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How do fire sprinklers work?

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Quick answer

A fire sprinkler is a heat-activated valve. A glass bulb or fusible link holds the water back until a fire raises the temperature to the head's rating; then the seal breaks, water rushes out and hits a deflector that spreads it as a spray. Only the heated heads open, so the system targets the fire directly.

Fire sprinklers work by using heat to melt or break a heat-sensitive element in each sprinkler head, releasing pressurised water only from the heads directly above a fire. Most sprinklers in a building stay closed, so water is concentrated where it is needed.

Step by step

  1. A fire starts and heats the air and ceiling above it.
  2. Heat reaches the nearest sprinkler head and its glass bulb or fusible link reaches the rated temperature.
  3. The bulb shatters or the link melts, releasing the seal and opening that head.
  4. Pressurised water flows through the open head and strikes the deflector plate.
  5. The deflector spreads the water into a cone-shaped spray that cools the burning material.
  6. Water flow trips a flow switch that sounds the alarm, and falling pressure may start a fire pump.

Key things to understand

  1. Each sprinkler head is sealed by a heat-sensitive element, such as a glass bulb filled with liquid or a fusible metal link.
  2. When a fire raises the air temperature around a sprinkler to its rated range, the bulb shatters or the link melts, opening that head.
  3. Water flows through the open head and strikes a deflector plate, which spreads it into a cone-shaped spray over the fire.
  4. Water flowing in the pipes trips a flow switch or alarm valve, sounding the alarm and, in some systems, starting a fire pump.
  5. Only heads heated by the fire open; the rest remain sealed, so water use is far lower than in a deluge system.

Labelled diagram

  1. Sealed head

    A glass bulb or fusible link holds the water seal closed.

  2. Heat reaches bulb

    Fire raises the air temperature around the sprinkler head.

  3. Bulb breaks

    The heat-sensitive element shatters or melts at its rated temperature.

  4. Water flows

    Pressurised water pushes through the open head.

  5. Deflector sprays

    Water hits the deflector and spreads as a cone over the fire.

How a fire sprinkler head opens and sprays

Real-world examples

  • In a hotel corridor, a small wastebasket fire heats only the sprinkler directly above it, which opens and sprays water while the other heads stay sealed.
  • In a warehouse with a hot roof space, sprinkler heads rated for higher temperatures are used so normal summer heat under the roof cannot open them.
  • In a residential apartment, a single sprinkler head above a kitchen fire can control the flames until firefighters arrive.
  • In a computer room, a pre-action sprinkler system keeps pipes dry until a fire detector confirms a fire, reducing the risk of accidental water damage.

Common mistake

✗ Myth: All the sprinklers in a building go off at once when one fire starts.

✓ Fact: Each sprinkler head opens independently when its own heat-sensitive element reaches the rated temperature. Usually only the head or heads directly above the fire open, so water is concentrated on the fire and water damage is limited.

People also ask

What temperature do fire sprinklers activate at?
Most sprinkler heads are rated to open between about 57 °C and 74 °C (135–165 °F) for ordinary occupancies. Higher-rated heads, up to around 140 °C (285 °F) or more, are used where the ceiling is normally hot, such as near heaters or in some warehouses.
How much water does a fire sprinkler use?
A single residential sprinkler head typically flows about 40–75 litres per minute (10–20 gallons per minute), while a fire hose can flow several hundred litres per minute. Because only the heads near the fire open, a sprinkler system usually uses far less water than firefighting hoses.
Can a fire sprinkler freeze in winter?
Yes, water-filled pipes in unheated spaces can freeze and burst. To prevent this, buildings use dry-pipe systems, where pipes hold compressed air until a fire opens a valve and lets water in, or they use antifreeze solutions and insulation.
Why don't all the sprinklers go off at once?
Each sprinkler head is sealed individually and opens only when the heat-sensitive element reaches its rated temperature. A fire usually heats only the heads near it, so the others stay closed and water is directed at the fire.
What is inside a glass bulb sprinkler?
The bulb contains a liquid that expands when heated. At the rated temperature the liquid exerts enough pressure to shatter the glass, which releases the seal and lets water flow.
How does the water spray cover the fire?
Water exits the open head and hits a deflector, a metal plate shaped to break the stream into a wide, umbrella-like pattern. This pattern wets the burning material and nearby surfaces, cooling them below the temperature needed for burning.
Do sprinklers need electricity to work?
The sprinkler heads themselves are mechanical and open from heat alone, so they do not need electricity. However, the alarm, fire pump and water-supply controls often rely on electrical power or a backup supply.

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