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How to Bleed a Radiator (and When It Will Not Help)
Cold at the top, warm at the bottom? Trapped air is the usual culprit, and it takes ten minutes to let it out.
Dana Whitlock · Oct 23, 2026 · 5 min read · 4 steps
Air collects at the top of radiators over time, especially after the system has been drained or topped up. It takes up space that hot water should fill, so the radiator heats unevenly and the boiler works harder to warm the room.
Bleeding releases that air. You need a radiator key, a cloth, and a few minutes per radiator.
Check that air is the problem
With the heating on, carefully feel the radiator. If the top is noticeably cooler than the bottom, it almost certainly has trapped air. If the bottom is cold and the top is warm, the problem is more likely sludge, which bleeding will not fix.
Bleeding, step by step
Turn the heating off and let the radiators cool first — the water inside can scald.
- Fit the radiator key to the small square valve at one top corner
- Hold a folded cloth under the valve to catch drips
- Turn the key a quarter to half turn anticlockwise; you will hear a hiss
- When water starts to come out steadily instead of air, close the valve
- Work from the radiator nearest the boiler outward, and downstairs before upstairs
Check the pressure afterwards
On a sealed (combi or system) boiler, bleeding lets a little water out, so the pressure drops. Look at the gauge on the front of the boiler: most want around 1 to 1.5 bar when cold. If it has fallen below 1, top it up using the filling loop, following the boiler manual.
If it keeps coming back
Needing to bleed the same radiator every few weeks means air is getting into the system somewhere, or the system is producing gas as it corrodes internally. That is worth a visit from a heating engineer, who may recommend a chemical clean, a power flush, or a magnetic filter.
This is general home-maintenance information, not professional advice. Building codes, permits, and safe practices vary by location — consult a licensed contractor, electrician, or plumber for work beyond your experience.