Maintain it · practical guide
How to bleed a radiator (and why it goes cold at the top)
In one line: Trapped air is the usual culprit for a radiator that is hot at the bottom and cold at the top — a ten-minute fix with one pressure check at the end.
If your home has radiators and one is hot at the bottom but cool at the top, it is holding trapped air. Air gathers at the top of the radiator and insulates exactly the part that should be hottest. Bleeding — letting the air out — is a ten-minute job needing one cheap key and one careful check afterwards.
The signs and the setup
Cold at the top, warm at the bottom: air. Cold all over while neighbours on the same system are hot: a valve or system problem instead — that is not a bleeding job. To bleed, you need a radiator key (a couple of coins or a flat screwdriver fit some modern valves), a cloth, and a small bowl. The heating should be off and the radiators cool — bleeding a hot, running system sprays and sucks in more air.
The bleed, step by step
At one end of the radiator's top edge is the bleed valve. Fit the key, hold the cloth and bowl beneath, and turn slowly anti-clockwise about a quarter to half a turn. You will hear a hiss — that is the air leaving. When water starts to seep steadily (not spray), close it: hand-tight plus a small nudge is enough. Work around the house radiator by radiator. The commonly-suggested order is to start with the radiators furthest from the boiler or upstairs first; systems vary, and none of it is sensitive enough to fear — the point is simply to do every radiator, not just the noisy one.
The step that matters afterwards: pressure
Bleeding releases pressure as well as air, and boilers are fussy about theirs. After bleeding, check the boiler's pressure gauge: most sealed systems want the needle roughly in the 1 to 1.5 bar zone when cold (your boiler's manual states the exact band). If it has dropped below, repressurise using the filling loop per the manual's instructions — two minutes with a tap-like lever. If the radiator is still cold at the top after a proper bleed, or you bleed weekly, the system has a deeper issue (corrosion sludge or a failing pump) and that is a heating engineer's visit. General information, not professional advice — and the boiler manual beats any website for your exact model.
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General information, not professional advice. Homes differ — if a job is beyond your confidence or the guide’s boundary, that is what tradespeople are for.