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Radiator cold at the bottom but hot at the top

By Declan Barry, plumber, Stockport. Updated 2026-09-10.

A radiator that is hot at the top and cold at the bottom is full of sludge: iron oxide from the inside of the system that settles at the bottom and stops water circulating. It cannot be fixed by bleeding. The radiator needs flushing, and if most of the house is the same, the system needs a power flush and a magnetic filter.

What to do

  1. Check it is not air

    Air collects at the top and makes a radiator cold at the top, hot at the bottom. That is the opposite problem and bleeding fixes it. Cold at the bottom is not air.

  2. Feel the other radiators

    If one radiator is cold at the bottom, it can be taken off and flushed with a hose in the garden. If most of them are, the sludge is throughout the system and flushing one radiator will not last.

  3. Check whether you have a magnetic filter

    A canister on the return pipe near the boiler, usually black or silver. If you have one, it needs cleaning yearly. If you do not, that is why the sludge is building up.

  4. Do not add more inhibitor to a sludged system

    Inhibitor stops new sludge forming. It does not remove what is already there. Adding it to a dirty system is money down the drain.

When to ring a plumber

  • One radiator cold at the bottom: I can flush it off the wall in about an hour.
  • Several radiators, or a boiler that is noisy or cutting out: a power flush cleans the whole system, and a magnetic filter fitted afterwards stops it coming back.

Questions

How long does a power flush take?

Most of a day for an average Stockport semi. Every radiator is flushed in turn until the water runs clean, then the system is refilled with inhibitor.

Will a power flush damage old radiators?

Very rarely. Old radiators that are already corroded thin can weep afterwards, and I will tell you if yours look at risk before starting. Usually they are fine and heat far better.

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