Initiating Devices and Detection Circuits
Spacing and placement
Spot smoke detectors typically have a listed spacing of 30 ft on smooth ceilings up to 10 ft high, and every point on the ceiling must lie within 0.7 times that spacing (21 ft). Higher ceilings, beam pockets, and sloped ceilings require reduced spacing, with detectors placed in the high rows near the apex. Keep detectors at least 4 in. from side walls and at least 3 ft from supply diffusers so air does not blow smoke away. Heat detectors commonly have a 50 ft listed spacing and follow the same 0.7 rule.
Manual and duct devices
Manual pull stations go on the egress path within 5 ft of the exit door, mounted 42 to 48 in. above the finished floor, unobstructed. Duct smoke detectors are required for larger air-handling units (over 2,000 cfm under the mechanical code), installed downstream of filters and before humidifiers and branch takeoffs, with remote test stations and service access.
Traps
- Applying the 0.7 rule only at walls instead of across the whole grid.
- Mounting a pull station at 36 in. or 60 in.
- Treating a duct detector as area smoke detection; it provides shutdown and annunciation, not general coverage.
- Placing a detector directly in front of a diffuser.
- Forgetting that a conventional zone short reads as an alarm, not a trouble.
Rule of thumb
Sketch the grid, then measure the worst-case point (usually the center of four detectors). If the half-diagonal exceeds 0.7 times the listed spacing, add a row or column.