hi kevin.
the problem with your system is the expansion vessle fitted in to your boiler possiblly needs to be recharged with compressed air. usual pressure is 1 bar or 15 psi, you can do this using a foot pump or a bicycle pump would do.
the vessle is usually located at the rear of the boiler if its a combination or perhapes in the airing cupboard if it a conventual system.
the vessle is coloured red.
hope it helps..
terry ellis, January 2008
