Why Is My Radiator Cold at the Bottom?
Why sludge causes cold bottoms, what bleeding can and cannot fix, and when to call an engineer.
Last updated: 2026-09-01
Cold at the top means trapped air. Cold at the bottom means restricted flow, usually sludge (magnetite) settled inside the radiator.
Safe checks
Bleed the radiator. If the water runs black, or the bottom stays cold afterwards, sludge is likely. Check whether one radiator or many are affected, and whether the boiler is noisy (kettling) or the pressure behaves oddly. All of that is useful detail for an engineer.
Fixes
Individual badly sludged radiators can sometimes be removed and flushed through. Widespread contamination across many radiators points to power flushing plus inhibitor and a magnetic filter. TRV and balancing faults mimic sludge symptoms, so diagnosis confirms the cause before any work is quoted.
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Related advice
- Why Is My Radiator Cold at the Top? — Trapped air is the usual cause of a radiator that is hot at the bottom and cold at the top. Here is how to bleed it properly.
- How to Bleed a Radiator Properly — A safe, step-by-step method for bleeding radiators, including what to do when a valve is stuck and how to restore pressure afterwards.