Vaillant ecoTec 831 stays on CH with no call for heat

[JSHarris]

Absolutely definitely not any blockage either in the narrow opening to the pressure sensor or in the space behind the pressure sensor in the block. The hoses were clean inside, too (I still have one of them that was removed when the F.75 kit was fitted).

This fault developed within 4 weeks of the new boiler having been fitted, and looking back I'm fairly sure that was the first really cold night we had afterwards.

It persisted through at least three pressure sensors changes.

This isn't random "Vaillant knocking" on my part, it's a bit of a mix of a design limitation with this model (fixed by Vaillant by their release of the F.75 mod kit) and, more importantly in my view, incompetence, and perhaps even deceit, on the part of the installers I chose.

Yes, this boiler has incurred more than it's fair share of problems, but most of which have resulted from it not working well with our particular low flow resistance system, the only true faults with the boiler have been the leaking hoses (which was simply because the crimped hose clamps around them were loose, according to the fitter who fixed it) and the more recent fault which seem to be within the PCB somewhere.

 

[Steelmasons]

After a lot of detective work I eventually found the cause for our F.75 problem. The rads all have TRVs with the LSVs all wide open (as suggested by the installer). The house is a bungalow, with pretty short and direct pipe runs to the boiler. What was happening was that if the temperature dropped at night enough to cause all the TRVs to open fully, then the flow resistance was so low that the pressure sensor wouldn't reliably detect the pump kick. The work around was to partially close off all the LSVs so that the system flow resistance increased, and this partially fixed the F.75 problem. The snag with this was that the LSVs needed to be closed so much that in very cold weather we were struggling to get enough heat. Luckily someone here pointed me in the direction of the "F.75 mod kit" and that fixed the problem, such that we can now run the system with the rads properly balanced, as the repositioned pressure sensor in the kit fixes the problem.


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No disrespect but what a load of tosh.

Possibly find the filter was/is blocked (canoe) , how did you check for a faulty pressure sensor? , has someone been playing around with integral by-pass?

Lets hope the Bunny boys skip over this thread otherwise their gonna have a field day.

 

[JSHarris]

I can understand that the most common reason for this problem is a blockage that stops the pressure sensor seeing the pump kick. For a year, and around a dozen call outs, our installer worked on this basis. He replaced the pressure sensor (the original advice being that the orange one fitted had been superseded by a modified one with a black body). He took the hydro block out and cleaned it, including the difficult to get at area behind the pressure sensor (although commented at the time that it seemed to be clean).

The second firm we got out to look at it (after the warranty had run out) did much the same, replacing the pressure sensor, cleaning things and generally not being able to find the cause.

The reason I'm 100% certain of the cause now is based on this evidence, from multiple tests (and is based on advice I had from here):

1. There has never been any evidence of dirt in this boiler.

2. The first occurrence of the F.75 fault was only a month after the boiler had been fitted, when it would still have been clean (there's a magnaclean in the return and the system was power flushed beore the boiler was fitted).

3. It's been cleaned and had several pressure sensor changes and yet has thrown up an F.75 the day after a sensor change and every time the overnight temperature drops a lot in winter.

4. The F.75 fault could be initiated at will by simply opening every rad valve, when the system would always fail and throw an F.75. Conversely, closing a few rad valves would ensure it always started, no matter what.

5. Since fitting the F.75 kit last Autumn the boiler has never thrown up this fault, even with all the rad valves wide open.

6. It worked continuously and without a fault for the whole of last winter, the first time ever that it's run for more than a few weeks in winter without throwing an F.75.

All of these factors have convinced me, beyond any doubt whatsoever, that dirt has never, ever been the problem.

 

[Steelmasons]

12 months later and you still have this problem?

 

[JSHarris]

12 months later and you still have this problem?

No, that's the whole point. Thanks to advice from here I got our current repair chap to fit the F.75 kit last year and finally, after a several winters of having the boiler shut down after every particularly cold night, it has worked fine for a whole winter.

The "sticking in CH" problem occurred around this time last year, but was fixed by turning off the power for a few minutes and then turning it back on. At the time, I thought it might be, perhaps, just a glitch, as the problem didn't recur through the whole of the last heating season. On the basis that it generally seems better to leave well alone I didn't touch it between then and now.

Unfortunately the "sticking in CH" problem came back when we turned the heating on for the first time this year, and seems to have got worse, as it is now randomly sticking with either the CH on, no matter what the demand, or sticking with it off. It seems that over the Summer what I thought to be a one-off glitch may have developed into a hard fault.

