Radiator cold at the bottom? It's probably sludge.

What's happening inside the radiator

Central heating systems develop rust and debris over time. Water plus steel plus oxygen plus heat produces iron oxide sludge, dense black silt that settles at the lowest point of the system. In a radiator, that's the bottom.

Once sludge coats the bottom of the radiator, hot water can't circulate through it. The top stays hot (via conduction), the bottom stays stone cold. You end up heating only half the radiator's surface, wasting gas and leaving the room cold.

Diagnosing how bad it is

Run your hand down the front of the radiator while it's fully on. Start at the top, slowly move down. The point at which it suddenly goes from hot to cold is the top of the sludge layer. The bigger that cold area, the more sludge.

If only the bottom 20cm of a single radiator is cold: usually flushable in isolation. If multiple radiators show the same pattern: the whole system is affected and needs a proper power flush.

The quick fix that isn't

A lot of YouTube guides suggest closing all other radiators, running the cold one hot, then opening the valves to force water through. This does help briefly, but the sludge re-settles in days. It's a patch, not a fix.

The proper fix is either single-radiator flushing (for one problem rad) or a full power flush (for systemic issues). A full flush adds an inhibitor to prevent the sludge coming back, so the problem stays fixed for years.

When itʼs engineer territory

If one radiator is cold at the bottom: it may clear with a weekend of balancing. Give it a try first.

If multiple radiators show the same pattern, or you've tried balancing and it's still cold: book a power flush. System-wide sludge is only clearing with proper chemistry and a flushing rig.

If the boiler is also making banging or kettling noises, the sludge is in the boiler's heat exchanger too. That's urgent, leave it too long and the heat exchanger fails (£400+ part).