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

1 Like

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.

Really?! I’m not familiar with Vaillant, controls but I would be very surprised if there’s no way to set the max temperature.

That sounds like a good plan, I’ve also disabled legionella on my Samsung heatpump since it insists on using the immersion even though R32 in my Samsung is able to get the tank to 55C. I heat to 45C daily since my tank gets a high turnover.

Everything about DHW is configurable: target temp, temp of water being added to reach target temp, timing of when to create it etc.

Except for the legionella run.

(And it’s impossible to create a schedule for creating hotter water on one day than others. Then one could create one’s own legionella run.)

I do find it astonishing that we are still using tanks that store potable water rather than pass the incoming cold through a coil especially when the heating circuit passes through a coil instead.

1 Like

Isn’t it a matter of heatpumps not being able to provide the necessary flow (volume per second) at the right temperature precisely when it’s requested?

Not just that, but also the number of compressor starts/stops would increase significantly. And that’s one of the primary factors determining the service life?

Unless by coil you’re referring to the immersion/resistor heating. That could work. But is wildly inefficient: COP 1.

1 Like

Nah, the suggestion here is storing hot water from the heat pump directly in the cylinder, and then having a coil that heats up potable water later on request, using the heat stored in the cylinder. I believe that phase-change thermal storage devices (e.g. Sunamp) work in a similar way.


Oh, wow, very interesting! That sounds indeed very interesting. Sounds like that would also diminish legionella risk!

I’m guessing it’s not yet widely deployed because of the additional complexity and/or because it’s fairly novel?

P.S.: I’d definitely appreciate a link or two to learn more about this! :nerd_face:

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…

1 Like

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