Vaillant ecoTec 831 stays on CH with no call for heat

[JSHarris]

My troublesome 4 year old Vaillant EcoTec 831 has now developed another problem that has me stumped.

The boiler stays firing in CH mode even when the thermostat isn't calling for heat, resulting in things getting a bit warm in the house. These are the things checked so far:

1. Thermostat checked, contacts definitely open when not calling for heat.

2. 3/4 disconnected on boiler to ensure that boiler wasn't seeing an accidental wring fault in the wired thermostat connection, boiler still runs in CH mode as if being called.

3. First diagnostic mode indicates that thermostat isn't calling for heat (indicates 0 in diagnostic selection d.8 )

4. Resetting boiler has no effect, radiator symbol remains on when boiler restarts and boiler fires, even with no call for heat.

Boiler can be "fixed" by turning the power off for at least ten minutes, ten turning the power back on, when the boiler will return to normal operation. However, after an hour or two it will get "stuck" again, with the CH always on.

DHW works fine, the only error codes logged are all the old F.75s that were before the "F.75 repair kit" was fitted (at significant cost). Pressure is normal (half way up the grey band), the diverter valve (also replaced within the last year following another failure) looks to be working fine, the system has a Magnatec fitted in the return (and that looks clean inside).

I suspect this latest fault may be within the PCB, as I can't see how the display can show that there's no call for heat from 3/4 yet the boiler is still happily firing in CH mode. When it is firing with no call for heat the only ways to stop it are to either turn the CH temperature knob fully ACW or to turn the boiler off.

Before I lash out on a new PCB I'd find it really useful if anyone here could see if there's something I've missed in trying to pin down the cause of this latest problem.

 

[hertsboilers]

On 24V side is there any connnecton on 7, 8 and 9 terminals?

When heating is on while not in demanding, is there is a radiator sign on display?

 

[Agile]

His bullet point "4" says that ***********
I agree that it does sound like a PCB problem assuming he has removed wires from 3/4 and there are no other external connections to the boiler.

As its so close to being in warrantee and not a part expected to fail so soon perhaps a complaint to the makers might result in a free issue part perhaps?

Tony

 

[JSHarris]

Thanks, folks.

This boiler has been a pain from a couple of months after it was installed. It had a 12 month guarantee from the installer, not, as I found out much too late, the three year warranty that Vaillant provide. The F.75 fault started a couple of months after installation, the installer tried changing sensors (three were changed I think) to no avail. Then the rubber hoses developed leaks, causing our microwave to blow up (it was under the boiler and, unseen, water dripped into the back of it). Next the diverter valve failed (outside the 12 month guarantee). Then, thanks to help on here earlier this year I found out about the "F.75 repair kit" and got that fitted, which fixed the long standing and regular F.75 failures we'd been having for over three years.

I can confirm that there's no 24V side connections, just thermostat/programmer contacts connected to 3/4. The radiator symbol is on even when there's no call for heat. I did lift the 3/4 connections at the PCB to test for a possible wiring fault. The diagnostics report no call for heat with the CH running, so it looks as if the front end side of the PCB is detecting the thermostat call OK but something within the PCB is causing the CH to stay on as if being called.

So far this thing has cost me as much in repairs as a new boiler, so I'm not best impressed by it. We bought it because everyone seemed to say that Vaillant were pretty reliable, but we obviously got a "Friday job" one, as we've had a couple of dozen call outs to fix it, only around half of those being whilst it was under guarantee, and most of the call outs haven't resulted in a fix at all (the F.75 problem was sporadic, and might only occur once every month or two in winter).

It looks like we'll have to shell out for a new PCB, which makes me wonder whether we might well be better cutting our losses and just scrapping this thing and getting a new boiler of a better make. I'm guessing we're looking at around £200 or so for a replacement PCB to be fitted, which added to the several hundred we've already paid out for repairs over the last three years seems a bit like throwing good money after bad.

 

[JSHarris]

More testing this morning seems to show that it must be the PCB, I think. I thought that turning the power off, waiting for ten minutes and then turning the power back on had fixed it, for an hour or so.

Testing it this morning seems to show that it can also get stuck on no CH, even when the thermostat is calling for heat. It's almost as if it's getting a bit deaf internally to the thermostat calls, they reach the PCB OK, show up on the diagnostic display correctly (d.8 = 0 for satisfied, d.8 = 1 for calling for heat) but for whatever reason the boiler is often just ignoring the calls and staying in whatever mode it was in last.

