Vaillant 831: CH stopped DHW ok

[cjn1957]

After just over 2 years of excellent continuous service my boiler has stopped working. DHW is fine but on the CH side I get a S.0 status most of the time, even when there is clearly a heat demand, so the radiators remain cold. D40 and 41 both give the same temperature reading, e.g. 67C at the moment, though the return pipe beneath the boiler is cold.

Not sure whether this is relevant or related, but there is a dried up green/black residue in the bottom of the boiler and a similar looking stain down the wall beneath, which first appeared about 3 weeks ago. I haven't been able to see any similar deposits on any of the pipework within the boiler. Everything looks pristine.

Any thoughts anyone?

 

[Andygasman2010]

S.0 means there is no demand into the boiler. Timer/room stat fault. To confirm this put a link between terminals 3 and 4 on the PCB. If all this is alien to you just get an RGI in.

 

[cjn1957]

Thanks for your quick response. The link has always been in place. I have a vrt360 connected to ebus +/- terminals; it needs the link to be present on 3 and 4. I'm pretty sure the vrt360 is working fine, but, just in case, I've tried with it disconnected. It didn't make any difference.

Whenever the temperature in the flow and return has dropped, eg when the boiler has been switched off or when some DHW has been drawn off, the boiler runs through the s1-s4 sequence, heating up the CH water (within the boiler at least). It very quickly gets the water back up to temperature, say 67C and then stops (first overrun, then anti-cycling, then S0). The pipework outside the boiler never gets hot. Is it feasible that there is a problem with the diverter, so that the water is being cycled within the boiler? If I'm not mistaken, I think the boiler has a bypass circuit that would normally come into use if all the PRVs in the system were closed. I should add though that, according to the diagnostic code d.35, the valve is operating as you would expect - =0 normally, =100 if DHW is drawn off, then back to =0. But maybe that's not the whole story.

Sorry to be so long winded. Any further thoughts?

 

[Bunnyman]

Look at the divertor valve if there is corrosion or water directly under it u can c the spindle through the cut out on the side , switch from water to heatin watch spindle see if it moves in an out

 

[cjn1957]

There are strains from dried up CH water on all the bottom surfaces of the boiler and on the wall nearby (beneath the boiler, to the right) but I don't think these originate from the diverter. These happened about three weeks ago, so they are almost certainly related to the present problem - maybe they are from the pressure relief valve.

The latest news is that both yesterday and today the CH has worked for a few hours. This occurred on both occasions after there had been periods of continuous DHW use, i.e. people having showers. After a few hours it packed up again.

I have been trying to observe the movement of the diverter valve but only seen movement on one occasion. Even during the times in the last two days when the CH was working for a while, I couldn't see any movement, even though d.35 was changing between 0 and 100 when you'd expect it to. I presume this means the diverter is sticking in both positions, though more often than not, in the position where the CH doesn't work. Does this make any sense? Could this situation have led to the pressure relief valve relieving itself? Does the entire diverter assembly have to be removed to remove debris or check whether it really is stuck etc?

 

[The831Bunny]

Almost certainly a DV fault. If the spindle has brown water stains obvious when you shine a torch in the inspection hole on the side, then I would get the valve replaced pronto. If a leak is just left water can seep into the motor and in turn damage the PCB. This is then a costly job not easily diagnosed.