No hot water but the heating works: likely causes (combi and system boilers)
- Checked September 2026
- 4 min read
- By PROS AI editorial team
Quick answer
The short version, before the detail.
If the radiators heat but the taps run cold, the boiler is firing, so the fault is in how hot water is made or sent to the taps. On a combi that is usually the diverter valve, which swaps the boiler between heating and taps. On a system or heat-only boiler with a cylinder it is usually the timer, the cylinder thermostat or a motorised valve. Check the controls and pressure first.
What is the likely cause?
Most common first, with how to tell. The first three apply to a combi, the rest to a boiler with a hot water cylinder.
Combi: a stuck diverter valve.
How to tell: the radiators are fine but the taps run cold or only lukewarm, or hot water only appears while the heating is running. Common on older boilers, and it needs an engineer.
Combi: the hot water setting.
How to tell: the hot water temperature dial is turned right down, below about 30 degrees, or hot water mode is switched off.
Combi: low pressure.
How to tell: the gauge is below 1 bar and a low-pressure warning is showing, although the heating may still limp on.
Cylinder: the hot water is used up or the timer is off.
How to tell: the water was hot earlier and is cold after baths or showers, or the hot water programme is not on at this hour.
Cylinder: the cylinder thermostat.
How to tell: the dial on the side of the cylinder is set low, or the cylinder never gets warm even when the programme is on.
Cylinder: a motorised valve that is not opening for hot water.
How to tell: the boiler and pump run for heating but the pipe to the cylinder coil stays cold. Engineer work.
Any boiler in a freeze: the condensate pipe.
How to tell: gurgling and a shutdown, though this usually stops the heating too.
What can I safely check myself?
Nothing below opens the boiler, the cylinder or the immersion heater cover, and nothing moves the lever on a motorised valve.
- Boiler settings. Make sure hot water mode is on and the hot water temperature dial is turned up. Check the timer or programmer has a hot water period running now; a power cut or the clock change can shift it.
- Run the taps. Open a hot tap fully for a minute, then turn it on and off a few times while the heating is running. On a combi this sometimes frees a diverter valve that has stuck.
- Pressure. Cool, the gauge should read 1 to 1.5 bar. Below 1 bar, open the filling loop to 1 to 1.5 bar and close both valves.
- Reset once. If a fault is showing, press reset once and wait two to three minutes.
- Cylinder controls. Turn the cylinder thermostat dial up and give the cylinder an hour on the hot water programme. If your cylinder has an immersion heater with its own switch, switch it on: it heats the cylinder electrically and is designed as a backup for when the boiler is not doing the job. Use it as a stop-gap, not a fix.
- Condensate pipe. In freezing weather, find the white plastic pipe outside and thaw it with warm, not boiling, water, then reset once.
When do I stop and call an engineer?
- If you smell gas, leave the property, do not switch anything on or off, and call the National Gas emergency line on 0800 111 999 from outside.
- If water is leaking from the cylinder or the boiler near sockets or the consumer unit, switch off at the main switch only if it is dry and safe to reach.
- Stop if the checks above have not brought the hot water back. A diverter valve, a motorised valve, a cylinder thermostat and anything behind the boiler casing are engineer jobs. If the immersion heater trips the RCD when switched on, leave it off and call an electrician.
- Gas work must be done by a Gas Safe registered engineer. Check the seven-digit number on their ID card at gassaferegister.co.uk, or text Gas and the number to 85080.
What does it cost in London?
A weekday call-out with diagnosis in London is £60 to £125, often waived if the repair goes ahead, and labour is £80 to £140 an hour. A diverter valve, the usual combi fault behind heating that works without hot water, is £150 to £390 with the part. A motorised valve or cylinder thermostat on a system boiler is charged at the hourly rate plus the part.