What opens a home fire sprinkler head when there is no fire: space heaters, heat lamps, shop lights, and the things that never will

Most of the accidental discharges we clean up after in Northern Virginia had no fire anywhere near them. They also, almost without exception, had no mystery. Something hot was placed close to a head, or something hard hit one. Owners worry about the wrong list, the candles and the cooking smoke and the hot shower, and ignore the short list that actually does it. This article is the short list, with the physics behind it, so you know what to move and what to leave alone.

How a head decides

A residential sprinkler head is a plug held in place by a glass bulb or a solder link. The bulb holds a liquid that expands with heat, and at the rated temperature it shatters and the plug drops. Nothing else is involved. No sensor, no wire, no connection to the smoke alarms. The heads in almost every Northern Virginia house are ordinary temperature rated, which means the bulb lets go at about 155 degrees F, and the standard allows them anywhere the ceiling will not exceed 100 degrees F in normal use. Hotter spots get an intermediate head rated 175 to 225 degrees, and the temperature ratings article covers where those go and what the standard says. The point for this article is simpler: to open a head without a fire you have to get the air at the bulb, at the ceiling, to 155 degrees and hold it there. That rules out almost everything in a house and leaves a handful of things that can.

Things that can do it

A radiant space heater aimed upward. The kind with glowing elements throws heat in a line, and a basement ceiling in a Springfield split level sits at seven feet. Set on a shelf or tilted up at a chair, with a head three feet away, the bulb can reach its rating in an evening. Fan forced heaters on the floor almost never do it, because the heat spreads and cools before the ceiling. A heat lamp. Bathroom ceiling heat lamps are installed into the ceiling, which is the one place a head cannot escape them, and a reptile tank lamp on a tall dresser under a head is the same thing with a different excuse. A halogen work light clamped to a joist during a remodel, which is the one we see most in the summer, because the crew hangs it near the ceiling where the light is useful and leaves it on through lunch. A heat gun during paint stripping on trim near the ceiling line. A propane patio heater under a covered porch that has a head in it. A dryer vent that broke loose in a utility closet and blows on the head above the machine, which is close to the line rather than over it and gets there on a long cycle.And a sauna or a steam shower, which is not an accident, it is a design error. Those rooms get an intermediate head or the room is laid out so the head sits outside the heat, and a sauna with an ordinary head in its ceiling will open it the first time the stove is run hard.

Things that cannot

Smoke. Burnt toast, a smoky oven on Thanksgiving, a cigarette, a fireplace that puffs back when the damper is closed. Smoke sets off the smoke alarm and does nothing to a bulb that needs heat. The myths article covers why this one survives every year. Steam from a shower, because a shower runs around 105 degrees and the steam loses heat the moment it leaves the stall. Candles, unless a candle is placed inches under a head, which is a fire risk for other reasons. A hot summer day, since attics get a head rated for it and living space never comes close to 100 degrees. Sunlight through a skylight, which the temperature ratings article covers, since the standard already places a hotter head there when the geometry calls for it. An oven, a stovetop, or a range hood running under normal cooking, as the head over the kitchen sits far enough from the stove and the ratings article explains the exceptions. Cold, which freezes pipe and cracks fittings but does not open a head, and the freeze article explains why a freeze break looks like a discharge and is not one.

The other half of the accidental list: hits

Heat is half of what opens heads. The other half is contact, and it needs no temperature at all. A bulb is glass. A ladder swung in a stairwell, a mattress carried up to the second floor, a closet shelf installed into a sidewall head, a hanger loop of Christmas lights, a basketball in a finished basement. The plug drops the moment the bulb breaks, and the head flows exactly as it would in a fire. The living with sprinklers article lists the daily rules, and the head guards article covers the cage that solves the low ceiling case. A recessed concealed head with a cover plate is the other answer, since there is nothing to hit until the plate drops in a fire.

What to move today

Walk the house with the list above and look up. Any radiant heater gets pointed at the floor or across the room, never up, and goes nowhere near a head closer than the far side of the room. A heat lamp in a bathroom ceiling is fine if the head in that bathroom sits outside its throw; if the two are within a few feet of each other, call us before winter, because the fix is a different head rating or a relocation and not a hard one. Tell every contractor about the heads before the work lights go up, which the contractor article covers alongside the pipe hits. Replace the dryer vent clamp. Keep the sauna head on the list for the next inspection.

If one opens anyway

Close the control valve on the riser, open the main drain, and then read the after it activates article for the next hour. One head flowing is less water than most owners expect, which the discharge article puts in numbers, and the replacement is a short visit with a matching head. Before the new head goes in, we want to know what opened the old one, because a head that opened from a heat source will open again next winter if the heater goes back on the same shelf. That question is on our inspection report for the same reason.