BoilerFaultCodesNo hot water but heating is working?
If your radiators are hot but the taps run cold, the boiler itself is firing — it just can't send hot water to the taps. On a combi boiler that points to one component 90% of the time: the diverter valve. Here's how to confirm it and what it costs to fix.
What does a diverter valve do?
On a combi, the diverter valve switches the flow of hot water between the radiators and the taps. When it sticks in the "heating" position (common), hot water can't reach the taps even though the boiler is running fine.
Five checks before you call an engineer
Check the hot water dial / setting
It sounds obvious — but the hot water dial on the front of the boiler can get knocked to zero, especially on wall-mounted units within reach of kids or pets. Turn it back up. On newer boilers check the app or panel for any "hot water off" setting.
Check the pressure gauge
Low pressure (below 1 bar cold) prevents hot water demand on many models. If pressure is low, see our repressurise guide. A pressure fault often shows as both heating and hot water off, but sometimes only hot water fails first.
Run the tap for 30 seconds
Turn a hot tap fully on and listen. You should hear the boiler fire up within 5–10 seconds. If it doesn't fire at all, the flow sensor may have failed. If it fires but water stays cold, the diverter valve is stuck.
Try a different tap
If only one tap is cold — and the others are fine — the problem isn't the boiler. It's a blocked cartridge or frozen supply at that specific tap. Most common on rarely-used taps or outside taps in winter.
Check for fault codes
Look at the boiler display. Even if there's no obvious error, a fault code may appear when you try to run hot water. Note it down and search for it on this site before resetting — resetting clears the code.
The likely culprit: diverter valve
If you've confirmed heating works but hot water stays cold and there's no obvious fault code, you're almost certainly looking at a stuck or faulty diverter valve.
Other less-common causes
Stuck 3-way valve (system / regular boilers): If you have a hot water tank rather than a combi, the 3-way motorised valve may have failed mechanically or the motor burned out.
Failed flow sensor: Tells the boiler that a hot tap has been opened. If it fails, the boiler never fires for hot water.
Blocked plate heat exchanger: Limescale blockage on combis in hard-water areas. Hot water becomes progressively weaker before stopping.
Faulty PCB: On older boilers the control board may have failed. Usually accompanied by other odd behaviour.