Central heating switches off when cylinder stat up to temp

6 Dec 2016
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United Kingdom
Conventional gravity fed system with 3 port valve and HW cylinder stat - unless the cylinder stat is triggered, the CH pump will not run. Consequently, HW temp has to be up much higher than I would like, in order to heat the radiators. System is Danfoss AT tankstat and Danfoss programmer, and a Sunvic 2701 valve. This has been the way it operates for years, and it's about time it was sorted - is it more likely to be an original wiring error or a fault in the 3 port valve Sunvic head (which I don't recall ever replacing over the last 17 years)? I did change the valve body last year however. In summer months with CH off, I can have HW set at lower temp, and all works fine without the CH. Grateful for any advice or insight.
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When heating only is required, the motorised valve winds fully across to close off the hot water port. To get to that position, a 'hot water not required' signal is needed. This comes from either the hot water cylinder thermostat when the cylinder is hot, or the programmer when the hot water is selected 'off'. If this signal is not present, then the valve will not move to the heating only position, which it needs to do in order to operate a small microswitch inside the motorised valve, which inturn energises the orange wire that switches on the boiler / pump when only the central heating is required.

So my guess would be either 1) The grey wire to the motorised valve is not receiving the 'hot water not required signal', or 2) The signal is present, the valve moves to the correct position but the microswitch is not operating. Both would generate the fault you describe.

The only real way to check is for someone who knows how to use a multimeter and can do so safely to test the various components in isolation.

You could try switching off the hot water at the programmer and just have on the heating and see what happens then. If the 'hot water not required' signal from the cylinder thermostat has failed the programmer should still provide it and the heating may come on. If it does you know what the problem is. If it doesn't, it's back to plan A with the multimeter.
Thanks for your detailed reply- I've never been happy wielding a multimeter on A.C., so I will try your other suggestion and report back.
Ok, so with the hot water switched off on the programmer, the heating will not run with the tank stat either on or off. Does this indicate valve head at fault somewhere? I could just replace the head - the cost is not excessive.
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All that test shows is the valve is not getting (or responding to) the HW off signal. The next step is to carry out voltage testing (or call someone in to do that) to find the fault. It would be a waste of time, effort and money to replace the valve if the wiring either isn't in place, or the fault is a simple loose connection. Testing will take less than 5 minutes.

I assume you have checked you actually have a HW off wire connected to the programmer and cyl stat?
A simple way to assess your problem.
Can you indentify the CH only position.
With CH only selected, does the valve move to the CH only position.
If yes, the problem is most likely the micro switch so nothing comes out of the orange wire to fire the boiler.
If No, then either there is no power reaching the Grey wire of the valve or if there is power, the other micro switch is the most likely culprit.
Micro switches become burned and result in contacts not making when they should or not breaking when they should.
So first check,, does valve move to CH only position,

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