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 it works
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.
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.