Radiator Hot at the Top, Cold at the Bottom? Don’t Just Bleed It
A system flush and a magnetic filter can restore heat output and protect your boiler.
You’ve turned the heating up, but one radiator still won’t warm evenly. Where it feels cold is the useful clue: trapped air and sludge need different approaches.
Three things to know.
- Bleed radiators to fix cold spots at the top
- Look for black water when bleeding as a sign of sludge
- Install a magnetic filter to catch debris before it reaches the boiler
Cold top vs cold bottom
Cold at the top usually means trapped air, which bleeding fixes. Cold at the bottom or cold in the middle usually means sludge, which bleeding will not fix.
If the water from a bleed valve runs black, that is a strong sign of sludge in the system.
Why it matters
Sludge reduces heat output, makes the pump work harder and can block the narrow heat exchanger in a modern boiler. Manufacturers often require the system to be cleaned when a new boiler is fitted for the warranty to apply.
Treatment options
- Chemical cleanse: a cleaner circulates through the system, then is drained.
- Power flush: a pump pushes water and cleaner through each radiator at high flow.
- Magnetic filter: fitted on the return pipe to catch debris continually.
- Inhibitor: added after cleaning to slow future corrosion.
For appliance-specific steps, follow your boiler or product manual. Work inside a gas boiler must be left to a Gas Safe registered engineer.