No heating but hot water works: what to check
Quick answer
If your hot water is fine but the radiators stay cold, the cause is often in the heating controls rather than the boiler itself — a thermostat set too low, a programmer not calling for heat, or closed radiator valves. Check these first. If the boiler still won’t fire for heating once controls are confirmed correct, or it locks out repeatedly, contact a heating engineer.
Safety first
- If you smell gas or suspect a leak, leave the property and call 0800 111 999.
- Never remove boiler covers or interfere with any gas component.
- If water is leaking near electrical fittings, isolate power at the consumer unit only if it’s safe to reach, then call a professional.
Full checklist: Safety guidance
What you may notice
- All radiators are cold but hot water runs normally — may suggest a controls or demand issue rather than a boiler failure, since the boiler is clearly working.
- The boiler fires briefly then stops when heating is requested — can indicate the system isn’t maintaining a heating demand, possibly linked to a safety cut-out.
- Some radiators are warm and others aren’t — this is usually a separate issue — see our guide on one radiator not heating.
- A fault code appears only when heating is requested, not hot water — may point specifically to the central heating side of the system.
These are possible explanations, not a confirmed diagnosis. An engineer can confirm the exact cause on inspection.
Safe checks to make, in order
- Set the room thermostat clearly above room temperature. This confirms whether the thermostat itself is calling for heat. If nothing happens, the fault may sit further along the control chain.
- Check the programmer or timer is set to ‘on’ for heating. It’s easy for heating to be accidentally switched off or scheduled to a different time while hot water remains active.
- Open the TRVs (thermostatic radiator valves) on a couple of radiators. A TRV is the valve, often numbered, that controls how much hot water flows into a radiator. If they’re turned down or closed, those radiators won’t heat even if the boiler is working.
- Watch and listen for the boiler responding to the heating demand. You’re checking whether it fires up at all, and whether any fault code appears specifically when heating (rather than hot water) is requested.
- Check system pressure if you have a sealed system. Some boilers won’t run the heating circuit properly if pressure has dropped too low.
What you can do safely
- Turning the thermostat above room temperature and checking the programmer is set correctly.
- Opening a couple of radiator TRVs fully.
- Checking system pressure if you have a sealed system.
- Resetting the boiler once only, if your manual describes this and there are no danger signs.
When to call a professional
- The controls are clearly calling for heat but the boiler still won’t fire.
- The boiler locks out repeatedly when heating is requested.
- You notice signs of overheating, such as unusual smells or the boiler cutting out on a safety fault.
- Any leak is found near electrical fittings.
What to tell the engineer
- Whether the thermostat and programmer were confirmed correctly set before you called.
- Whether the boiler fires at all for heating, and any fault code shown.
- Whether hot water continues to work normally.
- System pressure reading, if you have a sealed system.
- Whether you’ve already reset the boiler, and the result.
FAQs
Why would hot water work but not heating on the same boiler?
Most boilers treat hot water and heating as separate demands internally, so a fault affecting one doesn’t always affect the other. A common cause is a controls or demand issue specific to the heating circuit, but an engineer can confirm this.
Could this be my thermostat batteries?
It’s possible — many room thermostats run on batteries and won’t call for heat if they’ve died. Checking or replacing the batteries is a safe, sensible first step.
Is it safe to keep resetting the boiler if heating won’t come on?
Resetting once, per your manual, is generally fine. If it locks out again straight away, stop resetting and get a professional to look into the underlying cause instead.
Should I open all my TRVs or just a couple to test?
Opening just one or two is enough to test whether the system responds. There’s no need to open every radiator valve just to run this check.
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