Vaillant ecoTec 831 stays on CH with no call for heat

[JSHarris]

Oh dear , seems the Bunny boys stumbled upon this thread after all.

Take heed JSHarris , two of the best vaillant guys in the business (831/bunnyman) have taken the trouble to reply to your thread , LEARN something from it.

Look, I have learned from it. In fact I learned from the advice given freely here a year ago and as a result the chap that is now looking after this boiler for me fixed the system, such that we've not seen an F.75 fault once in the past year. That problem is fixed, completely and 100% fixed. It's a non-problem now, and when the boiler is running the system works extremely well indeed.

The current problem is wholly un-associated with the pressure sensor and the blasted F.75 fault code. It seems to be something within the PCB that causes the boiler to continue to run the CH even when the programmer/thermostat is satisfied and even when the diagnostics are showing that the thermostat is satisfied. It also does the converse, in that the boiler will stay off even when the programmer/thermostat is calling for heat, and when the diagnostics indicate that this is so (d.8 = 1).

I can temporarily fix the fault by turning the power off, waithing five to ten minutes, then turning the power back on, which will often resolve the fault for maybe an hour or so. Unfortunately, ths fix doesn't last much longer than this, although when it happened last year turning the boiler off, waiting a while, then turning it back on, did fix the problem for the whole heating season.

I've got a new PCB on order, as based on what's been advised here and my own gut instinct I suspect that the fault must be within the PCB.

 

[JSHarris]

The system was repeatedly checked, cleaned, pressure sensors changed, etc, the F.75 fault persisted (remember this is a new installation).

The second firm that looked at it checked the pressure vessel, no problem found, no dirt found, the pressure sensors was changed again.

The F.75 fault persisted.

But, by your own admission the installers never checked the expansion vessel which I have always found to be one of the prime causes of F75 when its not correct.

The second engineers you say "looked at the expansion vessel". Unfortunately just looking at it, seeing if air comes out of the schraeder valve etc. is not sufficient.

The system pressure needs to be reduced to zero and the air pumped to the correct value for the installation.

What flow temperature are you using when you cannot get enough heat from the big rads?

Tony

I double checked the expansion vessel with a digital tyre pressure gauge, and the current guy and I did run through the empty the system, empty the pressure vessel, recharge the pressure vessel and re-pressurise the system routine (more than once, as I recall).

The flow temp is usually around 48 deg C at the boiler, but I increase it to around 55 deg C in very cold weather, which is when we had problems getting enough heat from the big rads in the living room. I know this is low, but if I increase the flow temp much more we have problems with the bedroom temperature going up and down like a yo yo, even with the LSVs turned right down. Not sure why, maybe the TRVs have too much hysteresis.

The bottom line is that we no longer have the slightest problem with the F.75 fault. the system works extremely well, the boiler is running at a low return temp (so condensing well) and we have masses of heat available (and, I have to say, a pretty impressive DHW output, something I was a bit worried about before we fitted this combi).

The current problem is solely the fact that the boiler gets stuck with either the CH on, or the CH off, irrespective of the programmer/thermostat call.

 

[Bunnyman]

I guess ur right on the pcb, but u need to get in second level diagnostics, this is pass number protected to prevent any accidents by non qualified people meddling , ur new engineer should know how to access it , that's hoping that the pcb is not knackered stock tho

 

[JSHarris]

I guess ur right on the pcb, but u need to get in second level diagnostics, this is pass number protected to prevent any accidents by non qualified people meddling , ur new engineer should know how to access it , that's hoping that the pcb is not knackered stock tho

Thanks very much for that, although it's not reassuring to think I may have just bought an expensive bit of knackered stock!

My understanding is that the procedure for fitting the new PCB is to disconnect the power, remove all the PCB connections, remove the PCB, fit the new one, replace all the connections, turn the power back on, then go into the diagnostics and check the DSN to see if it's 7. If it's not, go to d.97, increment it to 17, go to d.76 and set the DSN to 7, then exit the diagnostics and do a functional test.

Does that sound right to you?

