My observation about the vaiilant internal pump is that it’s fixes itself on heating and adjusts the DT to maintain output.
Sometimes as low as DT2 when warm outside, upto DT5 when very cold.
Point in case this morning. No deviation with the flow rate, it just fiddles with the DT
So perhaps stick heating pump in Auto, see what flow rate you get shown in the controller (and heat meter?)
Auto will then give you a ballpark flow rate what the heat thinks is best.
edit: although will the arotherm just go super fast / max as there is no resistance just into the buffer and back? So perhaps this isn’t a great metric of what’s actually required?
Then try and set the flow rate on the other side to match that? I don’t think you’ll need PWM, just a fixed flow should be ok. (as that what the Arotherm sort of does)
You could always fix the heating side too to a % setting to really ensure things are the same?
I’ve found that no matter what you do with pump settings and speeds, the Arotherm just gets on with it and still cranks out the heat at decent COP (for me anyway).
What size Vaillant do you have?
Finally, I created this flow rate chart for the various Arotherm models, if this helps at all?
