Occupied Retrofit: Detector Spacing Around a New Conference Room
A tenant improvement converts an open office area into a 40-foot by 30-foot conference room with a 9-foot ceiling and a 2-foot-deep soffit around the perimeter. The existing smoke detectors are spot-type photoelectric units on a 30-foot grid in the open ceiling. The contractor proposes to keep the existing detector layout and add one detector in the center of the new room. The room will have a projector mounted at the ceiling and a return-air grille near one corner. The AHJ has asked for a detector layout review because the soffit and the return grille may affect smoke entry. The building is fully sprinklered, and the tenant wants to minimize new devices.
1. Is one centered detector adequate for the conference room? Explain the spacing and placement considerations.
Read the worked answer
One centered detector may be adequate only if the spacing calculation supports it, but the reviewer must check the listed spacing for the detector, the ceiling height, and the soffit. A 40 by 30 foot room with a 9-foot ceiling is within many detectors' listed spacing, but the soffit creates a perimeter obstruction that can affect smoke travel, and the return-air grille can pull smoke away from the detector. The designer should calculate detector spacing using the manufacturer's listed coverage, verify the detector is not within the prohibited distance of the return grille, and confirm the soffit does not require additional detectors or a different placement.
2. What concerns arise from the return-air grille and the projector at the ceiling?
Read the worked answer
The return-air grille can create airflow that delays smoke reaching the detector or dilutes the smoke, so the detector should be located away from the grille per the manufacturer's instructions and the applicable code. The projector and its mounting may create a dead-air space or obstruction; the detector should not be placed in the wake of the projector's airflow or too close to the obstruction. The reviewer should require the contractor to show the detector location relative to the grille and projector and to confirm it meets the listing.
3. How should the existing open-area detectors be treated after the conference room is built?
Read the worked answer
The new walls change the space, so the existing detectors that were spaced for the open office may no longer provide the required coverage in the remaining areas. The contractor should re-evaluate the layout for the entire affected area, not just the new room, and verify that the existing detectors remain within their listed spacing and are not obstructed by the new construction. Any detector that no longer meets the spacing or placement requirements should be relocated or added, and the revised layout should be submitted for review.