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CH only works when HW on (no HW off on programmer)

Hi - first post here

Trying to resolve an issue with our system at the moment. We have a Potterton Suprima boiler with a Sunvic 207 XLS programmer. I’m only able to get CH to work when HW is active. There is no HW off wiring into the programmer which I imagine could be the issue after doing some research.

However, something that confuses me... when I send a call for CH only this activates the micro switch in the three port valve and turns the spindle. So far so good.... but it doesn’t fire up the boiler.

Just wondered if this would be a wiring issue (due to not having a HW off lead on the programmer) or whether this could be a physical issue with the valve itself.

I’m a total novice but have been studying the Y-Plan and looking at our setup for a number of hours! I imagine a HW off wire could solve the problem but obviously it’s a big job running wires from the wiring unit in the airing cupboard upstairs to the programmer next to the boiler.

I’m about to get a Nest installed so wanted to resolve this before going ahead with that if I could. Would a Nest make any difference to the issue I have if I were to go ahead with the current way of working (having to put HW on when I want CH)

Any advice or thoughts welcomed.
Many thanks


Staff member
S. Mod
Gas Engineer
Could be a port valve issue eg not moving all the way to heating so the micro switch isn't contacted
Could be a port valve issue eg not moving all the way to heating so the micro switch isn't contacted
The micro switch appears to make a good connection and I’m assuming it’s turning the valve adequately but it’s the fact it’s not firing the boiler which confuses me a bit.
Just checking, is there any way the CH could operate independently without the HW off wire? From all diagrams I see it looks to be an essential part.


Gas Engineer
Check on the back of the clock. Is the top dips witch set to gravity. If it is you could have had an old gravity system at some point and whoever installed the suprima set it up like this to save running a new cable


Gas Engineer
It may be a W-plan as opposed to a Y-plan. That is Hot water priority. If it is, you'll need to completely wire the system to Y-plan. It's not as daunting as you would think. Find a competent engineer who can wire if you're not sure.
Been mapping out the wiring on the system tonight and finally got to grips with it all. It’s a Y-Plan and I’ve found a blue wire tucked behind the programmer which isn’t connected to anything and a further blue wire in the junction box upstairs, also not connected to anything. Will test it prior to connecting tomorrow but I’m 90% certain that it’s the HW off (or at least I can use it for that)

Therefore I’ll be installing the Nest heat link where the original programmer was but then I need to work out how to connect T1 and T2 on the Nest stat to the heat link - I’ll save that for another post!!
You would be better off installing the heat link next to wiring center and disconnecting the old programmer. If the blue turns out it’s not the same cable either end then you could install a relay to act the same as HW OFF.
All sorted now, thanks all for the advice and pointers

The blue wire was indeed the HW off and when I connected this up to the existing programmer the call for heat alone worked as expected. This was a big result for me!

I then intended on putting the Nest Heat Link next to the wiring center so I could use the original room thermostat wiring for T1 and T2 on the Nest.

However.... I came to find that all my live connections are located next to the boiler and fuse spare downstairs. So I’ve ended up putting the Heat Link where the programmer was and I’m using the Nest thermostat wirelessly in the living room. Probably a better location for it really but not what I originally intended.

All up and running well though which is great.