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.
Codes commonly shown with this
- F.24VaillantTemperature is rising faster than it should when the burner fires. That is a circulation problem caught a step earlier than an overheat.Get it checked
- F.83VaillantThe burner lit but neither sensor saw the temperature rise it should have. Either water is not moving through the boiler, or a sensor is misplaced.Get it checked
- E5 218Worcester BoschThe flow temperature sensor has measured water leaving the heat exchanger far hotter than it should be. The boiler shut down to protect itself, and heat is not being carried away fast enough.Get it checked
- A7Worcester BoschThe hot water temperature sensor has failed. Heating usually carries on as normal while the hot water runs cold, scalding, or swings between the two.Get it checked
When it is urgent
Take care
What it costs
Typical repair cost
£180–£450
Typical UK prices including parts and labour, updated September 2026. Your quote may differ.
Get an engineer to look at this
We do not pass enquiries to a repair firm, so there is no form here. Find a Gas Safe registered engineer through the Gas Safe Register, and ask to see their registration card when they arrive — it is the register, not the van, that tells you they are qualified.
Technical review
Not reviewed by an engineer
No engineer has read this page. Severity follows what the manufacturer publishes, and we would rather tell you that than let a byline imply a sign-off.