No, They Don't All Go Off

Each head is plugged by a glass bulb filled with liquid, or by a metal link. Heat from the fire expands the liquid until the bulb shatters, or melts the link. Both are calibrated to a specific temperature, usually around 155°F. A head that never gets that hot never opens, which is why one head can be flowing while the one beside it stays shut.

The large majority of sprinklered fires are controlled by one or two heads. That is the whole design intent: catch it while it is small enough that a single head over it is enough.

It is rare, and when it happens the cause is almost always physical — a forklift hitting a head, freezing, corrosion, or a head painted over during a renovation. Sprinklers do not respond to smoke, and a smoke detector going off does not open a sprinkler. They are separate systems that do not talk to each other.

This is the part that surprises people most. A sprinkler puts out roughly 20 gallons a minute onto a fire that is still small. A fire department attack line puts out closer to 175 gallons a minute onto a fire that has had ten more minutes to grow — and by then the fire has done its own damage too. The sprinkler is the cheap outcome.

Because the belief that sprinklers flood buildings is the reason maintenance gets deferred, valves get left closed after unrelated work, and storage gets stacked into the spray pattern. The misunderstanding costs real money, and it costs it quietly.