I'm puzzled by the fact that I can't get into the second level diagnostics, too, and wonder if it's related to the fault on the PCB in some way. If I go to d.97 in the first level diagnostics, increment the value to the password of 17 and press the i key nothing happens, yet according to the manual this should get it into the second diagnostic mode.

 

[The831Bunny]

Very little of my income is generated from regular customer ecoTec breakdowns...

 

[Steelmasons]

Leave the second level diagnostics to the engineer (if a problem at all) , you shouldn't be 'playing around' , especially DSN variants , a good thing it is not accessible to you.

 

[JSHarris]

Very little of my income is generated from regular customer ecoTec breakdowns...

To be fair to the chap that looks after this boiler now, that's pretty much what he's said right from the off.

I suspect that fact that they are so reliable is one reason why fixing this has taken so long; it seems that a genuine F.75 fault (i.e. one caused for the reason that Vaillant made the F.75 kit) is pretty rare.

Our installers (not that I have much faith in them) said they'd never, ever seen a problem like ours, and they'd fitted loads of Ecotecs.

The (admittedly second hand, and allegedly from Vaillant themselve) reason for developing the F.75 kit was for the odd system like ours. Our boiler is pretty much in the centre of the house and the 22mm flow and return runs 4ft down the wall to tees, with the two branches off the bottom running to two big rads within a few feet, and the rest coming off further down each arm. In effect, we have a pretty large bore system, with big radiators that are running at a relatively low temperature. The net result is that the pump kick pressure pulse down in the hydroblock is too low if all the rad valves happen to be wide open, so the pressure sensor doesn't see the 0.1 to 0.2 bar needed to convince the electronics that the pump is actually running. By repositioning the pressure sensor so it can better see the pump kick directly at the pump outlet, the mod kit resolves the problem.

 

[JSHarris]

Leave the second level diagnostics to the engineer (if a problem at all) , you shouldn't be 'playing around' , especially DSN variants , a good thing it is not accessible to you.

Good advice, I'm sure, but in this case I believe I need to get in there when I fit the new PCB (but am happy to be told otherwise).

I'd appreciate being corrected if I've got the PCB install process wrong, but I believe the procedure is:

1. Disconnect the mains.

2. Open the case, fold down the control panel and unclip the PCB cover.

3. Undo all the PCB connections.

4. Remove the PCB and fit the new one, re-making all the connections.

5. Reconnect the power.

6. Go into the diagnostics menus (via i and +) select d.76 to check the DSN.

7. If the DSN isn't set to 7 (for the 831) then select d.97, increment to 17, go to d.93 and change the DSN to 7.

8. Exit diagnostic mode and do a functional test.

I believe, but again would appreciate being corrected if I've misunderstood, that the same PCB is used for several models, so needs to be set for the specific boiler that it's fitted to like this.

 

[Agile]

Yes, this boiler has incurred more than it's fair share of problems, but most of which have resulted from it not working well with our particular low flow resistance system,

You do seem very convinced that your system is somehow special.

My conclusion is that your system has never been properly balanced. Yes, LS valves settings ARE very sensitive. They need sensitive adjustments.

Like SM, I have never needed an F75 kit!

Whilst not all Vaillant engineers seem to agree, I find that the expansion vessel ( EXV ) setting is very significant. Even to the point that most F75 faults that I see can be mostly eliminated by adjustment at the EXV.

Did your installers have much to say about the EXV?

There is also an internal bypass inside the boiler and that also has an influence on balancing the system.

You should also have an auto bypass in your system. You have not mentioned on or its setting. Did the installers leave it out?

Tony

 

[JSHarris]

You may be right, and I certainly have little faith in the original installers, knowing what I know now. They certainly never touched the bypass setting and didn't check the expansion vessel pressure, plus their advice was to leave all the LSVs wide open and rely on the TRVs to balance the system (previously the system with the old system boiler had no TRVs, and had been balanced pretty well to give even room temps).

The second chap has been a lot more thorough and I know he checked the expansion vessel. Not sure whether he adjusted the bypass, but he did agree with the advice from here to balance the system with the LSVs and this did certainly almost fix the F.75 problem. The snag was that we couldn't get the two big living room rads balanced so that they could deliver enough heat in very cold weather and still keep the boiler from throwing an F.75 on a cold start in the morning.

