Heating Problems
Radiators Cold But Boiler Working? Causes and Fixes
Reviewed by our Gas Safe registered engineers · Last updated 21 July 2026
The boiler’s running and hot water’s fine, but the radiators stay cold — classic signs of trapped air, sludge, a pump or valve issue, or a system that needs balancing. Most you can start on yourself.
Quick answer: If the boiler works but radiators are cold, the heat isn’t reaching them. Trapped air (bleed the radiators) and sludge (needs a powerflush) are the most common. A radiator cold at the bottom but warm at the top points to sludge; cold at the top but warm at the bottom points to air.
Common Causes
Why radiators stay cold
Where the radiator is cold tells you a lot. The colour on the left shows whether it’s a safe homeowner check or a job for an engineer.
| What you feel | Likely cause | What to do |
|---|---|---|
| All radiators cold | Pump or motorised valve | Check the thermostat is calling; if so, book an engineer |
| Cold at the top, warm at the bottom | Trapped air | Bleed the radiator |
| Cold at the bottom, warm at the top | Sludge build-up | A powerflush usually clears it; book an engineer |
| Some rooms warm, others cold | System needs balancing | Balance the radiator valves; bleed the cold ones |
| One radiator always cold | Stuck TRV or valve pin | Check the TRV pin isn’t stuck; free it gently |
| Radiators cold and boiler kettling | Heavy sludge | Book a powerflush |
Safe Checks
What you can safely do
Safe to do
- Bleed radiators that are cold at the top to release trapped air
- Top the pressure back up to ~1.3 bar after bleeding
- Check the TRV pin on a stubborn radiator isn’t stuck down
- Balance the valves so far radiators aren’t starved of flow
Book an engineer if…
- Radiators are cold at the bottom (sludge needs a powerflush)
- The pump is silent or grinding when heating should run
- Every radiator stays cold despite the thermostat calling
Engineer Insight
What we usually find
The position of the cold spot is the giveaway. Cold at the top is air — a two-minute bleed. Cold at the bottom is sludge settling in the radiator, which a powerflush clears. If every radiator is stone cold while the boiler runs, we look at the pump and motorised valve. Adding an inhibitor after any flush keeps sludge from coming back.
Costs
What repairs typically cost
Typical North London guide prices — you’ll always get an upfront quote before any work starts.
FAQs
Cold radiators — your questions answered
Why is my radiator cold at the top but hot at the bottom?
That’s trapped air rising to the top of the radiator. Bleeding it with a radiator key releases the air and lets it fill with hot water again. Top the boiler pressure back up afterwards.
Why is my radiator cold at the bottom but hot at the top?
That’s sludge — corrosion debris settling at the bottom and blocking flow. Bleeding won’t fix it; a powerflush clears the sludge and restores full output.
How do I bleed a radiator?
Turn the heating off, hold a cloth under the bleed valve, and open it a quarter-turn with a radiator key until water (not air) comes out. Close it, then re-check and top up the boiler pressure.
Why is only one radiator cold?
Often a stuck TRV pin or a closed valve. Freeing the pin gently usually helps. If it stays cold, it may need balancing or, if sludged, flushing.
Do I need a powerflush?
If radiators are cold at the bottom, the boiler kettles, or the system is slow and patchy, sludge is likely and a powerflush is the fix. We’ll confirm before recommending one.
Related
Keep troubleshooting
Step-by-step for cold-at-the-top radiators — the most common fix.
Read guideHeatingHeating On But House Still ColdWhen radiators are warm but rooms won’t heat up.
Read guideCentral HeatingPowerflush GuideWhat a powerflush does and when cold-at-the-bottom means sludge.
Read guideBoiler ProblemsBoiler Making NoiseKettling and sludge often go hand in hand with cold radiators.
Read guideHeatingCentral Heating Not WorkingIf the whole system’s down, not just some radiators.
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.
Cold radiators? We’ll get the heat flowing.
Book a Gas Safe engineer to bleed, balance, flush or replace the pump, or call and describe which radiators are cold — we’ll tell you what’s likely.
Gas Safe Registered 918817 | North London specialists | Upfront pricing