greater than 2,000 cubic feet per minute, “in the return air duct or plenum upstream of any filters, exhaust air connections, outdoor air connections, or decontamination equipment and appliances.”

NFPA 90A, Standard for the Installation of Air-Conditioning and Ventilating Systems, requires duct smoke detectors be installed “downstream of the air filters and ahead of any branch connections in air supply systems having a capacity greater than 2,000 ft3/min., 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 15,000 ft3/min and serving more than one story.”

Both codes require all duct smoke detectors be installed in accordance with NFPA 72, National Fire Alarm and Signaling Code. All three codes require all duct smoke detectors to be listed in accordance with UL 268A.

The smoke detectors’ purpose is to monitor the entire airflow conveyed by the HVAC system. Both documents also require access be provided to the installed duct smoke detectors for inspection and maintenance. While these requirements seem straightforward, why is there confusion regarding duct smoke detector installation and operation?

The reason we need these devices is to prevent the HVAC system from recirculating smoke from where the fire is occurring into nonfire areas of a building.

The issues begin with who supplies the detectors. In new building construction, duct smoke detector requirements are found in the mechanical contractor’s specification. It makes sense then that the mechanical contractor supplies the required duct smoke detectors.

But mechanical contractors have no electrical background and little knowledge of fire alarm systems and NFPA 72 requirements. They purchase 120V AC duct smoke detectors from their supplier and physically install them as shown on the mechanical drawings. Since they are probably not licensed electrical contractors, they will not connect the 120V AC power to the installed duct detectors. They may or may not coordinate the installation with the electrical contractor. Assuming they do ask the electrical contractor to connect the 120V AC power to the devices, they will have no knowledge of the backup power required by NFPA 72 if these devices are to be connected to a fire alarm control unit (FACU).

If the EC also is not knowledgeable regarding the backup power requirements, they simply connect to the 120V AC to ensure the devices work. Assuming there is no FACU or fire alarm system in the building, the actuation of a duct smoke detector shall activate a visible and audible supervisory signal at a constantly attended location. Also, smoke detectors used solely for closing dampers or for heating, ventilating and air conditioning system shutdown shall not be required to activate the building evacuation alarm.

The EC should interconnect the devices to the HVAC power to use the duct detector’s relays to shut the HVAC down when the detector alarms. The EC may also connect the detector to the fire alarm system as an alarm device if required by the AHJ.

The above scenario has two problems. The first is the lack of backup power supplied to the detector. Per NFPA 90A, this is only allowed when its sole function is for fan shutdown. If it is required to signal the fire alarm system, it should not indicate an alarm condition on the FACU.

The ICC Mechanical Code states, “The actuation of a duct smoke detector shall activate a visible and audible supervisory signal at a constantly attended location. In facilities required to be monitored by a supervising station, duct smoke detectors shall report only as a supervisory signal, not as a fire alarm.”

NFPA 90A requires that if duct smoke detectors are connected to the building’s fire alarm system and are used “to cause the operation of HVAC systems, smoke dampers, fire dampers, fan control, smoke doors, or fire doors,” then they must be monitored for integrity. This last requirement applies to 120V AC and 24V DC powered duct detectors.

All 120V AC-powered duct detectors connected to an FACU, and not to a standby generator circuit, are not code-compliant. All devices on a fire alarm system must operate when the building loses power. If a mechanical contractor supplies duct smoke detectors, coordinate the device’s power to ensure it matches and is included in the calculations of the FACU’s standby power. If you do not coordinate, plan on a delayed acceptance of your fire alarm system by the AHJ.

NFPA 72 also requires all duct smoke detectors (connected to the FACU) shall initiate a supervisory signal (not an alarm signal). The AHJ can override the last point, but they generally accept it due to the history of high false alarm rates from dirty duct smoke detectors.

Duct smoke detectors are not required in a building in three situations: 1) where the air distribution systems are incapable of spreading smoke beyond the enclosing walls, floors and ceilings of the room or space where the smoke is generated; 2) 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 and 3) is for fan units that remove air from the inside to the outside (total exhaust).

Another coordination item is when the FACU actuates the HVAC system for the purpose of smoke control, the automatic alarm initiating zones on the fire alarm system must be coordinated with the smoke control zones the duct smoke detectors actuate.

Finally, when are remote test station/alarm indicators or remote reset stations (essentially the same device) required? The remote test station simply shorts the contacts of the detector to ensure it is connected to the fire alarm circuit. If a remote reset station is needed, it simply resets the smoke detector after it has actuated. This device is only required when the duct detector is not resettable from the FACU. NFPA 72-2019 Section states, “Where duct smoke detectors are not resettable from the protected premises fire alarm system, a listed alarm/supervisory indicator with an integral reset switch shall be provided in an accessible location.”

All AHJs will require remote alarm indicators as a minimum for all concealed detectors, regardless of type.

NFPA 72 requires that in addition to ensuring air duct detectors sample the air stream according to manufacturers’ instructions, duct smoke detectors that use sampling tubes must be tested to ensure they properly sample the air stream in the duct using a method acceptable to the manufacturer or in accordance with published instructions. The frequency of all testing is spelled out in Chapter 14 of NFPA 72-2019.

Hopefully some of the smoke has cleared as to the requirements around duct smoke detection devices since you might be responsible for ensuring proper setup. As always, I strongly advise you to own and use a copy of the current NFPA 72.

About The Author

MOORE, a licensed fire protection engineer, was a principal member and chair of NFPA 72, Chapter 24, NFPA 909 and NFPA 914. He is president of the Fire Protection Alliance in Jamestown, R.I. Reach him at [email protected]





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