What to check depends on your type of boiler. A combi heats water instantly when you turn on a tap and has no hot water tank. A system or regular boiler heats a tank of hot water, usually in an airing cupboard.
Combi boilers
- Check the hot water setting on the boiler. It may have been turned down or switched off by accident.
- Check the pressure. Low pressure can stop hot water before it stops the heating. How to check and top up pressure.
- Try more than one tap. If only one tap is cold, the problem is likely with that tap or its pipework, not the boiler.
- Turn the tap on fully. Some combis need a minimum flow before they fire up for hot water.
If none of that helps, a common culprit is the diverter valve, the part that switches the boiler between heating and hot water. Replacing it is a job for a Gas Safe registered engineer.
System and regular boilers
- Check the programmer or timer. Hot water often has its own schedule, separate from the heating. Make sure it's set to on, or use the boost button.
- Check the cylinder thermostat on the hot water tank. It's usually set to around 60°C.
- Feel the pipe going into the tank when the hot water is on. If it's cold, the boiler isn't sending heat to the tank.
If the timer and thermostat are fine, the usual causes are a motorised valve or the cylinder thermostat itself. Both need an engineer. If you have an immersion heater, you can switch it on in the meantime.
Never take the casing off your boiler or try to repair gas parts yourself. That work must be done by a Gas Safe registered engineer. With boiler cover, your provider sends one for you. Compare boiler cover.
Have boiler cover?
Call your provider before paying an engineer yourself. A repair like this is exactly what cover is for.
No cover yet? A new plan won't pay for a fault you already have, because problems that exist when you join aren't covered and there's a waiting period at the start. It can protect you next time.
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Last updated 30 October 2026