Heat detector spacing and temperature ratings under NFPA 72
Pick the rating against the hottest the ceiling gets, keep off the corner, and only halve the spacing in the direction the code says.
Heat detectors follow their own layout rules in NFPA 72 Chapter 17. Some look like the smoke detector rules and some differ in ways that matter. This covers the rating, the mounting position, and how ceiling construction and height change the spacing.
Temperature rating and marking
The detector rating has to be at least 20 degrees F above the maximum temperature expected at the ceiling, and the temperature classification is picked against the maximum expected ambient. Both apply, not either.
Spot heat detectors that operate at a set temperature, and rate compensated ones, are colour coded to their temperature classification. Only where the detector's own colour matches the code colour is a contrasting ring, or the rating in numerals at least three eighths of an inch high, used instead. Every heat detector is marked with its listed operating temperature, and a field adjustable one with its temperature range.
From the field
You should be able to tell a 135 from a 200 without pulling it down. In boiler rooms, attics and kitchens, standard rated detectors nuisance trip constantly.
Spacing and mounting position
Heat detectors are laid out on their listed spacing, and every point on the ceiling still has to fall within seven tenths of it. A ceiling mounted heat detector sits at least 4 inches from the sidewall. A wall mounted one sits between 4 and 12 inches down from the ceiling. Smoke detection has no such sidewall clearance, and its wall band has no 4 inch minimum in current editions, so check the edition your AHJ enforces before carrying either rule across.
Joists and beams
| Ceiling members | What it counts as | Heat detector layout |
|---|---|---|
| Project 4 in. or less | Smooth ceiling | Listed spacing |
| Project more than 4 in., 36 in. or less on centre | Solid joist | On the bottom of the joists; half the listed spacing at right angles to the joists, listed spacing along them |
| Project more than 4 in., more than 36 in. on centre | Beam | No more than two thirds of the listed spacing at right angles to the beams |
| Project more than 18 in. AND more than 8 ft on centre | Beam | Each bay is its own area to cover |
Where beams are less than 12 inches deep and less than 8 feet on centre, detectors may go on the bottom of the beams. A girder running at right angles to the beams or joists counts as a beam only if its top is within 4 inches of the ceiling. Deeper than that, it is not a factor in detector location at all.
From the field
Measure the projection and the spacing before you decide which rule you are in. With joists only the across the joists direction is halved; halving both is a common and expensive overcount. In warehouses and gymnasiums the bay rule multiplies device counts fast, so catch it at design.
High ceilings
Heat detector spacing shrinks as the ceiling gets higher, because heat has further to travel and spreads as it goes. The reduction table in Chapter 17 covers ceilings over 10 feet up to 30 feet, and it is applied before any further reduction for beams, joists or slope. Above that range a table lookup stops being acceptable and an engineering evaluation takes over. The table does not apply to line type electrical conductor detectors or pneumatic rate of rise tubing, which follow the manufacturer's instructions.
Sloped ceilings
A row of heat detectors goes at or within 36 inches of the peak first. On a slope of less than 30 degrees, every detector is spaced using the height at the peak. At 30 degrees or more, everything other than the peak row is spaced using the average slope height. Either way, spacing is measured along the horizontal projection.
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