When we had the bathroom radiator removed and reinstalled to allow for the laying of the new travertine tile floor earlier this year the plumber never refilled the radiator system. All eight radiators are 75-80% full of air which is surely not a good thing.
I called around and was advised by a heating contractor that it would be best to bleed them myself, it's a time-consuming job and not difficult to do. He advised that we turn the system on which would circulate the water, hold a container under the valve when we do it and start at the bottom of the system and work our way up. He said we might need to go around top to bottom more than once to get all of the air out.
Tracking down a radiator valve key was a little challenging but Ace Hardware carries them, though my favorite Ace store in Seattle --Tweedy and Pop's -- didn't have them in stock. They promised me that they would order one in for me and have it as soon as Tuesday.