No heating unless hot water is on

[Xavicc]

I've got a strange fault with my y plan heating system, where the heating will not go on unless my hot water is turned off, this is more strange however as it is intermittent. I've got a st9400c controller which I suspect is the fault and I plan on upgrading with tado, is anyone able to shed a light on the likelyhood of the programmer causing the intermittent fault. I'm almost certain that its the programmer, as when I switch the hot water from 'constant' to 'off' the C/H works.
or am I wrong and could it also be the 3 port causing the issue?

 

[CBW]

It could be the motorised valve actuator, as this could stay in mid position and not be motoring over to heating only.

 

[Xavicc]

It could be the motorised valve actuator, as this could stay in mid position and not be motoring over to heating only.

Thanks, thats a good theory, that would make sense as to why its intermittent, am I best to replace the entire unit or just the powerhead, its around 14 years old and is a honeywell 3 port

 

[Xavicc]

I could also replace the synchron and see if that makes any diff before replacing

 

[CBW]

am I best to replace the entire unit or just the powerhead, its around 14 years old and is a honeywell 3 port

Well, it depends if the unit is needed. If not, it’s cheaper for the powerhead. If you remove it and can move the spindle using finger and thumb, or gently with an adjustable spanner then just the powerhead.

I could also replace the synchron and see if that makes any diff before replacing

if it’s the synchron, yes you could.