I moved to a new flat three weeks ago and it has a very old conventional central heating system with a cold water tank, hot water cylinder and a feed and expansion tank. When I moved in and saw the F&E tank (alone in the top of the wardrobe in my room) I wrongly though it was an old cistern for the toilet so I emptied it and closed the water valve. Of course, the next morning the heating started to do some estrange breathing noises and realising my mistake I opened the tank's valve filling it with water again. The problem now is that some air went into the system through the feed pipe an now the hot water cylinder sounds like filling a half empty metallic water tank every morning about 10 mins before the heating starts working. I have checked the radiators and they are hot on the top so they seem to free of air. My guess is that the air is trapped in the cylinder and that's what is making the system so noisy. Could someone offer any light on the problem? Is there an easy way to bleed the system? Many thanks.