potterton suprima and ch controller problem

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I have a holiday house fitted with a potterton suprima boiler and a roomstat. When I last visited I noted that the Central Heating hadn't been working and that the roomstat had stopped clicking. I checked he power supply to the roomstat and then replaced it with a new one which seemed to work OK, but the boiler shut down soon after firing up and showed a red lock out light. At this time the radiators were still cold but the DHW cylinder was hot. I called a CH engineer who suspected the PCB without investigating further and then briefly looked at the motorised three-way valve in the airing cupboard but I am not sure whether he did anything to it. Howevem the CH then started working. The CH Engineer seemed to think it was a faulty diverter valve and left having promised to come back in my absence and change it. After he left the CH heated up uncontrolled until I switched it off at the control panel, and I once again noted that the new roomstat had stopped clicking. I switched the system off when I left the house, returned home and waited for the CH man to call me to tell me he had changed the diverter valve and all was well. However, when he called he said he hadn't done anything as he had maybe changed his mind about the valve, had noted that the boiler seemed to be bypassing the roomstat, and once again suspected the PCB, but was generally stumped and would speak to Potterton and call me back, which he hasn't yet. Two roomstats failing one after the other seems too much of a coincidence to me and the replacement clicked fine for a couple of hours until the CH fired up. I have no confidence the CH engineer will identify a fix from his present position and am facing the prospect of returning to a freezing holiday house and starting again with someone new. Does anybody recognise these symptoms and able to advise the likely fault?

Thanks chaps

Sophierose
 
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The pcb on the suprima has a history of problems. The second version was not that much better than the first version. However the latest version which includes all the wires ready attached and encased in plastic box is far better. No problem over last two years.
The fact that the boiler operates suggests the pcb is not the problem.

Now the 3 port valve is the heart of controlling and directing the heated water to where it should be going.
The HW side is easy to understand and does not really involve the 3 port valve, except the valve is spring loaded. Without power to valve, the spring closes off CH and leaves HW port open.
The CH side is quite complicated.
Briefly the programmer supplies power to the room stat.
The room stat then provides the power to the valves white wire, which in turn supplies two micro switches.
The power drives the valve to mid position only, where the power is cut off but replaced from a different source so the valve can continue to CH position.
With the valve in the correct position, one of the switches feeds power to light the boiler.
You can see the boiler may not light (micro switch)
valve may not move from mid position (micro switch/wiring)
valve may not move from HW (motor/no power from room stat/ no power from programmer.
To me it sounds like your valve may only be going to mid position and no further, this way once HW is satisfied it is not going to CH as it should.
 
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:D Thank you so much for this. I'll speak to the CH Engineer to ask him to replace the three way valve immediately after Xmas with high hopes that this will solve the problem. However, why would the original and replacement roomstat stop clicking ? - or have I missed something? Once again, thanks very much for your help.
 
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