how to bleed a radiator

[waggstar]

If one wishes to bleed a radiator does one leave the water (main water feed into the central heating system) on or should it be switched off?

[Postdog]

Leave it on. Really, it is just a five second job for each radiator. Turn the key slowly, and air should hiss out. As soon as water starts to come out, turn the key back to seal the rad, then move on to the next.

[jno]

and try not to do it while the radiator is actually heating as the water can scald. I usually wrap a towel around the hand holding the key, just in case.