5 March 2026
How to bleed a radiator (and when it won't fix the problem)
Cold at the top but warm at the bottom usually means trapped air. How to bleed a radiator safely, and the sign that points to a bigger issue.
If a radiator is warm at the bottom but cold at the top, trapped air is the usual culprit, and bleeding it is a five-minute job you can do yourself.
How to do it
Turn the heating off and let the radiators cool. Fit a radiator key onto the bleed valve at the top corner, hold a cloth underneath, and turn it slowly anticlockwise. You'll hear air hissing out; as soon as water appears, close it again. Once you've done every radiator, check the boiler pressure and top it up if it has dropped below about 1 bar.
Cold at the bottom is different
If a radiator is hot at the top but cold at the bottom, that isn't air, it's sludge settling in the system, and bleeding won't fix it. That usually means the system needs a powerflush and a magnetic filter to clear the debris and stop it coming back.
Bled them and still cold, or seeing black water when you do? Give us a call on 07969 110679 and we'll take a look.
