Detector Spacing Around a New Duct and a Partial-Height Partition
You are reviewing a design for a small office tenant build-out inside an existing building. The ceiling is a smooth, flat 9 ft suspended ceiling throughout. The design shows spot-type smoke detectors on a 30 ft x 30 ft grid. The tenant added a floor-to-ceiling glass partition enclosing a 15 ft x 20 ft conference room; the partition has a 3 ft gap at the top because it stops at the suspended ceiling and the ceiling plenum above is continuous. The design also shows a smoke detector mounted directly on the side of a new return-air duct that serves the conference room. The mechanical contractor sized the duct at 18 in wide x 12 in high. The building is fully sprinklered. The reviewer asks you whether the layout is acceptable and what, if anything, must change.
1. Evaluate the 30 ft x 30 ft detector grid against the ceiling conditions described.
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The grid is too coarse and does not satisfy the standard spacing logic for spot smoke detectors.
Reasoning:
- On a smooth flat ceiling, spot smoke detectors are typically limited to a 30 ft spacing but that 30 ft is the maximum center-to-center spacing, and each detector's coverage is a circle of radius ~21 ft (the 0.7 x spacing rule). A 30 x 30 rectangular grid leaves the corners of each 30 ft square at a diagonal distance of about 21.2 ft from the nearest detector - right at or beyond the limit. A square grid at 30 ft on center is therefore marginal and generally not acceptable without checking the diagonal.
- The correct approach is to lay detectors out so no point on the ceiling is farther than the allowable radius from a detector; a 30 x 30 grid must be tightened (e.g., ~21 ft spacing) or detectors added.
- The glass partition stopping at the ceiling with a continuous plenum above is a judgement point: if smoke can pass over the top into the conference room, the partition may not act as a full-height smoke barrier, but you cannot assume the plenum is unobstructed. The safer call is to treat the conference room as a separate area requiring its own detector coverage unless the design demonstrates the partition is effectively full-height or the plenum is smoke-tight.
- The 3 ft gap means the 'room' is not truly enclosed; a detector serving the open office may not reliably protect inside the conference room.
2. Evaluate the duct-mounted detector on the 18 in x 12 in return duct and state the acceptance criteria.
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A detector mounted on the side of a duct is only acceptable if it is a listed duct-type detector with its sampling tube projecting into the duct airstream; a standard spot detector stuck to the duct wall is not acceptable.
Reasoning:
- Duct smoke detection is required where the duct is large enough to warrant it - generally ducts over 2,000 cfm or as required by the design. The 18 in x 12 in size alone does not establish the requirement; the airflow matters. If this return serves the conference room and exceeds the threshold, detection is required; if it is below threshold, it may be optional but, if installed, must still be correct.
- The device must be a listed duct detector installed per its listing: sampling tube across the duct, proper airflow direction, and access for testing. A spot detector on the exterior of the duct will not sample the airstream and is a false sense of protection.
- Duct detectors are not a substitute for area (ceiling) detection; they protect the duct/air-handling system and are for shutdown, not general area coverage.
- Acceptance: verify listing, correct tube orientation relative to airflow, remote test station and indicator where required, and that the device actually samples (test with smoke at the sampling tube and confirm alarm/shutdown).