I dont know your type of boiler or system, but it sounds like your 2 way valve. Does it have a lever on the side? If it does give it a push, does it now heat the other pipes? if it does the valve is shot. there are 2 ways of thinking is it mechanicley shot or is its electicaly shot, if by moving the lever it works, you can replace the gubbins with out distubing the plumbing
Colin, January 2009