If your radiator is warm at the top but stubbornly cold along the bottom, it is almost always trying to tell you something. The good news is the cause is usually well understood, and in many cases you can diagnose it yourself in a few minutes before deciding whether to call anyone out.
A radiator that is hot at the top and cold along the bottom is the classic sign of magnetite sludge, a black, gritty iron oxide that forms inside steel radiators over the years. It is heavier than water, so it settles in the bottom of the panel and blocks the hot water from circulating through the lower section.
Cold patches in the centre and bottom, rather than evenly across the radiator, point to sludge rather than air. If several radiators are affected, or your boiler is short-cycling and your heating bills are creeping up, the whole system is likely carrying a build-up that needs dealing with at source.
Before assuming the worst, rule out the simpler issues. Trapped air usually sits at the top of a radiator, so a cold top with a warm bottom points to bleeding rather than sludge. A radiator that is cold all over may simply have a valve turned off or a stuck thermostatic head.
For a single affected radiator, we can isolate it, remove it and flush it through outside, which clears most localised build-up. Where several radiators are cold at the bottom, a power flush is usually the right answer. This pushes water and cleaning chemicals around the whole system at pressure to shift the sludge that a single flush cannot.
As a guide, a power flush in the Essex and London area typically runs from around £350 to £700 depending on the number of radiators and the state of the system. We always finish by dosing the system with inhibitor and, where sensible, fitting a magnetic filter so it does not return.
Sludge is a maintenance problem, not a one-off fault. The single best protection is a magnetic filter on the return pipe near the boiler, which catches iron oxide before it settles in the radiators and is easy to clean during an annual service.
Keeping the correct level of central heating inhibitor in the system is just as important, as it stops corrosion forming the sludge in the first place. If your boiler was installed without a filter, retrofitting one is a small job that pays for itself in efficiency and longer radiator life.
You can try opening both valves fully and bleeding any trapped air, but cold patches low down are usually sludge, which needs the radiator removing and flushing or a system power flush to clear properly.
It is not dangerous, but it wastes energy and makes your boiler work harder, so the sludge causing it can shorten the life of your radiators and pump if left untreated.
Most domestic power flushes take between three and six hours depending on how many radiators you have and how heavily the system is sludged.
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