Drayton MA1 3-port valve

but you would expect that to hold the valve in the heating position,not Mid

If there was a feed from both terminals, then in mid position.
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if there was a feed from both terminals,then after the central heating call the valve would drive fully open to heating then be held there permanently
only a call on the white wire can hold the valve in mid position

Hello Matt.

Engineer replaced the Cylinder stat.
He said that he had dealt with the exact same problem last week and had done the same - and has heard nothing untoward about it since.

He said that a thorough examination of the system's wiring would be quite a lengthy procedure - and said that if it was a wiring problem then the chances were that it would be a "constant" problem rather than an "intermittent" one. He did say that if my problem did recur then a full wiring examination of the system would then have to be done to trace its source. He said the wiring on my system would be difficult to examine as "everything is wired in from the back" and a major disconnection preocess would have to be undertaken to thoroughly examine it.

Hopefully it is the cylinder stat which was at fault - but I have a feeling that I haven't had the last of this problem.
Best wishes
Well - blow me down!
The problem has made a come-back, after exactly 2 weeks.
Exactly the same issues as before.

The only question that begs an answer ( given that it simply HAS TO BE a wiring glitch) is why the hell the engineer who called did everything he could to avoid having to look at the wiring centre box.

Is it potentially that complicated that he had to replace the cylinder stat rather than investigate the possibility/probability that the wiring is at fault here.
Is a multi-meter difficult to use? Are we talking borderline rocket science here? I mean , for god's sake!!!! So - new 3-port, new cylinder stat. The only component which remains to be replaced is the electronic programmer! Is it possible that they will replace that as well, as a possible cause of the problem, rather than check the bloody wiring?!

And while we are on that question - is it indeed possible that the programmer could be a cause of the problem as described rather than a wiring fault? It would be handy to know if that would be too far fetched before I call them out yet again!!!!!!!!!!!!
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Is your terminal box a normal 12 piece connector block, or is it a prewired wiring centre? Is it also close to the 'Y' plan and cylinder stat? If so, then checking the wiring is not too difficult to pick up and check. Given that it mostly works, especially after powering down, then it is reasonable to assume it is wired correctly, to identify the individual connections. If it is a prewired wirng centre, then swap it for a standard junction box, it's much simpler check.

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