Vaillant Arotherm Owners Thread

1.5kW at -3C? Almost passivhaus?

My heat loss survey was 5.4kW at -2.5C

You can read all about my design and how we sized it here:

and a shorter summary here:

I will be very interested to see the heat meter trace from the 3.5kW unit. Just to see ‘how low’ it can go, in terms of electric in and heat out.

The 3.5kW is the smallest unit on the market I think. but I have heard it’s just a downrated 5kW unit. The ‘golden’ doc suggests there isn’t much difference in the ‘floor’ of both 3kW and 5kW units. It’s more than that the smaller unit can’t reach as high.

Yeah, it’s on 65%
I had to leave it here as I had issues with my heat meter when I ramped the flow rate up.


I have at last worked out what controls the Vaillant heat pump cycle timing; this may have been obvious to all but, just in case, here is my understanding in the hope it might help others. Thanks for contributors on What is Energy Integral - Air Source Heat Pumps (ASHP) - for the penny dropping moment, which I followed up with an hour watching the energy integral in Live Monitor on the Vaillant lcd and the pump power draw feed in Emoncms.

On the VWL35/6 at least, this is what happens:
Compressor starts at Energy integral of -60degmins (default setting, can be changed).

Every 30 seconds the difference:
(Flow temp) - (calculated target flow temp)
is calculated and added to the previous degmin value. So, if flow temp is 2C lower than target, -60degmin changes to -62degmin after 30secs.

The degmin value continues to get smaller (more negative) at an ever decreasing rate until the flow temp exceeds the target temp. At this point, the compressor modulates back to 25% (which appears a hard coded minimum) and, because the sign of the temperature difference has changed, the energy integral begins to increase.

Once the energy integral reaches 0degmin, the compressor goes into standby mode, with the energy integral continuing to drop every 30 secs.

Once the energy integral reaches -60degmin, the cycle starts again.

So the absolute value of the increment to the energy integral is clearly dependent on the balance between the rate of heat energy added by the compressor and the rate of heat loss in the emitter system. The balance of heat loss versus the modulated heat input controls how long the “on cycle” lasts, while the heat loss alone controls the length of the off cycle. Intuitively obvious once written!

Looks like the “heating start from” value may be an interesting tuning parameter, but that is a thought to follow up once my monitoring is more automated!

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Hi Andrew, I was using stock ebusd config files … which are auto loaded as ebusd discovers devices on the bus. A few weeks ago I decided to try and get the necessary feeds working to populate the emon ASHP application (I needed a live Yield value) and as a result now have the config files locally (using a modified GitHub branch from mwildbolz) .
Good luck with Vaillant - but I suspect the reason ebusd exists is because they wouldn’t make the data accessible

All data from emontx3 and ebusd. A wishful 30min CoP!

Thanks Steve. When I eventually get an Adapter 3 I should like to call on your experience!
EEBus specification files are corrupt on download, so making no progress there either.

@AndyS yes, NP

Me too, we’ll perhaps create another thread for that? Unless you wanna create that now so insights can be shared?

I’m using EEBus from Loxone. Works but is fairly basic what you can do with it. The main issue I have is that it doesn’t support cooling.

Vaillant appeared to promise a Modbus gateway in a press release 18 months ago but nothing is available to the public yet. Although a Vaillant UK rep suggested they are using it in some instances in the U.K, but wouldn’t give me any further details.

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@dfeist: Many thanks for contributing this lead to Loxone. Looks like they have a reasonable product line, but at a cost. At least their solution allows for everything to be monitored and controlled within the local lan/wlan. The case example at EEBUS Gerät suggests that, as you say, there is very limited data exposed via EEBus, yet Vaillant suggest their SMA EEbus integration allows full optimisation. Seems a bit of a mismatch there, to me.

Not sure what this is, but what is available via Loxone is the same as what is shown on that page you linked to. When I first ordered AroTHERM I was working on the assumption that the “Modbus Gateway” that was announced in 2020 would be available at some point.

See: New control units for more convenience – A new gateway for more control

There is some sort of Modbus gateway being used in some installations, but I can’t get Vaillant to give me any information on this, and my installer has also been successful in getting any details.

See Mark’s comment here: Redirecting...

I should probably give up asking/waiting for Vaillant to provide something and go down the ebus route, but I didn’t get on the waiting list for the ebus hardware yet.

When I was looking, there was a gshp solution from ISE  SMART CONNECT Modbus Vaillant - ise Individuelle Software und Elektronik GmbH . They also had a KNX + ebus adapter solution for older Vaillant controllers. Didn’t work with VR720 at the time.

Ebus adaptor 3 reservations are suggesting Q1 23 at the moment.

