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johnmdc, that’s absolutely brilliant and thank you for your help. Everything is working correctly now. I can’t believe it was that simple.

Thank you johnmdc. Can I confirm that this will give me two wires at Live on new programmer?

It seems the wire from T8 is doing something when connected to the new T4 as it allows the CH to function and I get some hot water even when HW is ‘Off’.

Thank you denso13. I found this online, but it doesn’t tell you what to do with wire at old T8. I tried not connecting (isolating) the wire and not connecting to ST9400c, but then I couldn’t even get Central Heating (and no Hot Water)!

Thank you CBW for responding. Photos of existing wiring on ST94000c and display with HW on ‘constant’ selected. Apologies, I meant Drayton Mid Motorised Position valve. Note: the numbered labels are as they were connected on ST966, with T8 connected to T4 on St9400c.

Hi, I recently replaced a 27 year old Honeywell ST699 Programmer with a new ST9400c. Whilst system delivers Heating and Hot Water, it will not deliver Hot Water when HW only is selected (despite green LED being illuminated) i.e. boiler does not fire. My wiring changes were as follows: ST699...