How to bleed a radiator — step by step
Over time, air naturally gets trapped in your central heating system — it rises to the highest points, which is usually the top of your radiators. When air pockets lodge in there, they act like a barrier, preventing hot water from circulating properly and leaving you with cold patches or entirely cold radiators. This is one of the most common heating complaints, particularly at the start of the season or after the system has been drained for maintenance. The good news is that bleeding radiators is a straightforward job that most homeowners can tackle in under an hour with just a bleed key — no special tools or plumbing knowledge required. This guide walks through the process step by step so you can restore full heat to your radiators and keep your home comfortable. If you're unsure about your system pressure or the filling loop, or if bleeding doesn't solve the problem, it's worth calling a Gas Safe registered engineer to rule out deeper issues.
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Turn the heating on and allow the system to fully warm up (15–20 minutes). This allows air to collect at the tops of radiators where the bleed valve is.
Starting with the radiator furthest from the boiler, insert the bleed key into the square slot on the bleed valve (usually at the top, back corner of the radiator).
Turn the key anti-clockwise slowly — a quarter to half turn. Hold a cloth beneath the valve. You will hear hissing as air escapes.
As soon as water trickles steadily from the valve — with no air sputtering — close it firmly clockwise. Do not overtighten; the valve seat is soft and can be damaged.
Work through all radiators from furthest to nearest to the boiler.
After bleeding, check the boiler pressure gauge. Bleeding releases water and drops the system pressure. Repressurise to 1–1.5 bar using the filling loop if the gauge has dropped below 1 bar.
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Frequently asked questions
How do I know if my radiator has trapped air?
A radiator with trapped air will feel warm at the bottom but cold at the top, or have cold patches across its surface. You might also notice it's noticeably cooler than other radiators in the house, even when the heating is on full. If most of your radiators are affected, it's a sign air has entered the system.
Do I need to turn off my boiler before bleeding radiators?
No — in fact, you should turn the heating on and let it warm up fully first so air rises to the bleed valve. However, always turn the boiler off before you begin bleeding to avoid scalding yourself on hot water.
What should I do if my system pressure drops too low after bleeding?
After bleeding, check your boiler's pressure gauge and use the filling loop to repressurise if it's dropped below 1 bar — aim for 1–1.5 bar. The filling loop is usually located beneath the boiler and consists of a flexible hose with isolating valves at each end; your boiler manual will show you exactly where it is.
Why is water still trickling out but my radiator is still cold?
If you're getting steady water flow but the radiator won't warm up, the problem may not be trapped air — it could be a blocked radiator, a stuck thermostatic valve, or a more serious system issue. In this case, stop and contact a qualified heating engineer to diagnose the problem properly.