Radiators cold at the bottom?
It's probably sludge.
Cold at the top usually means air. Cold at the bottom usually means sludge. Here's the difference, and what to do about each.
Cold at the top: air
Air collects at the top of a radiator. Bleeding it with a radiator key normally fixes it. On a sealed system, check the boiler pressure afterwards.
Cold at the bottom: sludge
Over the years, heating water and steel radiators react and form a black sludge of rust and debris. It settles at the bottom of radiators and restricts flow through pipes and valves.
Tell-tale signs: black water when you bleed a radiator, some rooms never getting warm, a noisy boiler, and the heating taking longer to warm up.
How it's fixed
A chemical clean circulates a cleaning agent through the system, then each radiator is flushed to clear the loosened sludge. The system is refilled with clean water and inhibitor to slow new corrosion.
A magnetic system filter on the heating return catches new debris before it reaches the boiler.
Cold spots, noisy pipes and slow-to-heat rooms are usually sludge. A chemical clean flushes it out and gets the system flowing again.
Cold spots in
your radiators?
Tell us how many radiators you have and we'll quote a chemical clean before any work starts.