Vaillant Arotherm plus compressor blocked during DHW run

Hardware Heatpump
1

Hi all,

I have installed a Vaillant arotherm plus VWL75 A/6 heatpump for my parents and they have used it for a little bit less than a year and it has worked fine so far. Since about 2 weeks the heatpump does not turn itself on anymore after a DHW run. After doing some investigations into the issue I noticed that when the flow temperature of the unit reaches about 55 degrees the unit turns off and the compressor is blocked for about ~63700 minutes.

When I reset the unit the heatpump turns on again as the blocking time is reset/cleared. Only to block the compressor again during the next DHW run when the flow temperature reaches about 55 degrees.

So far I have checked the circulation pump, the diverter valve, the magnetic filter and the control module (VWZ AI). The diverter valve operates normally as expected and can be operated trough the VWZ AI module. The circulation pump reaches about 1700 liters per hour when set to full power. The magnetic filter was also relatively clean, so there does not seem to be blockage of the hot water/heating circuit.

I also tried swapping the VWZ AI module with my own to see if the problem was in the VWZ AI module but that also did not change the issue. So therefore it seems to me that this either a configuration issue with the thermostat or a hardware issue with the heat pump itself

A factory reset also did also not offer any permanent relieve (this was actually the first thing I tried) it initially did make the issue dissapear for about 2 weeks but then it returned.

I am not really sure what to do next. Therefore I would greatly appreciate any suggestion, advice or ideas from you.

2

Hi Henry,

That is really interesting. I’m facing a really similar situation with an aroTHERM plus VWL 105/6.

The heat pump worked flawlessly for around 10 months and since July I had three « phases » of DHW failure.

To make it simple, situation is like yours : after running for couple of seconds or minutes if I’m lucky, the compressor got blocked due to minimal flow not reached. At that exact time, I can see on the inside unit that reported flow turned from 1800 to 0l/h. Which is incorrect because I can see flow with outside flow meter.

First time, I was able to recover from this after ± 12h and an hard reset on full system. I suspected configuration issue.

Next time, impossible to have it on even after hard reset. Because it failed for long time, i have been able to find out failure occured always when water was reaching 35c, so floor heating and cooling was working fine. After couple of days, it started to work again. I haven’t been able to identify any root cause to this.

For the third time, DHW failed for more than a week. At some point, yesterday, it worked again for no specific reason… 3 successful runs so far.

Whenever outside unit was blocked, inside unit did not pick up and got reported as « blocked » also.

At this stage, I’m waiting for Vaillant’s guy to come in but with this fantom issue I’m afraid he’ll be clueless.

My feeling, and because I tested couple of hardware items like you did, is that the issue is software related. Or I might be wrong and It could be either flow sensor (vortex) or main board failing…

What is the current status on your side ? I would be really interested to know if you made progresses and if a link can be found between your issue and the one I’m facing.

Could you tell me more on how you identified the « blocking period » you mentioned ? Also, do you have any ebusd instance running on the bus ?

3

Hey did you get anywhere on this? I’m having similar issue.

4

Hi

Yes, the flow sensor (vortex) has been replaced and it fixed the issue.

1 Like
5

Hi all, I am still having this issue. Vaillant are meant to be coming, but I feel they might not be interested as they said the flow rate maxmium is too low (1750l/h) and wanted the installer to recify this - ive only had the unit three weeks. Does anyone have an idea on the below? For me it is NOT a flow issue, I have an idea of something else but I have three weeks experience. I am pretty anxious having spend a shed load of money and it’s going to be -7C on Friday.

Issue described below in chronological order:

Heat pump working fine

Heat pump does a domestic hot water DHW run

Either during of after DHW run it stops

Heating doesn’t restart

On Vaillant interface it gives errors – but no errors on app or sensocomfrot

Under live monitor

- Heating compressor blocked

- Anti cycle time 63773 min

Sometimes fault codes, sometimes not

Target flow is often low and equal to current flow temp (not as it should be on curve)

Fault codes

- F703 585 520 519 73

- F703 Sensor fault low pressure

- F585 Sensor fault Capc. Outlet Temp

- F520 Sensor Fault Building Circuit flow temp

- F519 Sensor Fault Building Circuit Return Temp

- F74 Building Circuit water pressure

Sometimes one ‘reset’ makes it work again, sometimes not. Sometimes needs lots of resets.

6

@Satwelsh I saw you comment elsewhere with helpful knowledge, do you have an idea?

7

I’ve found the exact same issue discussed on renewables heating hub:

8

Are you getting ~1750l/h with the pump power at 100%? If so, you have a flow rate problem, likely the primary pipework is too narrow/too long/too circuitous/all of the above.

