Vaillant Arotherm Owners Thread

Timely post, having got my overall COP sorted I’m nagging my installer about HW. I’ve got 73C at the end of my lunch cycle today with target of 54C. Compressor ends up at 65.9% but Power remains on Max. To be honest as you’ve indicated I don’t think the settings will give you much difference in either efficency or power consumption for the difference in time to heat.

Suspect its the Conf. DHW build. pump set to 100. This is where some definitions in the manual would help! Its either a misnamed parameter or needs explaining!

After playing about with various fixed pump speeds on the DHW side, I’ve had it stuck back on Auto for the last month or so.

I get same behaviour in Auto. Am awaiting a response from my installer, who to be fair since I’ve started throwing screenshots and questions at him on email has come back with resolutions.

Any chance you were doing your legionella run at the time? Even though the documentation says it should heat to 60ºC once and then keep it at >=60ºC for an hour, it actually heats it to 70ºC… :sweat_smile:

My installer called Vaillant about this and they confirmed that this is normal. Looks like a bug in their documentation.

70C is an overkill for Legionella

60C is sufficient, this can be reduced to 40C-50C if the tank gets a high turnover. You’ll get a higher COP by reducing your DHW max temp to 60C

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I’d love to. But I can’t. Vaillant does not make this configurable.

I also do not know my average daily turnover unfortunately.

My 190 liter tank (uniTOWER) is configured to 45ºC with 5º hysteresis. Based on your diagram, I think I’ll:

  • change it to 48ºC and keep the 5º hysteresis — which should keep me in the dormant/slow growth parts
  • disable the legionella run

It definitely is destroying efficiency. That’s crystal clear.

6 posts were split to a new topic: DHW storage and Legionella Risk

Heat batteries are quite commonly used, their main advantage is they take up less space than DHW tanks. The main manufacturer is @Zapaman has a Sunamp heat battery with his ASHP.

The main disadvantage with heat batteries is there are a bit more expensive than DHW tanks and require a higher flow temp to recharge than a DHW tank, so they will result in slightly lower heat pump COP. However, this may somewhat negated by the lower thermal storage losses.

Heat batteries are a great solution for flats or apartments where space is limited.


Nope, haven’t done a legionella run for about 6 months now. I should really go back to my installer to speak to the Vaillant guru.

In other news I got an esera pcb that I’m going to hook up to the interface to see what I can pull from that. I’ve also jumped onto the Octopus Agile tariff so will play about with home assistant to see if I get that integrated with the Octopus API and Vaillant either through the SensoApp or the PCB. With the Agile tariff and some automation that will make my bills 50% or less compared to a boiler.

I’ve got an SMA Home Manager 2.0, which talks to Vaillant heat pumps via EEBus. When there is sufficient PV surplus, it heats the tank an extra 5°C.

Per the above, my default target is 48°C. Today surplus PV heated it to 53°C.

This was a few hours later:

EDIT: 99% solar heated:


I am still looking for an installer for a heat pump who will install a direct system without hydraulic separation.
The latest visited last week and he seems to be good but is saying that he thinks there is need for a buffer tank and a separate circulation pump as the Vaillant heat pumps do not have powerful enough circulation pumps for a retrofit. He is saying they are really designed for new builds.
This seems odd to me as most of their business must be retrofits.
Should I be looking at a different heat pump manufacturer with better circulation pumps?
Any advice?

Almost every Arotherm on the heatpumpmonitor chart is a retrofit

Now there ‘could’ be a valid reason to separate and have a secondary pump.

But you’d only come to this decision after doing all the calcs and the measurements.
I think the aim should always be no buffer, no separation unless the calcs say otherwise.

It seems like many installers just skip the calcs and default to separation to cover their backsides or cos they don’t understand why they shouldn’t?

Where in the UK are you? Any joy from the heat geek map?

Thanks for your comments. I have tried the heat geek installers and they are all too busy at the moment. I am based in SE Wales so I have tried Bristol as well as the South Wales installers.

The latest installers have promised me the calculations so I have told them I will have to be convinced by them…
I will keep plugging away…

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The best installers always will be too busy. Beware those that are not busy!


Unfortunately the ones you are looking for cannot cover more than an hour radius.

This is exactly what you are looking for.

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Hi Vaillant owners, I have a question that I cannot find any answer online, yet.

What compressors do Vaillant use in their HPs? I know they are scroll, but are made in house by Vaillant or are something else from other manufacturers?
Thank you for anyone that is able to say or post a photo of the label.

How well does the Mixergy tank work for you?
How do you use it? I imagine you would use low cost electricity to fully heat the tank and then just keep it topped up until the low cost period is available again.
Is that the right usage model? Do you feel the mixergy is worth the extra cost?

It looks like I have finally found a couple of good installers so just considering what tank to go for.

I’m really happy with mine, it’s been in since April last year.
Having an absolutely consistent output of HW temp right down to the last drop is great.
My solar PV trickles into it; at this time of year I can recharge an entire 210l tank by lunchtime. In off season I charge it on off peak overnight to 70-80% or so.
I’ve recently had a HP installed but for now have just connected to Mixergy via the normal boiler primary connections. Not great for COP I know, but the heat exchanger bits are a little rich (500 quid or something) and I’m doing so well with PV divert that the value isn’t there for me right now. I probably will upgrade to the heat exchanger in the future if our winter HW turns out to be more than expected.

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If I had my time again installing the heat pump and cylinder, i’d just go for the standard Vaillant cylinder.

The Vaillant 250L is about £1500 whereas the Mixergy is £1700, then you need the £400 heat pump plate kit and on top of that, if you want to PV divert you need the £160 kit for that.

There is no way you’d get those savings back imho with a heat pump.

I’m not seeing any different (or better) COP with my Mixergy setup on my Arotherm than others being monitored with standard cylinders.

Yes, it’s nice to be able to see % full and there is a great Home Assistant plugin, but you can get temperatures from any other cylinder with some temp probes.

If you didn’t have a heat pump and you had the mixergy plumbed into both gas and electric, then there could be some advantage to let the brains choose the cheapest source and make use of time of use tariffs. But this seems like small subset of users.

Heat pump owners are going to get COP of 2:1 or 3:1 and above in warmer weather. So little point in using direct electric (1:1) unless you have so much solar going free that you can use that via a PV diverter.

Another though, many more components on the Mixergy to go wrong. Circuit boards, raspberry pis, heat pump and pv divert boards.

Final thought… as the Mixergy relies on the cloud, what happens if Mixergy ever go bust? What are you left with then?


Thank you both for your replies.
I have struggled to see the value proposition of Mixergy when I have a heat pump.
It will make sense for me to use the HP for the hot water and sell any surplus power. I also have concerns about the complexity. The heat pump kit has another pump as well as the other complexities.
I will keep thinking