Could be a number of things, first check to see if there is water in the F&E tank, check to see if there is air in the system, should be a manual or auto air vent in the pipework at the side of the cylinder or it could be a fault with the motorised valve, do you have a seperate one for the HW, most systems have one 3 port valve which on rare occasions can get stuck in the heating position, if you have two 2 port valves then the HW one may have failed, often there is a small lever sticking out th side which can be manually moved to the open position, good luck hope this helps.
Martin, October 2008