Can you monitor a DHW run with your ebusd dongle and post the resulting graph with flow, return and DHW temps, as well as flow rate?

9

Yes 1750l/h.

Have you looked at the linked thread? It’s exactly the same issue and was electronic.

I unplugged the ebusd incase it was the cause, so it’s still unplugged, but issues persist. And the issue is now persistent and outside of DHW cycle also.

I don’t see how this is a flow issue (though our flow could be higher), because it worked for a few weeks, and still works sometimes. It’s completely irregular. Though I am pursuing the installer to come back and fix the flow issue - but it seems that’s not going to be easy.

10

Have you completely powered-down and power-cycled the entire system? Outdoor unit, all controls etc?

11

Yes for 10 minuts. I had to do it yesterday a few times, and today. Eventually after hitting reset on the machine a lot of time it worked again.

Vaillant Spain are coming thuraday to look.

12

Vaillent changed the circuit board in the machine today and it’s working again. Let’s hope it stays that way. And their independent action tallies with that fork the discussion above.

1 Like
13

Glad it’s sorted, hope you weren’t too cold in the mean time!

14

Good thing I kept the big log burner for emergencies!

Let’s hope its fixed for good. It’s now snowing outside and going to be down to -7C tonight and not much above 0 till Tuesday.

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15

Hi all, sorry for the late reply. @brice I managed to contain the issue. I have swapped the main electronics board from my parents heatpump with my own. Surprisingly enough ever since that swap both units have been running without issue (so far, fingers crossed on that statement). Which is a really strange therefore I am still uncertain what the actual root cause of the issue is. It seems to me that the issue is most likely in either wiring connections or hard/software related. Other than that I don’t have much to go on.

The blocking period can be found in the installer menu/level on the VWZ AI module. Hold both the top buttons shortly and then navigate to the installer level if it asks for a code fill in 17. Then under monitor you can go trough a whole bunch of sensor values that it is reading from the heatpump. I know your issue is already solved but in case you were still wondering this is how I could find the blocking period.

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16

My issue has not returned since the internal PCB was changed. I’ve now got 500h on the unit.

Interesting this PCB is clearly the identity of the machine too as the serial number changed and stored values for run time etc were zero when new board was put in.

17

@JamesH @Henry1 I wonder if you could give some more detail on the systems which had this issue as I’ve just started seeing it with our heat pump that was working fine for the best part of a year.

There was a leak from the buffer tank which was fixed and I wondered if that introduced air into the system but it was ages ago.

Other than that I do wonder if it could be caused / made worse by interference on the EBUS line.

All the fault codes are related to being unable to read the sensors (if you go into the test menu and look at the sensor values all the erroring sensors are just not displayed).

In addition the Anti cycle time 63773 min is not wildly far away from an unsigned shorts maximum value (65535) so I wonder if there is a message on the ebus that sets the cycle time which can get corrupted?

Anyway I’ll be phoning in an engineer tomorrow but always interesting to try and figure out what is going on.

My install does not have the EBUS lines done to spec, un-earthed shielded cables, no crimps on the wire ends, but if you already investigated that I think I’ll just agitate for a new PCB!

18

Sorry to hear this. What a nightmare.

Just call vaillant, it’s a known issue (I feel). On the phone to UK vaillant that said software error to my symptoms immediately.

I’m in Spain, and on the phone they dismissed me a bit, but I said go away and talk to head office and think about it. They then promptly turned up with a few PCBs, diligently checked everything and then swapped the main PCB in the unit. Mines now done 1000h without and recurring issue.

The thread on renewableheatinghub lists the symptoms perfectly.

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19

@jms301

Initially when I was troubleshooting I also suspected that the issue might be caused by interference leading to corrupt values being present on the Ebus leading to the huge anti-cycle times. Thus I replaced the flexible wiring with solid wiring but this did not change the behavior I was observing.

I did not really investigate the wiring further after that because no changes were made to the cabling and the system ran OK since January 2025. Also I never set up Ebusd monitoring for that system. So unfortunately I can not comment on whether or not there were corrupted messages being sent over Ebus.

It seems to me that there can be multiple failure modes (bad flow sensor, bad contact from PCB to sensors, Faulty PCB). Which in turn lead to the huge anti-cycle times that we all observed.

As you mention not being able to see any of the error sensors in the test menu I suspect that a failing PCB is the the most probable cause of your issue. Hence agitating for a new PCB indeed seems like the right way forward.

If that doesn’t work the issue might be caused by bad contacts (I read one post where the engineer replaced the entire cable harness) or failing sensors.

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20

Thanks both.

Vaillant tech support agreed and they’re sending a technician out Thursday so they are at least very quick! I will ask them about other questionable elements of the install when they show up.