Cold at the top, hot at the bottom
This usually means trapped air. Air collects at the top and stops hot water filling the radiator. Bleeding it with a radiator key normally fixes it, as our guide to how to bleed a radiator explains. Turn the heating off and let it cool first, and check the boiler pressure afterwards.
Cold at the bottom, hot at the top
This is more often sludge. Rust and debris from the system settle at the bottom of the radiator and block the flow. Bleeding will not fix this.
Cold in patches
Cold bands or patches across a radiator usually mean sludge or scale building up inside. It is more common in older systems and tends to spread over time.
How to tell if it is the whole system
Switch the heating on and give it 20 to 30 minutes. If several radiators have cold patches, the house is slow to warm up, or the water is black when you bleed a radiator, the system probably needs cleaning. If only one radiator is cold all over, the cause may be a stuck valve instead, so see why radiators stay cold when the boiler is on.
What fixes sludge
A power flush cleans the system. Our guides explain what a power flush involves and whether you need one. A magnetic filter afterwards helps keep debris out.
Cold patches that keep coming back? Ask our Gas Safe engineers about a power flush.
Ask about a power flush
