Radiator cold at the bottom? Here’s why, and the fix

Why sludge collects at the bottom

The water in your heating slowly corrodes the steel inside radiators and pipes. That corrosion produces magnetite, a black, magnetic sludge. Because it’s heavy, it settles along the bottom of each radiator, where the water moves slowest.

Hot water still flows across the top, so the radiator feels warm up there, but the bottom stays cold. Left alone, the sludge builds up and the cold area creeps higher.

One cold radiator or all of them?

One radiator cold at the bottom usually points to that radiator or its valves: sludge has gathered there, or a valve is partly blocked.

Several radiators cold at the bottom, especially with black water when you bleed them, means sludge is spread through the whole system. That’s when a full powerflush makes sense rather than treating radiators one at a time.

Is it costing me money?

Usually, yes. When sludge blocks the radiators, the boiler has to run longer to get the rooms up to temperature, and you’re paying for heat that never reaches you. How much depends on how bad the build-up is, so we won’t put a number on it. What people notice after a flush is that the house warms up faster and every radiator is hot.

How to fix it: one radiator vs the whole system

A single radiator can be taken off the wall and flushed out with a hose. It helps, but the sludge in the rest of the system soon finds its way back.

A powerflush cleans the whole system in one visit: a machine pushes water and a cleaning chemical through the boiler, pipes and every radiator at high flow but low pressure, flushing each radiator in turn until the water runs clear. See how a powerflush works.

How to stop it coming back

Three things keep a system clean after a flush: inhibitor in the water to slow corrosion, a magnetic filter on the boiler’s return pipe to catch new debris, and an annual boiler service where the filter is cleaned and the inhibitor checked. More on magnetic filters.