Trapped air rises
Air in a heating system collects at the highest point of each radiator, stopping hot water from filling the top. The radiator feels cool at the top and normal lower down. Upstairs radiators are often affected first.
How to fix it
Bleeding the radiator releases the trapped air. On sealed systems, check the boiler pressure afterwards and top it up if it's dropped below the normal range.
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If it keeps happening
Air returning repeatedly can point to:
- A small leak letting air in
- Corrosion inside the system producing gas
- A pump or system design issue on older open vented systems
- Low inhibitor levels in the system water
