In most systems, water remains in the piping until needed, leaving the fire sprinkler system “on call”.
Sprinklers are heat activated and will go off when the air temperature around it reaches or exceeds the preset temperature; most commonly it is 155 degrees F (or 74 C).
There are also cases in which the sprinkler head has activated in extremely cold weather, such as near an open window or door.
At activation temperatures, the liquid expands or contracts inside the glass vial, breaking the glass and releasing the plug that holds back the water.
Fire sprinkler systems release anywhere for 35 to 150 gallons of water per minute with the average about 60. A 1.75″ fire hose sprays 150 gallons of water per minute.
Only the sprinkler closest to the fire will activate. Approx. ninety percent of fires are contained by the operation of just one sprinkler.
One activated sprinkler head can send a torrent of water in a 12’ diameter circle, enough to fill a bathtub in less than a minute. The average 5 foot bathtub holds 25 to 45 gallons.
The water from a sprinkler head is laced with black and smelly dirt and debris from sitting for months or years inside the pipes.