If your radiator is hot at the top but cold at the bottom, the problem is almost certainly sludge — not trapped air. Here's why and what to do about it.
Why it happens
Over years of use, iron oxide particles (from corrosion inside your steel radiators) circulate in the heating water and gradually settle at the bottom of radiators. This layer of black sludge acts as an insulating blanket, preventing hot water from heating the lower half of the radiator.
Bleeding won't fix it
Unlike a cold top (trapped air), a cold bottom can't be fixed by bleeding. The sludge needs to be physically removed — either by isolating and flushing the individual radiator, or preferably by a full power flush of the entire system.
DIY radiator flush
An individual radiator can be flushed by isolating it from the system, removing it from the wall, taking it outside, and flushing it through with a garden hose. It's messy work but effective. Wear old clothes.
Power flush for whole-system sludge
If multiple radiators have cold bottoms, the sludge has spread throughout the system and a professional power flush is the most effective solution. After a power flush, fit a magnetic filter to prevent future build-up.
How to prevent sludge
The best prevention is a magnetic filter on the return pipe, filled with system inhibitor. A filter catches iron oxide particles before they can settle. We fit Adey MagnaClean filters as standard on all new boiler installations.