Radiators cold in places?
Where a radiator is cold tells you a lot. Cold at the top usually means air you can let out yourself. Cold at the bottom usually means sludge, which needs an engineer.
What the cold patch tells you
- Cold at the top, warm at the bottom: trapped air. Bleeding the radiator usually fixes it. Worcester Bosch, Baxi, Viessmann, Ideal, Vaillant
- Cold at the bottom, warm at the top: usually sludge building up inside. The system needs cleaning by an engineer. Worcester Bosch, Ideal, Baxi, Viessmann
- Just one radiator cold: usually air, or a valve on that radiator that's stuck or turned down. Viessmann, Baxi, Ideal
How to bleed a radiator
You'll need a radiator key (from any DIY shop) and a cloth.
- Turn the heating off and wait for the radiators to cool down. Worcester Bosch, Baxi, Viessmann, Vaillant
- Hold a cloth under the bleed valve, the small square fitting at the top corner, to catch drips. Worcester Bosch, Baxi, Viessmann
- Put the key in the valve and turn it slowly anticlockwise. You'll hear air hissing out. Worcester Bosch, Baxi, Viessmann
- When water starts to come out, close the valve again. Viessmann, Baxi
- Check the boiler pressure. Bleeding lets pressure out, so top it up if it's now low. our pressure guide has your brand's figure. Worcester Bosch, Baxi, Viessmann, Vaillant
If one radiator stays cold
- Check the valve on that radiator is turned on and set where you want it. Vaillant, Glow-worm
- If the pin on a thermostatic valve is stuck, Baxi and Viessmann both say to get an engineer to free it if you're not confident. Baxi, Viessmann
Call a Gas Safe engineer if
- Radiators are cold at the bottom: the system probably needs flushing, which is an engineer's job.
- Cold spots stay after bleeding.
- You find yourself bleeding radiators often, or you spot a leak.
- The pressure keeps dropping after you top it up.
The names after each step are the manufacturers whose own guidance backs it. Every boiler is different, so check your manual too.
Common questions
Why is my radiator cold at the top?
Trapped air collects at the top of the radiator. Every manufacturer we checked says bleeding it is the fix.
Why is my radiator cold at the bottom?
Usually sludge. Worcester Bosch, Ideal, Baxi and Viessmann all link it to a build-up inside the system, and Vaillant says flushing it must be done by a heating engineer.
Do I need to top up the pressure after bleeding?
Often, yes. Bleeding lets out air, and the pressure drops with it. Worcester Bosch and Baxi both say to check the gauge afterwards.
Sources
- Worcester Bosch: How to bleed a radiator
- Baxi: Bleed a radiator in 8 easy steps
- Viessmann: How to bleed a radiator
- Ideal: Radiator cold at the bottom
- Vaillant: Why are my radiators cold?
- Baxi: Why are my radiators cold?
- Viessmann: Why is my radiator cold?
- Vaillant: Why is my radiator not working?
- Glow-worm: Hot water but no heating
- Worcester Bosch user instructions (bleeding and leaks)
- Viessmann: Boiler pressure too low