right
BTW, you don’t take the defrost cycles into account on your system leading to a wrong COP
Installers are following the Vaillant “schemateque” , always with indoor unit indeed making the installation cost higher overall
We now have a potentially interesting new development on this issue!
It seems that a couple of 5 kW units are achieving higher max output capacities if the pump speed is set to max rather than auto.
Note in particular these two tests on two different systems with flow rates at 22-24 L/min:
The outputs are substantially higher than most of the other tests, that have system flow rates of 12-14 L/min. It’s not always consistent e.g the 18.9 L/min test that only achieved ~4.9 kW
Two systems is not enough to be conclusive, it would be great to have some help to verify if this pump settings makes any difference for others at the limit with their 5 and 7 kW Arotherms this winter!
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Maybe would @Zarch would be able to help with this?
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Does this very much depend on the pipework in the house and how restrictive / unrestrictive to flow it is?
Just done a quick test on my 5kW (i’m usually on Auto).
DHW Pump
823 lph in auto (13.7 lpm)
985 lph at 100% (16.4 lpm)
But in hot water I’ve pushed close to 1200 lph (20 lpm) when testing, cos of the bigger mixergy plate I have.
So is this only a solution if you only have lovely free and open pipework and are capable of pushing more flow rate. I’m obviously restricted at 985/16lpm on the heating circuit?
The 5kW usually shoots for around 865 doesn’t it? 865 x SHC x DT5 = 5kW
If you went higher flow rate, you’d obviously give yourself a higher output with all other things being equal.
We know the 5kW can push 7kW to 8kW output in Normal hot water mode. So why would heating be any different? As long as you could get the flow to make the numbers stack up?
Note: these flow rate figures are from the sontex heat meter. The Vaillant controller (via ebus) is happily showing 865 when i’m seeing 823 on the sontext!!! 
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Thanks @Zarch that’s interesting.
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Did you see any benefit in increasing the flow for hot water @Zarch
I also have a Mixergy with the plate heat exchanger.
I just adjust the DHW flow rate to try and get around DT5.
You need to change the flow rate if you change the DHW mode because of the different kW output between the modes (Eco, Normal, Sweet Spot etc).
But when I did a big deep dive into flow rate/COP, i barely saw much performance difference whether I ran 60% or whether I ran 100%. The vaillant just seemed to get on with it regardless.
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I thought about this last winter. I believe that if you increase the pump speed, and subsequently narrow the DT, you will get slightly longer run times between defrosts, due to the return being slightly warmer. These longer run times would translate to a marginally higher net output.
Thanks Zarch this is really interesting.
This thread really is super useful to have as it perfectly represents the situation I find myself in with my 5KW Arotherm Plus.
It runs beautifully 95% of the year just fine, but on those cold days (when you need it most) IE <= 0C it really struggles to achieve the given flow temps.
I’m tempted to try and set the pump speed to 100% and test to see whether the unit does any better at sustaining / growing the flow temperature.
A quick way to help a heatpump on cold days is to set the DHW tank to use only its emersion heater (hopefully on off peak electricity).
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Maybe it’s better performing again under -5° when air is dry again. I didn’t face that case yet