The good news is that I've just managed to locate a brand new PCB (via a well known auction site) for a good price (far, far less than the £235 I've been quoted to replace it). Looking at it, the job seems straightforward, just disconnect the power and unplug a few connectors. As this doesn't seem to involve touching the parts of the boiler that must be dealt with by a Gas Safe person I'm guessing that it might be OK to just swap the board over myself without incurring any repercussions. If so, then it looks like I can save more than three quarters of the cost of this repair, which will be most welcome given the trouble this boiler has been.

Thanks again for helping to confirm my suspicions that the PCB is the most likely cause.

 

[Agile]

The fact is that there is no other boiler much more reliable than a Vaillant.

The F75 is primarily caused by dirt in the system. I would guess the installer did not properly power flush or otherwise clean the system.

You don't elaborate on why you did not get a three year warrantee. I suspect something to do with the installer or perhaps neither of you registered the installation. Probably not notified to Building Control either!

The hoses were a weak point and have been redesigned.

Changing the PCB yourself may mean that you have to set it up which would usually be done by an engineer although with a lot of research you might be able to achieve that yourself.

Tony

 

[JSHarris]

Thanks, I was aware of the good reputation for these boilers, which is why we opted for one. The F.75 wasn't caused by dirt in the system, it turned out to be a problem related to the boiler not being able to cope with a low flow resistance, apparently a design fault that can be fixed by fitting the "F.75 mod kit" (which in our case does seem to have fixed it).

Our installer did power flush the system for a couple of hours or so before the boiler was fitted (they left the flush going over lunch, IIRC). They also fitted a Magnaclean in the return, adjacent to the boiler. We had two or three pressure sensors changed in an attempt to fix the F.75 fault, to no avail. No dirt was ever found when changing sensors, either.

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.

The warranty was a bit of a con (or maybe an error) by our installer. I wasn't aware that Vaillant offered a 3 year warranty (someone here told me about it, IIRC) and after 1 year our installer started charging for call outs (they'd been out maybe a dozen times in the first year). The hose leaks were dealt with under warranty, but this didn't run to compensation for the fairly new microwave that was damaged by the leak. The diverter valve failed after a couple of years, IIRC, and again wasn't treated as a warranty repair by the installer (cost us a fair bit as I recall, and resulted in another leak).

Looking through the set up it seems that all I need to do is go into the diagnostic mode and check the DSN. If it's not set to 7 then I need to go into the second diagnostic level and set it to 7. This brings up another interesting quirk with the current PCB. If I go to d.97 and increment it so it reads the password, "17", then press "i", it doesn't enter the second diagnostic level. Makes me wonder if this may be related to the fault we're seeing.

 

[Agile]

I have to disagree with your installer about leaving LS valves fully open!

In my view the system should be properly balanced. Not only to equalise temperatures but to set temp differentials across each rad of about 15 C to ensure the boiler is operating efficiently.

Power flushing requires the flow to be through each rad in turn and reversed a few times and is a very energetic operation and is not achieved by leaving the pump on during lunch!

It almost seems to me that your installer intentionally set up the system to increase the number of charged visits.

In my view you did not need the F75 kit. I have never needed one to cure that fault code.

I still don't understand why you did not get the three year warranty. But if you happily paid the installer to repair after the first year then obviously he is not going to turn away the work.

Tony

 

[denso13]

It almost seems to me that your installer intentionally set up the system to increase the number of charged visits.

 

[JSHarris]

Knowing what I know now I'd disagree with the installers advice, too!

I didn't pay a lot of attention to the power flush, but they did reverse the flow direction at least once, I think. They hooked the big tank and pump up to the flow and return pipes on the wall, where the boiler fits, and they did flush out a lot of black gunge. The Magnaclean seems to always have a thin layer of black stuff on it whenever it's cleaned during the service, but the engineers who've looked inside the boiler when replacing pressure sensors or the diverter valve have all remarked that it seems clean enough, and that dirt probably wasn't a problem.

