How to Bleed a Radiator (And Which Key You Need)
How to tell if your radiator needs bleeding
If the top of the radiator stays cold while the bottom heats up normally, or you hear gurgling/clanking from the pipes, trapped air is almost always the cause. Bleeding it releases the air pocket so hot water can circulate through the whole radiator.
Standard vs universal keys
Most UK radiators from the last few decades use a standard square-socket valve that any basic key will fit. Older homes or radiators from certain manufacturers sometimes use a slightly different fitting, which is where a universal set with multiple sizes becomes useful.
Always have something to catch the water
A small amount of water (sometimes discoloured) will come out once the air has escaped and before you retighten the valve — have a cloth or small container ready. Kits with a built-in catch tray remove the guesswork here.
Check your boiler pressure afterwards
Bleeding radiators can cause your central heating system pressure to drop slightly. Check your boiler's pressure gauge afterward and top up if needed — most boiler manuals explain how, and it's a simple job most homeowners can do themselves.