No hot water but the heating still works
When heating works and hot water does not, the fault is almost always in the part of the boiler that switches between the two rather than in the boiler itself.
What it usually means
A combi boiler heats your radiators and your taps from the same burner. What decides where the heat goes is the diverter valve: when you open a hot tap, a flow sensor tells the boiler, and the diverter valve swings across to send hot water through the plate heat exchanger instead of round the radiators.
So when heating works perfectly and hot water does not, you have narrowed the fault down considerably before anyone has even opened the case. The burner is fine, the pump is fine, the gas supply is fine. What is not working is the switching, the sensing, or the plate exchanger that transfers heat into your tap water.
The one exception worth ruling out first is scale. In hard water areas the plate heat exchanger furs up gradually, so hot water gets weaker over months rather than stopping suddenly. If your hot water has been slowly declining rather than failing overnight, that is where to look.
How to narrow it down
Work through these in order.
Did it stop suddenly, or fade over months?
Sudden failure points at the diverter valve or the flow sensor. Gradual decline over months, especially in a hard water area, points at limescale in the plate heat exchanger.
Does the boiler fire at all when you open a hot tap?
If it fires, the flow sensor is working and the fault is downstream — the diverter valve or the plate exchanger. If it does not fire at all, the boiler is not being told you want hot water, which points at the flow sensor.
Is the water lukewarm rather than cold?
Lukewarm usually means the diverter valve is partially stuck, sending some heat to the taps and some to the radiators. Completely cold with the boiler firing suggests the plate exchanger is blocked.
Is it worse at one tap than another?
Then the boiler is probably fine and the problem is at that outlet — a blocked aerator, a failed thermostatic mixer cartridge, or a partly closed isolation valve.
When it is urgent
Take care
What it costs
Typical repair cost
£180–£450
Typical UK prices including parts and labour, updated August 2026. Your quote may differ.
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