Vaillant Arotherm Owners Thread

Thank you for posting this information.
The COP figures do not seem to be consistent with the SCOP quoted by Vailliant.
I am considering the Arotherm 7Kw and it has a quoted SCOP of 3.91 at 45C flow. If I use the COP numbers from this document I get a SCOP of about 3.2.
I assume they are using the European Average climate (Strasbourg) for their calculations.

Any thoughts on this?

How to measure and show COP without a heat meter

ie, just using the data from within the Arotherm Plus and compared to OpenEnergyMonitor heat meter data.

Below are OpenEnergyMonitor / heat meter figures recorded on my 5kW Arotherm Plus during December 2022, where temperatures in Sheffield dropped to -3C (average 4.3C across the whole month).

All figures / COP shown are for both heating and hot water combined.

OpenEnergyMonitor / Heat Meter figures

  • Electric input: 647kWh
  • Heat Output: 2195kWh
  • COP: 3.39 (2195 / 647)

Measure COP Using Vaillant Data

Vaillant explain on this document how to measure COP.

The gist being:

So we need to know where to get “energy yield” and “power consumption” from.

I have the Sensocomfort room controller and you can get the figures from there.

  • From the front menu showing “Control”, “Information” and “Settings”, choose “Information”.
  • From there, choose “energy data”, which shows you “environmental yield”.
  • This is the “energy yield” mentioned in the Vaillant calculation.

(perhaps this difference in terms is lost in translation to English?)

Like me, if you take this snapshot on the last day of the month, you can see the total for that month (current month). Alternatively, just do it when your into the next month and look at “last month”.

For Power Consumption, again from Energy Data, choose the “Power Consumption” option.
In here, the data can be seen as

  • Heating Only
  • Hot Water Only
  • Cooling
  • Installation (ie, all the above added together)

As the Yield figures aren’t broken down into the separate elements, i’m not sure how useful the broken down power consumption figures are?

So for us to make a calculation we need to head into the “Installation” option to get the total power used.

So for “Current Month” we’ve got the following figures:

Environmental Yield: 1427kWh
Power Consumption: 631kWh

Remember the Vaillant calc was (Yield + Power) / Power, so
(1427 + 631) = 2058 / 631 = 3.26 COP

Note: You can also get the monthly Yield figures from the heat pump controller, but there is no way of getting monthly power consumption from the controller. So it’s really tough to make your own calcs from the data on the controller.

But the controller does show you its own COP figures, which it calls “Working Figure”

Here are the Working Figure screen grabs for the same day (last day of December) from the Heat Pump Controller (VWZ AI).


The bottom line is that the COP from openenergymonitor / heat meter setup is pretty close to the built-in Vaillant COP figures (using both the Working Figures from the controller or using the raw data from Sensocomfort controller).

So on this evidence, the built-in information on the Vaillant Arotherm Plus is likely good enough for most regular folk.

  • Heat Meter COP: 3.39
  • Sensocomfort COP: 3.26
  • Controller: COP 3.2 heat (2.4 hot water) - AKA “Working Figure”

But whilst the COP are around the same ballpark, we notice that the raw data from the Sensocomfort isn’t exact when compared to the heat meter.

Electric Input (3% out?)

  • Heat Meter: 647kWh
  • Vaillant: 631kWh

Heat Output (7% out?)

  • Heat Meter: 2195kWh
  • Vaillant: (1427+ 613) = 2058kWh

But close enough for most people not to care? :man_shrugging:

Hope this helps. :+1:


Thank you - a very interesting analysis.

  • When Vailliant quote a SCOP I presume they calculate this using the method outlined in EN 14825 using the average Climate Zone (corresponding to Strasburg). My point was really that using this methodology I cannot see how the SCOP quoted can be attained from the COPs in this document

  • I presume the advantage of the Open energy monitor is that it provides more underlying information such as flow and return temperatures and much information on the change of performance over time.

  • It is interesting to look in more detail at the Sensocomfort heating controls which offers zone controls. I have quite a large property and would like zone control maintained but some of the installers I have spoken too are very against it. (I currently have the Honeywell Evohome system with my gas boiler)

I suppose SCOP is a really tough thing to predict as the weather does what it wants and doesn’t follow any set pattern year on year.

All I can say is that my installation hits the quoted COP values from the Vaillant technical documents at the stated flow/outdoor temp.

ie 7C outside at 34C flow I get COP of ~4.6
and 0C outside at 41C flow I get COP of ~3.0

We’ll see come April/May time what the real heating season SCOP will be for this winter.

Yep, it also allows you to make system tweaks and instantly see the effect.

Heat pumps work best with lots of flow and space to move around in. Cutting off rooms/rads etc will crash performance. As you can see with mine, you just let it run, best on weather compensation so the flow temp adjusts versus outside and it puts just enough heat into the rooms to match the heat going out.

Having seen this before, a evohome telling the heatpump to turn it on and off is a disaster waiting to happen. Best to use any rad TRV as limiters only, not for them to make on/off calls.

You can still zone sections off on the sensocomfort, floors etc. But you still want lots of flow to breathe and let the system just keep pumping away.

Thanks a lot - I really appreciate the advice. It is a different world with Heat Pumps!!

Fantastic! I have been hunting a this comparison for a while.

Although i still suspect my vailant heat yeild figures are over estimating. I have a heating working figure for jan so far of 6.8!

The pump has only been in since september so i have had to use degree days to compare to my previous anual gas use. And this is showing the house is using 50% more heat than we did on gas. House is a degree or 2 warmer and we have reduced our wood burner use and there has been less solar gain for the period. But with a ‘cop’ this high and the ‘incresed usage’ i suspect mine is not estimating heat out to your accuracy. Tempted to fit a heat meter to settle it once and for all…

Well insulated mid terrace. 7kw arotherm (over sized i think). Ufh and rads. Heat curve 0.55.