The F.75 problem was definitely, beyond any doubt at all, caused by the low flow resistance of our sustem. It got to the point where I could always make it happen by just turning all the TRVs and LSVs so they were wide open, letting the system cool down, when it would be guaranteed to do three start attempts then throw an F.75. Simply going around and partially closing the LSVs fixed it 100% (except it then meant we didn't get enough heat from a couple of the rads nearest the boiler). The second engineer we had out (having lost faith in the installer) confirmed this as the cause, and he had checked the pressure sensor for any sign of dirt a few times (and replaced it again to be sure), before ordering and fitting the "F.75 mod kit". The mod kit is far and away the best fix, as it has allowed the rads to be balanced properly. It's noticeable that the boiler always fires first time now, too, whereas from new it has always taken two or three attempts to fire, presumably because it struggled to see the pump kick.

I simply had no knowledge of the Vaillant warranty, as the paperwork only mentions the installers warranty. Had I known about it I'd have taken it up with the installers, but by the time I found out it was too late to do anything other than get bloody angry at them.

 

[Agile]

The F.75 problem was definitely, beyond any doubt at all, caused by the low flow resistance of our sustem. It got to the point where I could always make it happen by just turning all the TRVs and LSVs so they were wide open, letting the system cool down, when it would be guaranteed to do three start attempts then throw an F.75.

Yes, but a low flow resistance will create a high flow rate and that in turn means a low temperature differential across the system.

That in turn means a high return temperature and the boiler does not condense much and can be 12 % less efficient!

A correct differential is achieved by closing the LS valves to reduce the flow rate and give a differential of about 15 C if possible!

Boilers are condensing when the return is below 55 C. To get much heat out of the rads you need a flow temperature of about 70 C.

Tony

 

[JSHarris]

Accepted, but in our case we massively increased the insulation in the house at the same time as changing the boiler, so found that we could get the house warm enough with a much lower flow temperature than before. In effect our rads are now over sized for the heating requirement by a fair bit, as they were put in when the house had draughty single glazing and no wall or loft insulation.

We normally run with the flow set at 48 deg C, which gives about 46 deg C delivered to the rad flow side (measured). The return tends to be around 40 to 42 deg C, so the boiler should be condensing well, I think. In really cold weather I turn the flow temp up to maybe as high as 55 deg C, giving a return temp of maybe 48 deg C, but so far we've never needed to go higher than this (I have a data logger and temperature display recording the flow and return temps, together with the outside and inside temps, just for fun, really).

 

[Agile]

Well in that case having the correct differential will not aid your efficiency much. But it would prevent F75 errors caused by too low a system resistance without needing to but an F75 kit!

You could have more fun by fitting Weather Compensation!

Unfortunately, with a Vaillant, you have to buy their expensive unit unlike some other boilers where you just connect an external sensor.

Tony

 

[JSHarris]

With the benefit of hindsight I think I should have opted for weather compensation, but at the time we were spending so much money on getting the new boiler, additional wall and loft insulation and new high performance double glazing that I was looking to try and keep costs down.

I tried balancing the system and adjusting the LSVs to try and increase the delta T before getting the F.75 kit, but the two largest radiators (which have the biggest impact on the flow resistance) are those nearest the boiler and the LSVs on them are hyper sensitive when shut down enough to try and get a higher delta T and increase the system flow resistance a bit. I couldn't ever get to a 15 deg C delta T when the system was warm and running, no matter what. All the radiators are too big, really, especially the two big ones nearest the boiler, but changing them for smaller rads wasn't something I really wanted to do, because of the redecorating etc, plus the slight complication that the pipes are set in the concrete floor slab, so not that easy to reposition.

Fitting the F.75 kit seems to have been a 100% fix, though, which is presumably why Vaillant made it available.

I'm going to keep my fingers crossed that changing the PCB won't be too much hassle and will fix the current fault. I've ordered it, so with luck should have it fitted before the end of the week.

 

[Steelmasons]

Fitting the F.75 kit seems to have been a 100% fix, though, which is presumably why Vaillant made it available.

.

100% fix?? , I would suggest it was other issues causing F75 , ie: blocked pressure sensor/hoses/filter etc

See these boilers everyday and none of them require the F75 kit , if all that was fitted was an F75 kit (which comes with sensor and harness) then I guess it was a pressure sensor issue , quite possible the rubber hose was choked , obviously this might not have been checked when changing the tube/hoses for the F75 kit.

Too many threads bashing Vaillant for some reason or another , most of these threads are from DIYers who haven't a clue about the boiler so the easy way out is to bash the manufacturer.