A DUCT SMOKE DETECTOR is an HVAC-system device that can prevent flames and smoke from being spread or fanned by the system. In the event of smoke, the detector sends a signal to shut down the fan. Its presence is required by code in most circumstances. Normally the determination is made by city building department officials when a permit is applied for.
The HVAC contractor Jessy Perets told me that our sanctuary’s HVAC system lacks duct smoke detectors. He also pointed out that our twin HVAC units were installed in a visibly low-budget manner, apparently without a building permit. Which has led me to wonder whether or not a pair of duct smoke detectors were technically required in our case.
For a fire safety measure, the fact of requirement (if it indeed exists) would argue for installation after the fact (which would cost $1,275 as quoted by one contractor). Conversely, if our case is an allowable exception, that would argue for leaving things as they are. At its June meeting, the board adopted that approach and asked the B&G Committee to find out whether duct smoke detectors are required by code in our sanctuary.
I have a provisional answer, which I lay out below the jump.…
Summary
My impression is that duct smoke detectors are not required in our sanctuary’s HVAC system. Our situation appears to fall under at least one of the allowable exceptions.
The above answer should be considered provisional until confirmed by an HVAC professional, such as a mechanical engineer, or a building department official.
Applicable Codes
Building-related codes for the City of Los Angeles reference existing state and national standards, by declaring the adoption of certain chapters therein, either wholly or with specified amendments. This is reflected in matrix adoption tables at the beginning of the relevant sections. The City’s Fire Code does not appear to be relevant with respect to HVAC ducts, except for citing the State Building Code (Title 24). Rather, the City defers to the State Fire Marshal, who has adopted NFPA 72 (2022 edition) as the applicable standard. According to the latter, the relevant standard seems to be found in NFPA 90A; and in the Building Code, the relevant section is the California Mechanical Code. Both of these are detailed below.
National Fire Protection Association (NFPA) 90A: “Standard for the Installation of Air Conditioning and Ventilating Systems” (2024 edition, p. 90A-19)
§11.4 Smoke Detection for Automatic Control
§11.4.2.1 “Smoke detectors listed for use in air distribution systems shall be located as follows: (1) Downstream of the air filters and ahead of any branch connections in air supply systems having a capacity greater than 944L/sec (2000 ft.³/min).…
California Mechanical Code (July 2024 Supplement)
§609.1 Air-Moving Systems and Smoke Detectors. Air moving systems supplying air in excess of 2000 cubic feet per minute (ft³/min) to enclosed spaces within buildings shall be equipped with an automatic shutoff. Automatic shutoff shall be accomplished by interrupting the power source of the air moving equipment upon detection of smoke in the main air supply duct downstream of both the fan and filters served by such equipment.
Exceptions:
…Automatic shutoff is not required where occupied rooms served by the air-handling equipment have direct exit to the exterior, and the travel distance does not exceed 100 feet.
National Fire Protection Association (NFPA) 72: “National Fire Alarm and Signaling Code” (2025 edition, p. 72-117)
[No independent information here; I include it for the sake of completeness.]
§17.7.6.3.1 “To minimize the recirculation of smoke, a detector approved for air duct use shall be installed as required by NFPA 90A and 17.7.6.4.2.”
§17.7.6.4.2 “Where the detection of smoke in the supply air system is required by other NFPA standards, a detector(s) listed for the air velocity present shall be installed in the supply air duct downstream of both the fan and the filters.”
Exception #1
Both NFPA 90A and the California Mechanical Code require that a smoke detector be installed only in a system whose fan is rated at over 2,000 cubic feet per minute (cfm). The idea seems to be that a relatively small fan will not significantly affect the spread of smoke.
In our system, each of the twin units supplies air via a multi-speed blower. According to the installer (L.A. Pros), each blower is rated at exactly 2,000 cfm.
Therefore I believe that our system is technically exempt.
Exception #2 (?)
The California Mechanical Code allows for a prompt path of egress, couched as a “direct exit” within 100 feet. What counts as “direct”? Unfortunately, I have not found a definition for that term. The mention of “travel distance” seems to allow for the prospect of passing through an intervening unconditioned space on the same level. Alternatively, it is alluding to a room so big that someone inside it might be more than 100 feet away from a doorway. (It happens that our sanctuary is about 104 ft long.) I am not sure which construal is meant.
Because our sanctuary’s lobby is not separately conditioned (having no registers or vents), in effect it is part of the same conditioned space as the sanctuary. If so, then for purposes of this standard, our sanctuary-plus-lobby has a “direct exit” to the exterior on the west side.
Our sanctuary space has two more exits to the exterior through two corridors. Here the argument for inclusion is even stronger: The ceiling of those corridors contain the main air return registers for the sanctuary’s HVAC. The system won’t work properly if the inner corridor doors are closed. Thus surely the corridors are part of the same conditioned space as the sanctuary, rather than being a separate room. Thus for purposes of this standard, our sanctuary clearly has two “direct exits” to the exterior on the east side.
Appendix: Another Code
The International Mechanical Code (IMC) is another existing standard. Although it was not adopted by the City of Los Angeles (or the State of California), I have consulted it for good measure. It appears that under this code as well, our sanctuary’s HVAC system might be exempted.
According to “Scenario 2” in a summary table on meyerfire.com, an online website for fire protection professionals, IMC §606.2 makes an exception where the “fan only serves a single space; not required where system cannot spread smoke beyond enclosing walls, floors & ceilings from where smoke is generated.”
In our case, each HVAC unit in the sanctuary indeed supplies conditioned air only to the sanctuary space—including its exit corridors, from which most of the return air is drawn. With regard to the lobby or the mezzanine, the HVAC system does not penetrate their walls with any ducts. Therefore it is not itself capable of spreading smoke into those spaces.
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