I have exactly the same set of fault codes appearing in my machine which is now 2 years old. Likewise power cycling and clearing the faults appears to fix it, but then it recurs. I also occasionally get a communications error.
Has anyone had any insight beyond that above. If its a faulty circuit board I would expect the faults to reappear each time, but they dont.
Im faintly wondering if its a marginal circuit board, failing when it gets very hot outside. The heat pump is on a south facing wall in direct sunlight so its possible the inside overheats!
That’s annoying. The swapping of the main PCB the second time (as well as first, I’ve had same issue twice) resolved the issue. I suggest calling Vaillant ASAP as the issue will only get worse/more frequent.
I had a demand to fit a voltage protection so if it’s low voltage then power will cut.
Interesting hypothesis, mine is south facing too and we have 35C plus. But the first time I had the issue was in November a month after machine was first fitted.
Its odd. The fact its intermittent suggests something analogue not digital. Maybe its comparing something to an analog reference value, hence why I wonder about overheating. Underheating could conceivably cause the same. I have applied a sunshade, lets see if it eliminates the error!
Thats really interesting. Mine has recurred this evening, the outside temperature and unit has cooled down without a doubt so probably not overheating. Interestingly it sounded like it was pushing water through the system at the time, even though nothing was supposed to be happening. Does the Vaillant run a ‘test’ run of the pump every so often? I hadn’t noticed it previously but then I wasn’t looking for it.
My concern is that, with the problem being intermittent, Vaillant (or anyone else) will change something, declare it fixed, and then a couple of weeks later it will start up again.
Did you or anyone else discover anything about the underlying cause/trigger etc?
When Vaillant came the first time I had to fight them. The second time I called up and told them the faults and the engineer turned up with the circuit board ready. I’m inclined to think they know about it now.
Thanks that’s really helpful. The first time was it that you had to fight to persuade them there was a fault, or was it that they resisted fixing it. I’m wondering whether to keep a log if it’s the former.
Neither really. It was that they hadn’t encountered this situation before so they had to test everything and before visiting were telling me it was a flow rate issue - I’d found a thread elsewhere where someone had the issue so knew it should be this PCB. Once they’d done their on site testing they agreed and changed the PCB no issue.
The second time the technician came with the PCB ready and tested then swapped no issue. He did comment, once is odd, twice is very strange, three times impossible and suggested fitted a low voltage trip as he thought this might be causing the issue.
Thanks, again that’s very helpful. In fairness almost everything is a flow rate issue it seems but my filters are clear and the flow rate is well within spec and this particular fault seems to be clearly identified even if the underlying cause is as yet not. It does feel like it must be traceable to a marginal component somewhere on the PCB. How very un-germanic!
I have some information to add from the original post. As I have access to two units one of which is working perfectly, the other one had the same issue as @JamesH reported. Same huge cycle time after DHW runs & same error codes.
Based on the information I read on the linked renewable heating hub forum, I swapped the PCB between the two units to see if the issue moves with the PCB. In my case both units ended working up after that swap.
The heat pump that did not exhibit the issue has been working perfectly and is still working perfectly with the swapped PCB. Not to long ago the issue has sadly reappeared on the unit that was exhibiting the issue before. Thus it seems that the PCB itself is not the issue but swapping it did help temporarily in my case.
In appendix F of the VWZ AI heating appliance interface a list of fault codes is given with the causes and remedies. For all the sensor related faults the remedy as stated in the appendix are:
Check and, if required, replace the sensor
Replace the wiring harness
As faults are given for multiple sensors simultaneously I personally do not believe that the sensor are faulty. How would multiple sensors all fail at once, especially the temperature sensors as those are passive components?
As such I have very recently also interchanged the cable bundle that connects to the sensors between the two heat pumps. After all when replacing the front PCB, one has to disconnect the connectors of the cable harness to do so and maybe in my case that might have helped.
I am currently on the other side of the of the country so unfortunately did not have time to check if it worked.
P.S. I also swapped the rear PCB but that did not help in my case.
@Henry1 Thanks, thats very helpful. Like you I find it completely inconceivable that 5 sensors fail simultaneously, so for me thats completely ruled out.
My impression is that its intermittent and possibly worse (in my case) when its hot than when its cool. Unfortunately all sorts of things could account for that, it could be a marginal ‘reference’ voltage, a comms error, a dodgy cable/connector, a dodgy power supply and doubtless several other things I haven’t thought of. Thus the more info available the better.
At present my heat pump isn’t doing much so Im not seeing anything useful. I have cancelled cooling to rule that out (not that I really suspect it) and DHW reheats only about once every 3 days. There should be a DHW run tonight which may turn up something. I frankly doubt, based on whats reported here, that Vaillant have a definite diagnosis. However I suspect like others here that they do know there is a problem.
I put an initial email into their email support who responded ‘it could be a comms error due to voltage fluctuations or induced voltage, but suggest you contact our technical team’. Its not induced voltage unless the bus, which is tied to a pipe and nowhere near any electrical feed, is sensitive enough to detect the water movement. I do monitor supply voltage with a Shelley device but Im not sure the temporal resolution is adequate to rule this out. That said the primary noise (at relevant frequencies) on voltage is likely caused by the heat pump itself and I would hope it would be immune to voltage fluctuations it causes!
@JamesH Hi. I’m new to this. I’ve gone full on solar, Battery and HP install. My arotherm was in for a month and I had exactly the same issue as discussed here, exactly. I called Vaillant UK and they sent an engineer. He said he know what it was immediately. I am told that Vaillant outsourced a wiring harness to a 3rd party and they were no good. Vaillant took it back under control and manufactured them themselves and he said he put one of the new ones in. All was well for a month and the unit did the same again. They sent the senior guy this time and he put a new harness in manufactured in 2026 ( the last ‘new’ one was 2025), replaced the PCB and replaced sensors. It’s working again now but should it fail again, my supplier have promised to give me a new heat pump.
Thanks that’s very interesting. I’m going to write to vaillant now. Do you have anything in writing from Vaillant discussing this known issue with the wiring harness?
If they know, they really should be contacting me to replace said wiring harness.
Interestingly my example of the same problem (or at least the same symptoms) has mysteriously disappeared now it has cooled down. Looking back to last year I also had some random faults (not the same) when it was very hot. Does my heat pump hate the heat as much as I do.
I’m afraid not, no but I got the distinct impression I was not supposed to know this. The senior guy today went away making sure I knew it was a 2026 harness. The reason is replaced some sensors (apologies I don’t know which ones without looking on the service sheet) was because of resistance. He said that there was a minor discrepancy in resistance and so matched sensors to be as close to each other as possible. Respectfully, the first guy disclosed something he probably shouldn’t and given he was a good bloke, if you do go to Vaillant with this as is your right, just tell them a little birdie told you ha. I wouldn’t want him in bother, no matter how remote the chances! @JamesH