By Mohammed Miah, Gas Safe registered heating engineer
If a radiator is warm at the top but cold across the bottom, it's almost always sludge, a build-up of rust and debris that's settled at the bottom of the radiator and is blocking the hot water from circulating through that section.
This is different from a radiator that's cold at the top but warm at the bottom, which usually means trapped air and just needs bleeding.
What actually fixes it
Bleeding won't fix a sludge problem, the debris is a solid deposit, not air. For a single affected radiator, taking it off and flushing it out can help. If several radiators across the house are affected, that's a sign the whole system has a build-up, and a system flush is the proper fix, it clears the entire network of pipework and radiators, not just one.
A magnetic filter fitted near the boiler afterwards catches new debris before it can build up again, so it's worth considering at the same time.