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Does your (or anyone else’s) ASHP modulate the heating pump speed (and in turn flow rate) when the compressor speed is reduced?

Multiple people on the FB group, including myself, have never seen any pump modulation but installers (who also work with other brands) seem surprised that the AroTherm wouldn’t modulate pump speed. What’s strange is that there is one guy who is convinced that his does modulate pump speed.

@AndyS @Zarch Do yours modulate pump speed? Or has anyone spoken to Vaillant about this?


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@dfeist Yes the compressor and circulation pumps modulate on my Arotherm+. Or rather they “step”. Compressor pump modulates to 25% as per post in Dec. above, with flow rate remaining the same initially. At some point, possibly after the hp is cycled off, the flow rate steps down. I can’t monitor this easily (have to use lcd on UniTower), so can’t say much more.

Fully agree with this. I run emitters via a heating manifold which initially had electronic activators driven by room thermostats. These activators are now all disengaged and hanging below the manifold, with the flows to each circuit tweaked via the individual flow adjusters to deliver the required heat output.
I have learned to disregard everything I ever thought I knew about wet heating systems (admittedly not much!) since the ASHP install. The key for ASHPs is definitely to match the emitter sizes to the heat loss for the room in question, then let the hp do it’s job.

And in a passive house, SCOP is low because the ASHP spends most of its time circulating with the heatpump off as no one produces units that modulate down far enough!

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Thanks for the details Mick. I agree that the figures differ between the two Vaillant sources. I asked Vaillant why but never received a response.

Mine reduces pump speed when the compressor is off (to about 2/3), but that is it. I’ve never seen any modulation when the compressor is on. Even when with the compressor running at 25% the pump speed is still 100%.

Is this an argument for using “expanded” in a passive house? I was previously using expanded but then moved away from it as, given the very slow reaction time of my in-slab UFH circuit, didn’t seem logical.

Sounds like what I am seeing then!

Room Temp mod is on expanded, if that is what you mean. The controller does not turn off the ASHP/circulation at the set desired temp: there is a small offset built in. It looks like the algorithm on the hp is balancing the heat supply just right to hit and maintain the “desired temp”. Only time the room temp hits the “off point” is when there is solar gain in the room where the controller lives.

@dfeist @Zarch
I’m wondering if this is a different control strategy by Vaillant. Mick noted

for a given output, dT and flow rate are fixed. If other manufacturers have dT fixed, they need to modulate flow rate. Vaillant appear to fix flow rate and let dT vary. I’ve never seen dT of 5C on my install in heating mode and Mick noted this too, so this is compatible with a different approach. Of course modulating both dT and flowrate is possible, but might it be a bit difficult to optimise?

When first installed the unit in October, I played lots with flow rates trying to chase DT5 at all times. I set flow rate to 100%, Auto, 75% and even messed with the max.rem.head setting.

Came to the conclusion above that the Vaillant fixes flow speed and lets the DT vary to get the heat output it needs. Regardless whether you set it to Auto or 100%, the flow rate is the same, ie as much as it can push through. DT can be as low as DT2 when warm outside.

The only way I have been able to increase the flow rate is to open up rad valves fully. And I intend to do more of this type of playing over the next month or so.

Obviously, we can’t monitor what the compressor is upto, we’d need to hook into eBUS for that. But we have to assume that the compressor (and fan speed) is modulating up and down as the outside temp drops and rises. As we’ve recorded with the heat meter, the flow rate remains constant.

Personally, I have seen DT5… it was -3C outside!! :smiley:

Hi @AndyS and @Zarch i have an oversided 7kw hp and should be 5 i suspect. I have a night set back of a couple of degree and the heat pump comes on every 20 to 30 mins in the night for 5 to 10 mins, due to the pump not being able to go low enough. (And the heat loss calc being over our real requirements)

My question is does it make sense to turn the pump off for a few, say 2 hour periods on mild nights to alow it to have some proper work to do in the gaps? Or is this just over working the pump with poor cops? I realise this might lead to a less stable inside temp.

To try to increase cycle on time and off time I have put the energy intergal up to 100 (max) and this has helped increase the time between cycles. With no noiceanle effect on confirt (The house has a lot of thermal mass.)

I’m now playing with the compressor hysterisis . But i need to check its not increasing cycle time at the expense of higher flow temps. I suspect a higher hysteris temp combined with a notch lower on the heat curve might correct this. Thoughts?

If you look at mine overnight, my setback does the same to some degreee. Runs for 15/20 mins and drops off, with about 2 runs per hour.

Could you drop the curve? So the flow temp is lower and not putting as much heat into the building?

Are you monitoring elec input to see these cycles?

This sounds a sensible option, but i’m not sure how you’d schedule this? Not sure the standard controls would allow this?

I’ve never dabbled in there, sorry.

Thanks @Zarch. I dont think i can lower curve further as the current curve is maintaining the house at the right temp.

I have wiser controls so scheduling would be fairly straight forward.

My hp specific electricity monitoring is via a snoff pow eleite. Unfortunately after buying it the usage date is averaged to the hour so not much than usage available. I spotted cycling on the solar inverter but this only has a 5min sampling rate. So when emonpi is available i will probably go that direction.

Interesting that the steady state floors from heat meter data on both installs bottom out at about 2350-2400W (datasheet claims 2100W but I think we can put that in the margin of error category).

2300W right now. 6C outside! 34C flow.