Smoke Detector Layout in a Corridor with Supply Diffusers
A 100 ft long, 8 ft wide corridor in a licensed assisted-living facility has a smooth 9 ft ceiling. Three 24 in × 24 in ceiling supply diffusers are located on the corridor centerline at 25 ft, 50 ft, and 75 ft from the west end. The corridor has full-height walls at both ends, no beams or soffits, and a constant-volume HVAC system. The design calls for one row of spot-type photoelectric smoke detectors on the centerline. The contractor wants to use the minimum number of detectors that satisfies NFPA 72 spacing and clearance requirements and still allows access for service. The owner has asked for detector locations measured from the west wall. The ceiling is accessible from a lay-in grid.
1. Determine the minimum number of smoke detectors and give approximate centerline locations measured from the west wall. Show the spacing checks.
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NFPA 72 permits 30 ft spacing for spot smoke detectors on a smooth ceiling and requires the first detector within 21 ft of an end wall. Detectors must also be at least 36 in. from supply diffusers. A three-detector layout at 15, 50, and 85 ft fails because the 50 ft detector sits at a diffuser and the 85 ft spacing exceeds 30 ft. A layout at 15, 45, 75, and 95 ft also fails because a detector at 75 ft would be on a diffuser. The minimum workable layout is four detectors at approximately 15 ft, 45 ft, 70 ft, and 95 ft from the west wall. Check: 15 ft from west wall; 30 ft to the next; 25 ft to the next; 25 ft to the last; 5 ft to the east wall. All spacings are ≤30 ft, all wall distances are ≤21 ft, and the 70 ft detector is 5 ft (60 in.) from the 75 ft diffuser, which is greater than 36 in. Confirm final locations against the diffuser throw and manufacturer instructions.
2. The contractor proposes mounting the detectors directly on the diffuser grilles to save time. Evaluate that proposal and state the correct approach.
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Mounting detectors on diffuser grilles is not acceptable. NFPA 72 requires spot-type smoke detectors to be located at least 36 in. from supply diffusers to avoid air dilution and nuisance alarms, and the detector must be installed per its listing and manufacturer instructions. A detector strapped to a diffuser is outside its listed mounting method, is difficult to service, and may not sample the smoke layer. It also defeats the purpose of the 36 in. clearance: the diffuser can push clean air into the chamber and delay response. The correct fix is to relocate the detector off the diffuser while preserving the 30 ft spacing and 21 ft wall rule, or use a listed duct detector if the intent is to detect smoke in the air handler. Document the final layout on the shop drawings.