Hot Water
Hot Water But No Heating? Causes and Fixes
Reviewed by our Gas Safe registered engineers · Last updated 21 July 2026
Hot water at the taps but cold radiators is the classic sign of a diverter valve problem — though a thermostat not calling for heat or a wrongly-set programmer can do the same. Here’s how to tell them apart.
Quick answer: If hot water works but the radiators stay cold, the boiler is heating fine — the heat just isn’t reaching the radiators. On a combi that’s usually a stuck diverter or motorised valve; it can also be the thermostat not calling for heat, or the programmer set to hot water only. Check the thermostat and programmer first.
Common Causes
Why the heating won’t come on
These are the usual causes. The colour on the left shows whether it’s a safe homeowner check or a job for an engineer.
| What you see | Likely cause | What to do |
|---|---|---|
| Hot water fine, radiators cold | Diverter or motorised valve stuck | Needs an engineer to free or replace it |
| Heating won’t switch on at all | Thermostat not calling for heat | Check it’s set above room temp and in heating mode |
| Programmer on, still no heat | Programmer or wiring fault | Check the settings; if fine, book an engineer |
| Radiators warm very slowly | Trapped air or sludge | Bleed the radiators; a powerflush may be needed |
| No heat after a power cut | Programmer lost its schedule | Check and reset the programmer schedule |
| Only some radiators cold | System needs balancing | Balance the radiator valves; bleed cold ones |
Safe Checks
What you can safely check
Safe to check
- The room thermostat is above room temperature and in heating mode
- The programmer is set to bring the heating on (not hot water only)
- The thermostat’s batteries are fresh
- Bleed any radiators that warm slowly or have cold tops
Book an engineer if…
- Hot water is fine but no radiators warm despite the stat calling
- You can hear the boiler fire but nothing circulates
- The pump is silent or grinding when heating should run
Engineer Insight
What we usually find
Hot water working but no heating is the textbook diverter-valve fault on a combi — the valve sticks toward hot water and won’t swing over for central heating. Before condemning it, we always confirm the thermostat is genuinely calling and the pump is running, because a simple setting or a seized pump can look identical from the tap.
Costs
What repairs typically cost
Typical North London guide prices — you’ll always get an upfront quote before any work starts.
FAQs
Hot water but no heating — your questions answered
Why do I have hot water but no heating?
The boiler is heating fine but the heat isn’t reaching the radiators. On a combi this is usually a stuck diverter valve; it can also be the thermostat not calling for heat or the programmer set to hot water only.
What is a diverter valve?
It’s the valve that decides whether the boiler sends heat to the radiators or to your hot water. When it sticks, you often get one but not the other — commonly hot water but no heating.
Can I fix this myself?
You can safely check the thermostat and programmer settings and bleed radiators. A stuck diverter, motorised valve or seized pump needs a Gas Safe engineer.
Why did it happen after a power cut?
The programmer can lose its schedule after a power cut, so the heating simply isn’t timed to come on. Reset the schedule; if the heating still won’t run with the stat calling, it needs looking at.
How much does a diverter valve cost?
Typically £150–£340 in North London including parts and labour, depending on the boiler. You’ll get an upfront quote before any work.
Related
Keep troubleshooting
The reverse problem — diverter, pressure and immersion causes.
Read guideHeatingCentral Heating Not WorkingPump, valve and thermostat causes of cold radiators.
Read guideHeatingThermostat Not WorkingIf the stat isn’t actually calling for heat.
Read guideHeatingRadiators Cold But Boiler WorkingAir, sludge and balancing when heat won’t reach the rads.
Read guideHeatingHeating Not WorkingThe full troubleshooting guide from no heat to cold rads.
Read guideFault CodesBoiler Fault Code LibraryLook up a heating fault code by brand.
Browse codesHead back to the Help & Advice Centre for more boiler and heating guides.
Radiators cold but taps hot? We’ll sort it.
Book a Gas Safe engineer to check the diverter, valve and pump, or call and describe what’s working — we’ll give you an honest steer first.
Gas Safe Registered 918817 | North London specialists | Upfront pricing