QUESTIONS
Why is my hot water working but not my heating?
Updated 07/26
By: Laura Macdonald
Reviewed: Content Team
There's usually a straightforward reason for this, and it's worth working through a few quick checks yourself before assuming the worst. If those don't fix it, the cause is likely to be a specific boiler component, and which one depends partly on what type of boiler you have.
The quick settings and thermostat checks to run first
Why your radiators themselves might be the issue
Which boiler fault is most likely, and why it depends on your boiler type
The difference a combi vs a system boiler makes to the cause
Roughly what a repair costs
When to call a Gas Safe engineer
Find a heating engineer near you
Check the settings
Start with your boiler's control panel:
Is it set to "hot water only"? This is a common setting to accidentally leave on, especially after switching it over for summer.
Is the timer set correctly? Check the day, time, and duration, since this is a frequent issue after clocks change for daylight saving.
Is your boiler pressure too low? Pressure should typically sit between 1 and 2 bar when the heating is on. Check the gauge on your boiler and your manual for the exact recommended pressure and repressurising steps for your model.
Check your thermostat
Is it set too low? Aim for somewhere between 18-21°C for a comfortably heated home.
Are the batteries low? Most thermostats show a low battery warning; check whether this is the cause.
Is it a wireless thermostat? Make sure it's positioned centrally in your home, away from draughts, direct sun, or other uneven temperature influences that could throw off its reading.
Check your radiators
Do you have thermostatic radiator valves (TRVs)? Confirm each one is set to a sensible temperature and hasn't been knocked closed.
Are some radiators lukewarm or cold in patches? This usually means they need bleeding to release trapped air or cleaning to clear built-up sludge.
If your radiators are the actual problem rather than the boiler, see our full guide on making radiators hotter for the complete troubleshooting list.
Possible boiler faults
If the checks above don't resolve it, the cause is likely inside the boiler itself. None of the following should be attempted unless you're a qualified Gas Safe registered engineer, since this involves gas-carrying components.
Which fault is most likely depends on your boiler type:
Combi boilers: diverter valve
A combi boiler uses a diverter valve to direct hot water either to your radiators or to your taps and shower, since it heats water on demand rather than storing it. If this valve is stuck, worn, or its motor has failed, the boiler can end up prioritising one function over the other, commonly leaving hot water working while the heating stays off. It's a common, well-understood fault and usually a same-day repair for a Gas Safe engineer. Our guide on freeing a stuck diverter valve explains this fault in more depth.
System or conventional boilers: motorised valve
If you have a system or conventional boiler with a separate hot water cylinder, there's no diverter valve; instead, a motorised valve (sometimes called a zone valve) directs water either to the cylinder or to the heating circuit. If this valve or its motor fails, you can get exactly the same symptom, hot water working but no heating, even though the underlying part is different. A stuck motorised valve sometimes clicks or hums audibly when it's trying to open, which is a useful clue to mention to your engineer.
Other possible causes (any boiler type)
Blocked burner: carbon build-up on the burner can restrict it and may need cleaning or replacing.
Worn ignition lead: like any component, this has a limited lifespan and can fail with age.
A faulty PCB (printed circuit board) or sensor: the boiler's internal "brain" can occasionally misread demand or fail to switch correctly between hot water and heating.
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How much does this cost to fix?
Cost depends on which component is at fault and your specific boiler model. For a general sense of budgeting, our boiler repair cost guide covers typical pricing for common boiler faults, including valve and component replacements.
Central heating engineer near me
Central heating engineers are important for keeping our homes warm and toasty throughout the year - as well as ensuring that our central heating systems are safe. Here's all you need to know.
Before you call an engineer
Frequently asked questions
This usually points to a stuck or failed diverter valve, which directs hot water between your taps and your heating circuit. It's a well-understood, repairable fault for a Gas Safe engineer.
No, diverter valves are specific to combi boilers. If you have a system or conventional boiler with a cylinder, the equivalent fault is usually a motorised (zone) valve instead.
No. These involve gas-carrying components and must be diagnosed and repaired by a Gas Safe registered engineer.
Check its position; wireless thermostats placed near draughts, direct sunlight, or a radiator can give inaccurate readings that affect when your heating fires.
Yes. Cold patches or lukewarm radiators are often air or sludge related, not a boiler fault. Bleeding or cleaning them is worth trying before assuming the worst.
Common faults like a stuck diverter or motorised valve are often resolved in a single visit, though parts availability can occasionally affect timing.
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