hi, the problem is the diaphragm in the diverter vavle, this look at a diffirential pressure between the heating system pressure and the domestic cold/ hot water pressure, when you turn on the tap the cold water/hot water being a biger pressure pushes the diaphragm, and at the back of the diverter is a micro switch ,240 volt be carefull not to put your hand around that switch cover, stiking out of the back of the valve is a small shaft and a disc on the end which pushes on the micro switch and in turn starts up the gas valve for the hot water,if the diaphragm is damaged ie not moving the shaft to operate the micro switch then you get no hot water ie will not fire up,for hot water,to check this have someone turn on the tap while you with the front cover off, look and listen for the click of the switch at the back of the diverter valve it looks like, a brass body with a 3to4" diameter diaphragm housing with 6 screws around the brass outer edges and the rest of the body has about 5 pipe connections.that will do for now just to prove the fault.to change this diaphram is anonther long story. do not play with the gas[bang]. bobby rob.
bobby rob, May 2006