WE HAVE all seen how sprinklers work in films.

As soon as there is a puff of smoke, dozens of hoses turn on and drench the entire building.

But this is far from reality.

When a fire starts in a building, only the sprinklers that sense the fire will turn on.

If a sprinkler senses the temperature rising to 57C, its concealed lid will fall off.

Once the mercury hits about 65C, liquid in the sprinkler nozzle expands and causes the glass bulb in the sprinkler head to smash.

This frees up the pressurised water to force out the sprinkler plug, spraying water across the fire area until it is turned off at the main valve.

And because sprinklers immediately cool fires once they activate, they prevent other nearby sprinklers from heating up and activating it too.

Fire chiefs estimate one or two sprinklers are usually enough to put out a fire in its early stages.

Some people worry about the damage sprinklers can cause to their home if they are activated, especially if they malfunction.

Though sprinklers use 60 litres of water per minute, this can be as little as one per cent of the water used by a typical fire hose.

Rooms protected by domestic sprinklers can usually be repaired and dried in just a few hours.

That is a small price to pay, fire chiefs argue, compared to the permanent damage fires can cause if they get out of control.

Fire services estimate it costs about £1,500 to install a three-bedroom house with sprinklers.