Vaillant Arotherm Owners Thread

Yeah that seems to be about the floor there. I guess you need conditions such that outside temp is low enough that the ongoing heat loss perfectly matches your minimum steady state output of the HP, which is a less common set of conditions than you might think (I’ve been trawling back through both your data sets to find a long enough ‘burn’ that matches this!). By coincidence, today might be that day!

It’s actually weird that the output floor drops as the temp drops. Right hand chart is output (left is COP).

1.7kW output at -2C outside!!! :rofl:

I assume that ‘red’ on the header row is just indicative of % fan reduction from maximum (120)?

Yeah, my Czech to English google search came to that conclusion!! :wink:

rps being fan speed.

Hugh, this episode of betatalk might be worth a listen.

Why not to use zoning, no buffers, no trv, 3rd party controls… just try to design a fully flowing ‘open’ system.

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Do you mean the “Compr.start heat. from” setting? This is -999 to 9 range according to my uniTower manual, but I have never tried to change from the default -60 (yet). Making this setting more negative should increase the time between compressor starts I think.

I don’t have a heat meter, but the here’s the heat pump app display of the rest of the data from my significantly oversized heatpump for the last 24 hrs.

So heating requirement is 1.5kw @ -3C outside temp (obviously lots less at current 7C) and at the current COP of about 5, the unit is delivering c. 2.3kw while the compressor is on in semi-steadystate mode after it modulates to 25%. This is the smallest Arotherm+ with 0.4 heatcurve and using Expanded room controller mode with a desired temperature of 21C (constant, no set back). The initial few hours shown are when thermal gain in the main room shuts off the heatpump (interesting convective overturn in the flow temp: the heat pump is 2m above the internal temperature sensors). dT is around 2C max (except reaching dT5 in the DHW cycle). Note that the heating cycles keep the room temperature just on or just below the 21C desired temp. As I commented up the thread, the hp seems to deliver just enough heat to meet the desired temperature, but not to exceed the 21.2C “HP off” temperature (21C+0.2K).

So, lots of cycles, with very limited run times as you see in your setback period. I am planning on playing with the “Compr.start heat. from” value to see if I can reduce the number of startups.

It would be so nice to know what each of the settings in the controller does, and have some guidance on what varying them will do to performance. For instance, I played with the Max. Flow Temp setting during the cold period, because it was set lower than the temperature than the heat curve indicated was required. Changing this obviously made a difference, although the set maximum was regularly exceeded both before and after the change. Vaillant could easily provide details and guidance advice if they chose!

Isn’t this compressor pump rpm not fan?

@AndyS . Cirrect i was refering to

My manual also says 9 to -999 but when i try to make changes it only alows me to go between -30 and -100 . Perhaps there is another setting that has an interplay? Or they have made changes that have not made it to the manual publishing dept?

You seem to have very similar cycles. My data resulutions are too low to really see what is happening.

Having called the ‘technical’ vaillant help desk the people who know the meaning of these terms are few and far between!

I see the same. The only reason I was bringing this up again was due to comments on the FB group; i) installers suggesting this was wrong ii) another owner saying he saw his modulate along with the compressor speed.

You can see compressor speed actually, it’s visible in the “live monitor” as a %, which can then be mapped back to the performance tables.

Don’t know how the compressor works (i.e. pump vs. fan), but the rpm in the performance tables maps to the “compressor modulation” percentage available in “live monitor” and is different to CH/DHW pump speeds.

Very interesting pod cast - thanks for posting it.

A couple of observations:

  • It advocates having the common areas (Living areas) with no zone control and then controlling the bedrooms to give lower temperatures if wanted.
    The rooms next to cooler rooms need to have big enough emitters to cope with the heat loss to the cooler rooms

    • this is what I am hoping to do.
      The problem I have with this is heating from other sources may make some rooms too hot. My living room is south facing so on a sunny winter day (like today) it gets a lot of heat from the sun and may then get too warm if there is no control of the temperature of the rads. The heat from appliances in the room can also drive the room to temperatures that are too high
  • The description of the different types of buffer tanks is also very useful. It looks like the use of a volumiser (2 pipe buffer) is fine but other buffers should be avoided.

All helping - thanks

I absolutely agree with the open system model, but there has to be some leeway to adapt to the property.

I’ve got TRV on south facing rooms for the reasons you mention and I also close some bedrooms off etc.

As long as you design the system with enough ‘remaining’ volume after the TRVs have potentially shut down, you should be ok.

Brendon on the podcast say 15-20L of system volume per kW of heat output to help stop cycling.
So on a 5kW with the minimum heat output of 2.5kW, that’s around 38L to 50L at all times.

