The water extinguisher is the simplest thing in the cupboard and still the right answer for the most common fire in most buildings. It is red banded, it is rated for class A only, and it works by taking heat out of the fire faster than the fire can produce it. Its limitations are as important as its uses.
How it works
Water absorbs a great deal of heat as it warms and more again as it turns to steam, so a water jet cools burning solids below the temperature at which they release flammable vapour. The steam also displaces some air at the seat of the fire. It is a cooling agent above all, which is why it is so effective on deep seated fires in paper, wood and textiles that other agents only knock down on the surface.
What it is for
Class A fires: paper, cardboard, wood, textiles, most office and domestic rubbish. In an ordinary office, school or shop this is the fire you are most likely to face, which is why water extinguishers remain the backbone of provision in those premises. Additive versions carry a higher class A rating for the same cylinder size by making the water wet surfaces more effectively.
Where it must not be used
Never on burning cooking oil or fat: water flashes to steam under the oil and throws burning oil across the room. Never on burning metals. Not on flammable liquids, which it spreads. And not near live electrical equipment unless that specific extinguisher has been dielectrically tested and the label says so, because a water jet can conduct back to the person holding it.
The red band
A red body with a red band, or with no coloured band at all, is water. This is the one case where the band colour matches the body, which is why some people assume every red extinguisher is water. Every extinguisher body is red; only the band identifies the contents, so check for a cream, black, blue or yellow area before assuming.
Questions people ask about what is a water fire extinguisher used for
What colour is a water fire extinguisher?
Red, with a red band or no coloured band. Since every extinguisher body is red, the absence of a cream, black, blue or yellow area is what identifies it as water.
How do water fire extinguishers work?
By cooling. Water absorbs heat as it warms and much more as it turns to steam, dropping the burning material below the temperature at which it releases flammable vapour.
Can I use a water extinguisher on an electrical fire?
Only if that specific extinguisher has been dielectrically tested for use near live electrical equipment and its label says so. Otherwise no, because the jet can conduct.