How to Bleed a Radiator (and Why Cold Tops Mean Trapped Air)

If a radiator is warm at the bottom but cold at the top, trapped air is blocking the hot water from filling it — and "bleeding" it (letting the air out) is a five-minute fix that restores even heat and cuts wasted energy. Four steps: diagnose, turn off the system, open the valve slowly, and close it when water appears.

How do I know a radiator needs bleeding?

Feel for a cold top with a warm bottom. That temperature split is the classic sign of trapped air: hot water fills the bottom but can't rise past the air pocket at the top, so the radiator never fully heats and the room stays cool. Cold spots, gurgling, or a radiator that's slow to warm all point to trapped air that bleeding will fix.

Should I turn off the heating before bleeding a radiator?

Yes — turn off the heating system before bleeding and let the radiators cool. Bleeding with the system running (and the pump on) can draw in more air or spray hot water, and you can't tell the air out from the water. Turning it off and letting it settle makes the job clean and safe.

How do I open the bleed valve?

Open the bleed valve slowly with a radiator key (or a flat screwdriver, depending on the valve) — turn it a quarter to half turn counterclockwise. You'll hear a hiss as the trapped air escapes. Hold a cloth or small cup under the valve to catch drips. Go slow; you only need to crack it open enough for the air to vent.

When do I close the bleed valve?

Close the valve as soon as water starts to dribble out — that means all the air is out and only water remains. Snug it closed (don't overtighten). After bleeding, check your boiler's pressure (bleeding can lower it) and top it up if needed per the boiler basics. Then turn the heat back on and confirm the radiator now warms evenly top to bottom.

Track the fix

Logging which radiators you bled (and how often air returns) can flag a bigger system issue. Okoniq Property Hub keeps it with your home maintenance records in one private place.

Frequently asked questions

How often should I bleed my radiators?

Typically once a year before heating season, and whenever you notice cold tops or gurgling. If air keeps returning quickly, there may be a leak or a system fault worth a pro's look.

Why does air keep getting into my radiators?

Frequent air buildup can mean a leak letting air in, corrosion, or a pump/expansion issue. Occasional air is normal; persistent, fast air buildup warrants a heating engineer.

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