Heating Problems
Central Heating Not Working? Causes and Fixes
Reviewed by our Gas Safe registered engineers · Last updated 21 July 2026
When the boiler runs but the central heating won’t come on — or the radiators stay stone cold — the cause is usually the thermostat, the pump, a motorised valve, or sludge. Here’s how to narrow it down.
Quick answer: If the boiler fires but no radiators warm up, the heat isn’t circulating. The usual causes are a thermostat not calling for heat, a seized pump, a stuck motorised valve, or sludge blocking flow. Bleeding radiators and checking the thermostat rule out the simple ones.
Common Causes
Why central heating stops working
These are the usual culprits, roughly in order. 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 |
|---|---|---|
| Boiler on, all radiators cold | Pump not circulating, or valve stuck | Check the thermostat is calling; if so, book an engineer |
| Radiators warm slowly or partly | Sludge or trapped air | Bleed the radiators; a powerflush may be needed |
| Heating won’t switch on at all | Thermostat or programmer | Check the settings, position and batteries |
| Some rooms hot, others cold | System needs balancing | Balance the radiator valves; bleed any cold ones |
| Hot water fine, no heating | Motorised (diverter) valve | Needs an engineer to check |
| Grinding or humming when it tries | Failing pump | An engineer can replace the pump |
Safe Checks
What you can safely check
Safe to check
- The thermostat is calling for heat (above room temp, in heating mode)
- The programmer is set to come on, with fresh batteries
- Bleed radiators that are cold at the top to release trapped air
- Check the pressure gauge reads around 1.3 bar
Stop and call us if…
- You can smell gas — leave the property and call 0800 111 999
- The pump is grinding, or the boiler is leaking
- Radiators are cold at the bottom and the boiler is kettling (heavy sludge)
Engineer Insight
What we usually find
When every radiator is cold but the boiler runs, it’s nearly always circulation — a seized pump or a stuck motorised valve — or a thermostat that isn’t actually calling for heat. Slow, patchy warmth usually means sludge, which a powerflush clears. We test the pump and valves directly rather than guessing.
Costs
What repairs typically cost
Typical North London guide prices — you’ll always get an upfront quote before any work starts.
FAQs
Central heating problems — your questions answered
Why is the boiler on but no radiators are warm?
The heat isn’t circulating. Usually the pump has seized, a motorised valve is stuck, or the thermostat isn’t calling for heat. An engineer can test each quickly.
What is sludge and how do I know if I have it?
Sludge is corrosion debris that settles in radiators and pipework. Tell-tale signs are radiators cold at the bottom, slow warm-up, and a kettling boiler. A powerflush clears it.
Can I fix central heating myself?
You can safely bleed radiators, check the thermostat and programmer, and top up pressure. Pump, valve and sludge issues need a Gas Safe engineer.
Why is my hot water fine but heating isn’t?
That points to the diverter or motorised valve — the boiler heats water but isn’t directing it to the radiators. It’s a common, fixable fault.
How much does a powerflush cost?
Typically from £350–£600 in North London depending on system size and how many radiators. You’ll get an upfront quote first.
Related
Keep troubleshooting
The full troubleshooting guide — from no response to cold radiators.
Read guideCentral HeatingPowerflush GuideWhat a powerflush does and when sludge is the real problem.
Read guideCentral HeatingHow Central Heating WorksPump, valves and flow explained — useful for diagnosing faults.
Read guideHeatingRadiators Cold But Boiler WorkingTrapped air, sludge and balancing — how to tell them apart.
Read guideHeatingThermostat Not WorkingIf the heating won’t switch on at all.
Read guideHeatingHeating Problems HubAll our heating and radiator guides in one place.
Browse guidesBrowse all Heating Problems guides, or head back to the Help & Advice Centre.
Radiators stone cold? Let’s warm them up.
Book a Gas Safe engineer to test the pump, valves and flow, or call and describe what’s happening — we’ll give you an honest steer first.
Gas Safe Registered 918817 | North London specialists | Upfront pricing