I’m sorry you are finding Vaillant difficult to deal with: this has been exactly my experience too. I can’t see any reason why Vaillant will not expose as much of their internal data as do their competitors. What have they got to hide I wonder :wink:

Amazing set of information, i have been looking for this for a while. I unfortunately don’t have independent heat monitoring on our 7kw arotherm. I do have independent electrical monitoring and can see Vaillant control unit under reposts by between 50 and 100w. so when i work out the COP i use my measurement of electricity consumption with Vaillants heat out put. have you compared the Vaillant’s claimed heat out put vs your open energy measurement? it would be really useful to know if their figures are of a meaningful accuracy.

Hello All,

I’m chancing my luck as a fellow Vaillant Arotherm owner as I’m having a nightmare that our installer is not able to troubleshoot.

Installation is an Arotherm Plus Monoblock 155/6. It was installed in the summer and has been working well ever since.

Two weeks ago, we had a power cut just after we left the house for the Christmas break. The neighbour restored the power, and within an hour of the power going back on I got an F.788 error through the Vaillant app. This indicates a problem with the high efficiency water pump for the heating circuit.

The installer took a look and said it had completely seized, and has done something to get it working again and the system is now displaying no error messages.

But the compressor will not start even when there are obvious calls for heat (13 degree house temperature, 15 degree domestic hot water cylinder).

He gave up for the evening and is going to have another go tomorrow. Is there anything obvious that could cause the compressor to not start. Is is possible that a day without power during the cold weather could kill it through frost?

Thanks for any suggestions. We go back in two days and are not relishing a house with no heating and hot water with our 3 year old kid.

I’m afraid i’m not technical to that extent regrading compressors.

All I do know is that there’s two ways of protecting the outdoor unit against frost.

  1. You have Glycol in your system, which acts like anti-freeze. So if the outdoor unit stops working, the working fluid won’t freeze. Things should hopefully start up again once power returns.

  2. Anti-Freeze valves on the outside pipe work at the lowest part of the run. You have these if you have only water in your system. In the event of the power going off, the valves drain before the water freezes. You would then need to refill the lost water in the system before switching the power back on.

Do you know which protection method the installer chose for you?

Alternatively, you could try the Arotherm Facebook page?

Thanks that’s very helpful. From looking at the manual Vaillant recommends glycol as the frost protection mechanism. I don’t know if the installers used this or just used water plus rust inhibiters. I would hope they followed the Vaillant recommendation which is repeated in several places with the warning ‘If you don’t do this there is no frost protection in the event of a power cut’.

Not sure if there are also anti-freeze valves that would have let water out. I’ll ask the installers to confirm.

I also don’t know what frost damage would look like. If the expansion of the water/ice has burst a pipe then there should be a leak now everything is above freezing. But is it possible that there is internal and invisible frost damage that could have killed components without causing a leak?

The installer is due back today to continue troubleshooting.

This is Vaillant covering their backsides i’d have thought.

A system with glycol is less efficient than pure water. Mine is pure water with anti-freeze valves.

You wouldn’t have anti-freeze valves with glycol. No point as you want to keep the glycol in the system to stop anything freezing.

Opposite with water, you want it out of the system so it doesn’t freeze.

With a glycol system, it is usually gylcol in the outside pipework only, then water indoors (rads etc) and they’d be a plate heat exchanger to transfer the heat across from the glycol pipework to the internal water pipework.

Vaillant like protection via a glycol down to -15C according to my approved installer.

Did this get solved @Alex_Stewart? I hope it was not major failure.

As an aside, @Zarch, there is a great article just published on which tests various installation configs. Bottom line seems to be not to use buffers!

Late reply, but this is very good question @BristolJoe and I was going to ask @Zarch exactly that.
I’d note that, in my case, the new EmonTx4 measured watts on the external unit do correspond closely with those reported by the Vaillant live monitor.
The Vaillant controller Yield Monitor reports a current month and a lifetime COP estimate (“working figure”) for both heating and DHW. Vaillant don’t give an algorithm for how this value is derived. I think it is a “heatpump” running figure, so may ignore the “pump off but circulating” periods. Mick, do you have any comment on how your measured COPs compare to Vaillant numbers?


Still no fix for my heat pump. The new water pump arrived a month earlier than expected and has already been fitted. This clears all error messages but the compressor still will not start. It runs for a few seconds and then stops. He says he has checked obvious things like purging, filter blockage etc. Unlikely the compressor is dead as it would surely generate an error message.

My system is water only but with anti-freeze valves. So I’m not sure if there is unseen frost damage from the power cut right at the end of the cold snap a couple of weeks ago. With no error messages it is very hard to diagnose.

The installer has ordered new circuit boards for the outside unit and the hydraulic station, and apparently has some additional tests to do when he next comes before fitting the new boards if nothing else works.

Ouch! Sorry to hear this. The HP industry is going to have to become as good at repairs/maintenance as is the gas industry. These are key appliances and weeks with them out of service is not acceptable.

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