How to bleed a radiator
If a radiator is cool at the top but warm at the bottom when the system has been on for a while, it probably means there is air trapped in it.
Bleeding the radiator will remove the air and restore the radiator to proper working order.
Step 1 - Turn TRV to high . . .
First, make sure that the manual or thermostatic radiator valve (TRV), at one of the bottom corners of the radiator, is turned as high as it will go.
You don't need a key for this: the valve head is turned by hand.
Step 2 . . . Or open manual valve
If there is no TRV, you must have a manual valve.
To fully open it, a manual valve is simply turned anti-clockwise by hand until it stops.
Step 3 - Run system
Run your central heating system for ten minutes, then turn it off.
Step 4 - Locate air release vent
You will find the air release vent at one of the two top corners of the radiator. It is recognisable from the square-headed vent screw in the middle of the vent plug. In some cases, the vent screw is set in what looks like a plastic insert in the air vent plug.
There may also be a slot for a screwdriver in the end of bleed screw but it's usually easier to use a key, which won't slip so easily.
Step 5 - Find an old cloth
Make sure you have an old cloth or rag to catch the fine spray of water you will get at the critical stage of the operation.
Step 6 - Fit and turn bleed key
Fit the bleed key onto the bleed screw and turn anti-clockwise. If the valve is stiff or has been tightened very firmly, this may require quite a bit of pressure.
Hold the cloth in your other hand, directly underneath the bleed valve. After turning the key a short way, you will hear air escaping from the valve.
Step 7 - The squirty bit
When water begins to squirt out freely, all the air has been released.
Step 8 - Re-tighten bleed screw
Quickly re-tighten the bleed screw, nipping it up fairly tightly, but not so tight that you damage the bleed screw.
If the corners of the bleed screw wear, it may no longer be possible to turn it and it will have to be replaced.
Step 9 - Re-adjust TRV
Turn your TRV or manual valve back to the desired setting.
Step 10 - Turn system back on
Turn your central heating system back on. You should now find that the radiator gets hot all over.