This is def something to do with the valve. As Andy advised put your hand round the back of the valve to feel the pin, it should have 1/2" plastic disc on the end that pushes the micro switch when the rubbers inflate with water. They are very fragile, I have had a brand new one break on me. If the disc is ok then it is def something to do with the rubbers - even though they are new you may have pierced them in putting them in & they aren't inflating. Try turing the hot tap onto full blast from closed to see if the pressure temporarily fills the diaphram & presses the switch to fire up. If there is a hole in the rubbers it will switch off as the pressure drops. It should only take a 1/4 turn on the tap to switch if the pressure is ok - this is a good indicator the rubbers have gone. The cetral heating will work because the valve is in the default position & doesn't need to switch over like DHW needs. Lee.
Lee
May 2006