BoilerTroubleshooting2026-02-26
Radiator Cold at the Top: Trapped Air
A radiator cold at the top simply needs bleeding to release trapped air.
Cold tops with warm bottoms is the classic sign of trapped air — an easy fix.
Why it happens
Air rises and collects at the top of the radiator, blocking hot water from filling it fully.
How to fix
With the heating off and cool, open the bleed valve until air hisses out and water appears, then close it and recheck boiler pressure.
Key takeaways
- Cold top = trapped air
- Bleed with a radiator key
- Re-check pressure after bleeding
- Cold bottom is sludge, not air
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