How to Bleed a Radiator (UK)
Bleed air out of central heating radiators. Bleed key size, cold-top diagnosis and repressurising.
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Diagnosis
Cold at the top, hot at the bottom = air in the rad. Cold at bottom, hot at top = sludge — power flush required.
Bleed procedure
Boiler off. Rad valve fully open. Insert bleed key (4 mm or 5 mm sq). Turn 1/4 anti-clockwise. Hold cloth beneath. Close as soon as steady water appears.
Repressurise
Bleeding removes water — check pressure gauge afterwards. Top up to 1.2 bar cold via filling loop.
Frequently asked questions
- Should I bleed with the pump on or off?
- Off — pump running pushes air past the vent instead of collecting at the top.
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