What actually happens when one sprinkler head opens in a DC rowhouse or condo, and the three movie myths that keep owners from installing one

The objection comes up on almost every retrofit estimate in the District, and it comes from the movies. Someone holds a lighter under a head and every head on the floor lets go, the office is soaked, the heist proceeds. Owners who believe that version picture a kitchen flare up turning into a flooded rowhouse, and they decide a sprinkler system is a bigger risk than the fire. The real event is smaller, slower, and more useful than the film version. This article walks through what a residential head does from the moment a fire starts to the moment the water stops, with the numbers a DC rowhouse or condo owner should actually hold in mind.

The first myth, that smoke sets them off

A residential sprinkler head is a plug held in place by a glass bulb or a metal link that lets go at a set temperature. Nothing in it senses smoke. The ordinary temperature head found in most DC rowhouse bedrooms and living rooms carries a red bulb, which breaks somewhere around the boiling point of water, and the ceiling has to reach that temperature at the head itself. Burnt toast, a candle, a smoky pan, a cigarette, and a steamy shower do none of that. The bulb colors article covers the ratings and where each one goes.

What does open a head without a fire is a heat source placed too close to it. A space heater on a shelf under a cellar head, a heat gun stripping paint near a ceiling, a commercial oven in a rowhouse kitchen with the head over the range. Those are placement problems, and the heat sources article covers each one.

The second myth, that they all go off at once

Each head opens on its own, and only the heads hot enough to break open. In a house fire that means the head over the fire, and sometimes one more if the fire has grown before the first head reached temperature. The record from residential fires across decades of fire service data is that the large majority of fires in sprinklered homes are controlled by a single head, and most of the remainder by two. The other heads in the house stay shut because the ceiling above them never got hot.

The reason this matters for a rowhouse is the party wall. A kitchen fire in the rear of a Petworth rowhouse opens the kitchen head. The front bedroom head, the cellar heads, and the heads on the second floor do nothing, because their ceilings stay cool. The water that lands in the house lands in the kitchen, where the fire is. The party wall article explains why stopping the fire in that one room is worth so much on a block of attached houses.

The third myth, that the water does more damage than the fire

A single residential head flows somewhere in the range of a garden hose, on the order of a dozen gallons a minute, in a spray pattern sized to cover the room. A fire hose that an engine company brings through the front door moves ten times that, and it arrives after the fire has had several more minutes to grow, which in a rowhouse means after the fire has reached the stair and the floor above. An owner choosing between sprinkler water and fire department water is choosing between a wet kitchen and a wet house with the kitchen gone.

The water from a head does make a mess, and a condo owner below a discharge will have a claim. The first 24 hours article covers the cleanup and the insurance article covers who pays. Neither one is a reason to leave a fire burning.

What the minutes look like

A fire starts in a wastebasket beside a couch at the front of a rowhouse living room. For the first minute or two it is a small fire, and the smoke alarm in the hall goes off. The hot gas layer at the ceiling builds. Somewhere between two and four minutes in, depending on the fuel and the room, the layer at the head above the couch reaches the bulb rating, the bulb breaks, and the plug drops out. Water from the pipe in the joist bay hits the deflector and sprays down and out over the couch and the floor around it.

The fire is knocked down within seconds of the head opening in most cases, and held in check in nearly all of the rest. The head keeps flowing. It will keep flowing until someone closes the control valve, because the head cannot tell that the fire is out. On a system with a flow switch and a monitored alarm, the flow also sends the signal that brings the engine company. The monitoring article explains which DC systems have that and which do not.

The engine company arrives to a controlled fire, a smoky room, and water on the floor. They confirm the fire is out, close the valve at the riser, and the water stops. The heads in every other room are untouched. A sprinkler contractor replaces the one head that opened, and the restoring the system article covers the rest.

What a DC owner should take from this

Know where the control valve is and do not close it during a fire. Owners who shut the valve to stop the water while the fire is still burning have turned a controlled fire back into an uncontrolled one, and in a rowhouse that is the neighbor’s problem too.

Keep the heads clear and unpainted, because a head that cannot open or cannot spray is the one real way to get the movie outcome, which is a fire that nothing stops. The storage article covers the clearance a head needs.

And weigh the retrofit question against the real event, not the film. A 13D system in an occupied rowhouse sits at the upper end of our published $2 to $7 per sprinklered square foot band, and what it buys is a single head over a single fire, flowing a garden hose of water onto the room where the fire started, in the minutes before anyone with a hose could get there.

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