New PCB in older Vaillant ecoTEC plus 618 R1 not responding to request for heat

4 Mar 2018
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United Kingdom
Firstly, I'm not going to attempt to do any rewiring my self - but I do have a Gas Safe plumber on the job currently (waiting for an internal pipe that is leaking a bit), and I want to get informed on our situation so when he comes back we can get it sorted asap.

I have a Y plan heating setup with the following:

Vaillant ecoTEC plus 618 R1, Condensing System boiler VU GB 186/3-5, G.C. No. 41-044-46
British Gas UP2, Drayton LP822 Dual Channel Programmer
Honeywell Mid-position valve
Danfoss Cylinder Thermostat
Honeywell T40 Room Termostat
Junction box

I recently put a new PCB in older Vaillant ecoTEC plus 618 R1 as the old one was bringing up the F62 fault code and also appeared to have three blown resistors on it (they had a brown smudge underneath them on the underside of the board).

Vaillant confirmed what PCB I needed and I bought it and put it in so we could get our heating up and running as quick as possible. I replaced it plugging exactly the same leads into exactly the same place.

First problem - the new board had a bridge between 3 & 4, which I'm led to believe is something that you only leave in if you don't have a programmer (or you want an always on kind of setup etc.).

But the main problem is - after putting the new PCB in and switching it all on the boiler shows a temperature (inside boiler?) and a little tap symbol, then after a bit the circle with triangle symbol (pump I think) comes on but only for about half a second, during which time it makes a short sound then just does nothing.

Pressing the i button shows the S31 code, which is "Central heating thermostat knob turned off or no heat demand by the eBUS control unit."

I can make the boiler fire up by briefly holding the plus and minus buttons, which I believe is some kind of function that installers can use to do some basic checks etc. It then reaches a specified temperature for a specified time (5 or 10 mins) then does nothing again.

So, it could be a faulty new PCB, or the new PCB requires a differently wired setup (I think this is the case), or something else. Either way, the boiler doesn't seem to be receiving or responding to any requests for heat.

I've checked our wiring and fortunately theirs a wiring diagram online here (on website on their 'Central Heating Wiring Diagrams' page - the 'Y-plan Wiring Diagram for Vaillant ecoTEC Boilers (old)' one) that matches my setup exactly (with the exception that the blue neutral wires are not included in the below diagram - which are all just connected to the neutral 3A connection). I've linked to the image below.

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I've sorted this now. After speaking to Vaillant about 7 times, one person there said an engineer would need to put some new links in, another said it hadn't been wired up to manufacturer specification then cut me off (I actually got cut off from them 6 times before getting it sorted). And finally one of their guys said that S31 means that the boiler is being asked for heat, but that the temperature dial for the central heating is turned all the way down.

Turns out, on my old PCB the CH dial was turned all the way up - as most people have it - but that dial was missing on the front - it had broken off and only the top one was there. I new our system only needed one dial because we have a cistern with a temperature dial on it, but I'd assumed that the one dial on the front of our boiler was the one we needed, and the missing one was missing because it was the one we didn't need. Turns out it was the other way around. And the new PCB came with the CH dial turned all the way down. Once we turned that up it started working.

I with the S31 description in the manual actually said what that Vaillant guy said - rather than the technical terms used in the manual. Well whatever.
All new PCBs come with the potentiometers set at minimum and any selector switches set at off

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