 

[Bunnyman]

Ur messing with safety features they r password protected for a reason mate to stop people dabbling

 

[JSHarris]

Ur messing with safety features they r password protected for a reason mate to stop people dabbling

Yes, I understand that and realise that dabbling around in the second level without understanding the consequences of some actions could stop the boiler working properly. However, in this instance all I've done is take a note of all the settings in my current PCB so that when I fit the new one I can make sure that it has the same settings. AFAICS, the only setting that may need to be changed for the new PCB will be the DSN.

There are other threads on this forum describing in some detail what each setting does, and the procedure for accessing the second level is given in those threads and is also in the manual for the boiler (with a wise warning), so it's not really a well-kept secret.

As someone who designed control and instrumentation systems and firmware for flight safety critical applications I'm perhaps better equipped than some to DIY this PCB change. Without wishing to seem arrogant in any way, I have to say that the supposedly "qualified" installer who fitted and commissioned this boiler wasn't exactly competent, and knowing what I've learned over the past few years with this thing I reckon I'm probably safer at swapping this PCB over than they would be.

Their (supposedly Part P qualified) sparky was positively dangerous. If I hadn't been watching when he delved into the old wiring centre to pull out the programmer/thermostat connection to run down to the new boiler there'd have been sparks flying, as he failed to isolate the power feed to it before starting work. He also left a spare live (bare ended) wire exposed inside the programmer/thermostat back box in the wall, something I only found when checking the thermostat contacts to try and diagnose the current problem.

 

[JSHarris]

Just to close this off, a new PCB was fitted just after that last post and the system now works faultlessly.

It fires first time, every time, whenever there's a call for heat, and shuts down instantly when satisfied (with just the normal pump over-run). It looks at long last as if all the problems I've had over the past few years since this boiler was first installed have now been resolved. I'm keeping my fingers crossed that it will now live up to the reputation for reliability these boilers have.

Thanks to all those here who gave positive advice, especially the thread giving details of setting the DSN, which turned out to be very simple and straightforward to do, as did fitting the new PCB itself.

The boiler has just been checked and serviced by the chap that I now have some faith in to do a good job (unlike the original installers, who I wouldn't ever use again, I regret to say) and he has confirmed that it's still as clean as a whistle inside. He cleaned out the Magnaclean filter and showed me the very fine layer of muck that had collected on the centre, but reckoned this was far less than he normally sees and suggested that the system itself is staying pretty clean.

As one or two here had suggested that the old F.75 problem was caused by dirt, and didn't need the Vaillant F.75 service kit to fix it, I asked him to just swap the two pressure sensors over to see if the boiler would now work with with a known to be good pressure sensor replaced in the original lower hydrobloc position (the hydrobloc filter was clean). When refilling the boiler from cold and firing it up, lo and behold it failed to fire after three goes and popped an F.75 fault again, as it always used to. Reconnecting the F.75 service kit configuration got it working as normal again.

So, at least for my installation, I know beyond any doubt that the Vaillant F.75 service kit is needed to make my boiler reliably fire from cold (and it was only ever firing from cold that caused the original F.75 faults we suffered with for a couple of years). Clearly Vaillant know that there are some installations that need the F.75 service kit fitted to work reliably, otherwise they wouldn't make it. It just grieves me that my original installer wasn't aware of this, wasn't able to resolve this problem and instead chose to give me the run around.

 

[Agile]

As you say the F75 kit must be doing something and so does have a place. However, I have always managed to cure the F75 using the normal procedures.

What you did not mention was the financial aspects of the apparently faulty PCB which you bought. Did you manage to get a refund or replacement from the seller?

Tony

 

[JSHarris]

I didn't buy a faulty PCB, I bought a new one to replace the faulty one that was in the boiler from new. The difficult bit was confirming the problem as being the PCB, as it was intermittent. It'd sometimes go for days, responding normally to room stat commands, then get stuck on for no apparent reason. When it did this the diagnostics would show no call for heat (code S.30) but the radiator symbol would be on continuously (not flashing as if it were on over-run) and the boiler would keep firing.

My guess is that there is something like a dry joint or intermittently faulty component on the original PCB, but despite having looked all over it with a magnifying glass trying to spot a dodgy solder joint in that area (around where the 3/4 terminals are) I can't see a problem, so am guessing it's probably a faulty component somewhere on the board.

No chance of a refund, as the boiler is out of warranty.