Vaillant eco tec plus 637 not communicating with thermostat

6 Jul 2004
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United Kingdom
Hi guys, came home from a weekend away to find out our house had been hit by lightening. We have no hot water/heating from our eco tec plus 637. The wired thermostat showed 'com err' (communications error). The boiler itself seems ok but is displaying errors S.0 (no request for hot water) and S.7 (pump over run). The control box green light is flashing slowly (ebus issues). We have replaced the thermostat and control box but the fault continues. My engineer is stumped. Would really appreciate any advice, thanks.
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Without doing tests on your boiler I don't see how anyone is going to be able to do anything other than guess at what may be wrong. That could result in expensive parts changing with no guarantee of success.

Vaillant do a fixed price repair for about £300.

The concept of forums like these is to ask questions and give answers. I am a member of many and do my best to offer help where I can. I have had an engineer in, and he's stumped. Even his hotline to top-line Vaillant support couldn't help. I'm here to see if anyone has ever experienced similar issues and what their outcome was.
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If you have ebus connected controls, after totally powering off the boiler I'd be inclined to disconnect them and then try a wire link between terminals 3 & 4 to see if that fires up the boiler.
Lol sorry top line vaillant support :eek: there no such thing I'm afraid only customer service with computer manuals.. Best be is the vaillant repair fixed. Price, u may need a pcb S0 is not so good
jackthom - yes, connecting wires 3 and 4 does make the boiler run. Does that mean the pcb would be ok?
If the boiler is behaving normally with that loop connection it means at least the pcb isn't totally wrecked. There must be some damage to the board though.

If necessary you could carry on using the boiler with a more bog standard room thermostat connected to terminals 3 & 4, provided you can be sure it's running as it should and not misbehaving (overheating for example).
Depends what external controls you are using and whether ebus is essential for your system.
Did the stumped engineer not assertain that?

The usual procedure with computers and heating systems is to go back and test in the most basic operating conditions.

Connecting terminals 3 and 4 is one of the first things the engineer did. He phoned Vallaint and they couldn't provide an answer re ebus/com err.

Having replaced the thermostat and controller, If I replace the pcb would that mean pretty much cover all electrics issues, or are there further electrics involved somewhere?

I really want the original setup all working as it should having already replaced the thermostat and control box, and not try a new thermostat if i can, thanks.
The wiring from the control unit goes direct to the boiler pcb, so provided the pair of wires are still properly connected, the fault would have to be with the pcb.

Unfortunately the board itself is around £160 (although some saving is possible via ebay) so with fitting costs added to this, the flat fee Vaillant would charge for getting the boiler fixed begins to look a bit more attractive.
£300 fixed fee from Vaillant would normally be ok, but having already spent £450, would prefer to buy a pcb from ebay and get my engineer to fit it and hope that cures it.

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