I'm not aware of an auto-bypass anywhere in the system. The system has 8 rads plus a towel rail, with four rads coming from one arm of the flow/return tee just below the boiler and three rads and the towel rail coming from the other arm of the tee. All the rads have TRVs but the towel rail doesn't, it just has a normal valve and LSV. The flow and return come straight up from a tee in the floor to the boiler, with a magnaclean in the return pipe, maybe 18" from the boiler.

Once we came across the F.75 kit and fitted it all the problems went away. The system worked reliably all through last winter, for the first time since it had been installed, so my view is that the cost of this mod kit was worth it just to save more call outs. Maybe we could have persisted with trying to find a balance compromise with those two big rads, and maybe we could have got it working OK, but economically it seems to have been better, and certainly less frustrating, to just fit this mod kit that was, apparently, designed by Vaillant to address just this problem.

 

[Bunnyman]

I'm sorry mate I'm pleased you solved the f75 but that's the biggest load of tosh I've heard. It as nothing totally nothing to do with trvs or any low flow rate etc absolute rubbish I'm sure you won't believe me regardless of any tests etc you have done, we all see thousands of these (831 steel mason etc) . The f75 kit as you call it was brought out for dirty systems, I suspect your expansion vessel was flat and absorbing the pump kick. The trouble is that when the fault continues and engineers who don't know what there doing cant fix it they start to flounder and grab at straws an that's what happened here. I've never needed a f75 kit in 2-3000 visits to ecotecs

 

[Steelmasons]

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.

 

[JSHarris]

I'm sorry mate I'm pleased you solved the f75 but that's the biggest load of tosh I've heard. It as nothing totally nothing to do with trvs or any low flow rate etc absolute rubbish I'm sure you won't believe me regardless of any tests etc you have done, we all see thousands of these (831 steel mason etc) . The f75 kit as you call it was brought out for dirty systems, I suspect your expansion vessel was flat and absorbing the pump kick. The trouble is that when the fault continues and engineers who don't know what there doing cant fix it they start to flounder and grab at straws an that's what happened here. I've never needed a f75 kit in 2-3000 visits to ecotecs

To reiterate, again.

The F.75 first happened 4 weeks after the boiler was installed.

The pressure sensor was removed, the filter behind it was checked for dirt and a new pressure sensor was fitted,

The fault persisted.

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.

Vaillant were consulted, help was sought here (and elsewhere) and it was suggested that we look at increasing the resistance by closing off the LSVs.

Lo and behold this pretty much fixed the F.75 problem, after around three years of hassle.

The snag was we couldn't get enough heat out of the two big rads nearest the boiler in really cold weather.

Fitting the F.75 kit fixed the problem, 100%. We could now rely on the boiler to fire up after a cold night and the heating ran well all through last winter for the very first time since it was installed.

Now, you may well think this is a load of whats-its, but the facts are that the one thing that sorted this new boiler out was that F.75 kit. had I known about it three years earlier we'd have had three winters with a fully working and reliable boiler, instead of waking up every few weeks to a cold house because the damned thing had thrown up another F.75.

If this was an old EcoTec, in a dirty system, then I'd have fully understood all the comments about dirt being the cause. However, after more than three years of trying to find any possible cause, and being so paranoid about making sure that everything was clean (even to the extent that I cleaned out the internals of the hydroblock with pipe cleaners during the last two occasions when it was removed to see if it was blocked), I'm absolutely convinced that this was not a problem caused by dirt, and nothing you guys who've never looked at the inside of this particular installation say will convince me (or the guy that has looked after this system for the past year or so) otherwise. even Vaillant agree as to the probable cause, FFS.

Finally, this thread has nothing whatsoever to do with the F.75 fault. That's fixed, sorted, and is absolutely not a problem any more.

The current problem with this boiler is that it gets stuck and seems to ignore either calls for heat from the programmer/thermostat or continues to believe there is a call for heat when the programmer/thermostat is satisfied.

The diagnostics show the correct programmer/thermostat call (so when the rad symbol comes up showing a call for heat, yet the programmer thermostat is showing that it's satisfied the diagnostics indicate 0, showing that the contact closure on 3/4 is being read correctly).

I'm reasonably sure that the PCB has to be the prime suspect, so have ordered a new one.

My understanding is that this will need setting to the right DSN for this boiler, which will mean going into the second level diagnostics, but I've been warned not to do this. If I'm right that this is the procedure to install a new PCB then I can't see how I can avoid going in to the second level to set it to the right boiler DSN.

 

[Agile]

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