Why Is My Radiator Hot at the Top but Cold at the Bottom?

Why sludge builds up

Central heating systems circulate water through radiators, pipework and the boiler continuously over years. As steel radiators corrode internally, they produce a black iron oxide sludge that settles wherever water flow is slowest, typically the bottom of radiators. Older systems, systems without a magnetic filter, and systems that haven't been flushed in a long time are most prone to this.

Mixing metals within a system, such as aluminium boiler components alongside steel radiators, without the correct inhibitor chemical in the water can also accelerate corrosion and sludge formation.

Why bleeding won't help

Bleeding a radiator releases trapped air, which sits at the top of the radiator, not sludge, which sinks to the bottom. Bleeding a radiator with a sludge problem simply lets a small amount of air out and does nothing for the actual restriction, though it can occasionally bring a small amount of discoloured water with it, which is itself a useful clue.

What actually fixes it

Signs the whole system, not just one radiator, needs attention

• Several radiators showing the same hot-top, cold-bottom pattern
• A boiler that's noisy or seems to be working harder than it used to
• Radiators that need frequent bleeding
• Discoloured water when you bleed a radiator
• Central heating that takes noticeably longer than it used to to bring rooms up to temperature

What you can check before booking a flush

Feel each radiator by hand from top to bottom to map exactly which ones are affected and where the cold section starts. This information helps an engineer judge quickly whether the problem looks isolated or system-wide, and speeds up the initial assessment when they arrive.

What this costs to put right

Heating engineer time for diagnosing and flushing radiators is charged at our standard £99 per hour daytime rate, with a one-hour minimum then billed in 30-minute increments. A single radiator removal and flush is generally quicker than a full system power flush, so it's worth having an engineer confirm the scale of the problem before assuming the whole house needs treating. Any parts required, such as a replacement magnetic filter, are quoted separately before they're fitted.

Local water and older systems in Cheam and Sutton

Hard water across Cheam and the wider Sutton area doesn't cause the black sludge itself, which comes from internal corrosion of steel radiators, but it does add limescale into the mix in systems with an unvented cylinder or older components, compounding the restriction. Properties with heating systems more than fifteen or twenty years old, or that have never had a magnetic filter fitted, are the ones we see this pattern in most often.

If your home still has its original 1980s or 1990s radiators and pipework, it's worth asking whether a filter and inhibitor were ever added when the boiler was last replaced. If not, this is a sensible thing to raise the next time an engineer is on site for a service or repair.