Honeywell controller has stopped working for HW (no control light comes on - no signal to boiler) but does still work for CH. Is this repairable or do I need a new one.

we had one of these and found out the battery inside had failed. the battery is none replaceable. did try to replace it but it lost its memory and was only good as a switch. think when the battery fails it loses it memory and things start going wrong. so would say change it. now thinking on it should of changed the battery why having the old one on and taken the old one off after installing the new one.

If this a honeywell valve you may need to change the motor in side the valve , sounds like its lost some of it power to move the inside of the valve to y postion .

Yes it could be something other than the motor but I think you are right yo start there.Good luck and thank you

diverter valve may not be seating on ch side letting water to pass.

this will be your diaphram inside your divertor valve.
the one that looks like a spaceship.
it will e split.

with y plan 3 connections on programmer HW on HW off CH on HW on to cylinder stat then HW call to Orange on Mvalve to power boiler and pump HW off to grey wire on Mvalve to close hot water CH to room stat common then heating call to white on Mvalve

have the batteries died have you got the programmer set back to timer control or is it still in the set mode

If the valve runs at other times, the valve is OK. you have a relay somewhere that is not sending signal to the valve when in that mode.

Trace down all the relays for that cycle and check voltage going int, coming out, and to see if the coil is enrgised.

HW valve was stuck in the open posiion and the micro-switch was not working so thats why boiler and pump were not starting. Bled system to below the level of the valve to be changed, and replaced valve with a drayton ( easy quick change of valve head if needed no screws )

Hi check the wiring to the cylinder stat is there power to it then is it going to the valve