You’d assume you’d only work to the minimum output as if you need more output, it would be cold and rooms would be open?

If you haven’t got this much volume in rads/ufh, then you’d consider putting in a volumiser (not buffer!) :smiley:

In a perfect world, you’d keep every rad/ufh open all the time so would have oodles of volume at all times. But the UK housing stock is far from perfect!!!

That’s my “non-heating engineer” take on things, anyway.


Heat Geek engineer Ken Bone brought this design flaw to my attention on Twitter.

Apparently it’s common that water/rain can get into the recess between the top lid and the cover and then into the electrics behind the cover.

One solution recommended by Craig Brookes Heating is to put a bead of silicone across the gap to stop water ingress from above.

Which I think I’m going to do once things dry out a bit.

Here’s a link to the Tweet and video

I’d be inclined to stick a strip of fairly flexible plastic to the lip of the metal top cover such that it simply fires any water over the gap and onto the plastic cover. Flexible enough to get to the screws underneath.

Or a bit of gaffer tape!


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I have been told by an installer I am speaking to that Vaillant will reduce their warranty if I use frost protection valves rather than Glycol. Does anyone have any other comments on this?

This does not sound right. Have you tried ringing Vaillant for confirmation?

Going the gycol route is not best for performance.

To help Arotherm owners that aren’t privy to this forum, i’ve re-written the “How to work out Arotherm COP” article onto my website.

Hopefully allows more people to find it and benefit.


Thanks I will give them a buzz
Congratulations on your feature in the BBC article on heat pumps.

Hi @Zarch, great thread thank you. As an owner of Arothermplus 12kw I wish I had stumbled across this earlier. I had mine installed in March but didn’t get the wiring of the controls sorted correctly until October after countless attempts with various electricians.

I’m broadly content with my heatpump and its performance, comparing my bills and usage with my old system shows about 15% saving with current rates (unadjusted for temp difference between years) BUT I have more insulation and am probably running a bit cooler. Reading this thread I need to flip my thinking and I’m bit disappointed with;

a) my installer (gave me advice based on comfort not efficiency) and
b) none of the manuals hint at efficient operation settings and finally
c) the amount of data you can get out of the interface and Sensocomfort is a different level of disappointmet considering the cost of a heat pump and cylinder. Like selling a car with a stop watch and a tape measure to calculate speed.

My system
I’m in a fairly well insulated 1920s 4 bed semi with 2008 double height extension on the back I’ve got UFH downstairs with a zone each for a large kitchen, lounge, front room and small utility. Upstairs and my hallway I have 10 Type 22 Rads and 2 towel rails on the old copper piping. I need to replace the only single glazed window in the house which is on the half landing. The original solid walls are untouched on the front.

Over December and January I’ve had the heat curve at 1.5 I dropped it down to 1.3 after the end of the recent cold snap and had just twigged I need to extend the pre-heat time when it gets colder rather than try to cycle the heat round individual rooms to heat them quickly.

The rooms retain heat fairly well and only once over December and Jan can I recall the kitchen being below 18C in the morning (set back was 17 which kicks in at 22:00 - 06:15). I leave my HW on 45C and it kicks in when required. So despite that my combined COP from the Sensocomfort panel is 2.9 for Heating and Hot Water.

Reading this thread I’ve dropped the heat curve to 0.7 and the target temperature on the Sensocomfort to 20C. Hey presto, today I’ve observed the heat pump be on at something less than 95% power consumption with flow temperature of 39C.

So can I clarify a few things I’ve picked up from the thread.

For maximum efficiency go for;

  1. Longer, colder heating runs on big loops (ideally below 50C and dT=5, though theres no control you can apply to the dT Arotherms do their thing on that)
  2. Set back temperatures to ensure the heat pump can get into a stable state of running.

Data capture

My Heatmiser thermostats give me good data on when zones are pre-heating, running and what the desired temperatures are. However I’m missing the target, flow and return temperatures and whether the HP is in Heating or HW mode. All this data is available as snapshots in the VWZ AI Interface there isn’t anything approved (or not) by Vaillant to log it, is that a fair summary?

I’m interested to monitor how my HP copes with different combinations of zones in play so I can schedule them efficiently, today I saw with my stat upsatirs calling for demand and 2 zones downstairs my HP was struggling to get up to target temperature. To do that I need the missing target, flow and return temperatures.

Whats the cheapest way of getting a data log for the flow and return temperatures? I can probably use the flow temperature to infer whether its HW or Heating.


Some DS18B20 sensors and a Wemos Mini with ESPHome or Tasmota.