Why is the water not heating up?

with you saying ch valve im guessing you have an s plan system with a seperate valve for ch and for hw.

if hw works when heating on but not independantly then it must be the hw valve is getting power to open up and allow water flow (or has failed to open position) but not sending power down orange wire to fire up pump and boiler. which is why it works with both on cos the ch valve is supplying the live circuit to the boiler.

maybe valve not moving over enough to hit microswitch or burnt out.

an engineer and a multi meter should be able to tell you in minutes whats up for definate

iain
May 2008