The water extinguisher is the oldest portable type still in general service and the one most British buildings hold most of. It is rated for class A only, which is the fuel almost every occupied building is full of: paper, card, furniture, textiles, waste. Its band is red, which on a red body means the band is the same colour as the cylinder and is read as a panel rather than a stripe.
How do water extinguishers work?
Water takes heat out of the fuel faster than the fire can put heat back in. That is cooling, and it is the only mechanism a plain water unit uses. Because the effect is thermal rather than chemical, the volume matters: a small unit discharged at a fire that has already involved a second item of furniture will not carry enough water to finish the job, and the honest reading of that is to leave.
The additive versions
Water units are also sold with an additive that reduces surface tension so the water wets and penetrates faster, and some of these carry an additional rating. The additive does not turn a class A unit into a class B unit. Read the label rather than the sales sheet, because the rating printed on the cylinder is what the provision was assessed against.
Where it must not go
Not near live electrical equipment, because water conducts. Not at cooking oil, because the water flashes to steam under the burning oil and ejects it. Not at flammable liquid, because the burning liquid floats on the water and spreads. Those three are the whole of the exclusion list and they are the reason a building with any of those risks holds something else beside its water units.
Identifying one on a wall
The label states water, the class A pictogram appears alone, and the band is red. Where a building still holds pre-standard units the whole body will be red with no contrasting band at all, which is the one case where an all-red extinguisher is unambiguous.
A water fire extinguisher used for what type of fire, and what it must never touch
A water unit carries a Class A rating and nothing else, which means ordinary solid combustibles: paper, card, textiles, wood and most rubbish. The rating number in front of the A is a measure of how large a test fire the unit put out, so a 13A unit is doing more work than a 8A of the same nominal size, and that number rather than the litres is what a designer adds up for a floor. Where a water unit must not go is the other half of the answer: never on live electrical equipment, because water conducts back to the person holding it, and never on burning oil or fat, because it turns to steam under the surface and ejects the oil.
Fire extinguisher water ratings, and what the number in front of the A means
The rating is a measured thing rather than a size. A unit is tested against a standard crib of timber and the number records the largest test fire it extinguished, so a 13A unit has done more work than an 8A of the same nominal capacity and a designer adds those numbers up against the floor area rather than counting cylinders. Two consequences follow for a buyer. A cheap unit with a low rating can mean more units for the same coverage, which costs more to service for ever. And a replacement chosen on litres rather than on rating can quietly reduce the protection the assessment signed off.
Questions people ask about water based fire extinguisher
What colour is a water extinguisher in the current standard?
Red, shown as a panel or band on a red body. It is the only agent whose band colour matches the cylinder, which is a historical inheritance rather than a design choice.
Can a water extinguisher be used on a computer?
No. The provision for a room with live equipment is carbon dioxide or a unit marked for electrical risk, and the water units in that room are there for the paper.
Why is water still the most common unit?
Because class A is the dominant fuel in almost every occupied building, water is the cheapest agent to refill, and it does the least damage to a building when it is discharged in error.
Is a paper and wood fire extinguisher just a water one?
In practice yes, and foam does the same job with a wider range. Paper, card and timber are Class A, the fire is deep seated in the material rather than on its surface, and what puts it out is cooling and soaking, which is what water does better than anything else per litre. The reason foam is now more common in offices is that it also handles a small liquid fire and is safer where somebody might reach for the nearest unit, so a building with a print room and a kitchenette often carries foam everywhere rather than two kinds by the door.
Is a water mist extinguisher a water extinguisher?
It uses water and behaves differently, which is the point. A very fine mist absorbs heat over a much larger surface area per litre and displaces some oxygen at the flame, and because the droplets are so small the stream is far less conductive than a jet, so many mist units carry a rating that permits use near electrical equipment and some carry a cooking oil rating as well. That combination is why a small premises can sometimes carry one type instead of three. Read the label for the ratings rather than the marketing, and keep a unit specifically rated for a deep fat fryer where the assessment found one.