On Energy Stats Uk (https://energy-stats.uk/vaillant-arotherm-weather-curve-information/) Mick Wall gives a rough way of setting the correct curve.

A more detailed equation (TFlow = a * (HC * (Tset - Tout))b + Tset) is given on the Open Energy Monitor forum. (https://community.openenergymonitor.org/t/vaillant-arotherm-owners-thread/21891/83?page=5)

Ive used that to reproduce a more accurate table of flow temperatures for curves at desired room temperatures at -3C OAT

I will paste it below for anyone interested in the future.

simplistically, with ours the higher the number the higher the electrical bill.

Drew 

Thanks Drew

that isn’t how I understand it. To me the curve sets the “gradient” on the weather compensation chart. The desired room temperature moves that gradient up or down the flow temperature axis. 

I’ve put gradient in inverted comments as I know that it isn’t a straight line. 

I might be wrong but coming from a system where I enter the min and max flow temps for different outside air temperatures, it makes sense in my mind. 

In that case, if too low a curve is selected, a room might never reach the desired temperature. 

Our present heat curve setting is 0.3. That’s been set by Vaillant using the adaptive heat curve setting. Next time the system fires up I’ll check what the flow temp is for you.  

drew

It is set on pure weather compensation at internal temperature of 22C. The unit runs 24hrs with a setback overnight. Theoretically that should be good for COP even if the electricity usage could be higher than running it for a shorter period.

I cant really drop the desired temperature or curve as the BIL's wife likes it warm. It was running at 20C initially but that didnt seem to help the COP greatly.

Your COP figure looks lovely. What sort of flow temperature are you using to get that high?

I have no way of monitoring if it is cycling, but the heat pump isnt oversized for the property so I would be very surprised it it was. Apart from a couple of issues I wouldnt have done (eg gycol), it looks like a good install so Im very surprised to see such a low COP.

How is the unit set up to operate?  Are they running it a few times a day, or running it manually?  We found ours works at its most efficient if it is running in manual mode(water and heating).  Just set the temp we want and leave it.  Might be worth a try, also try lowering the heat curve a little.  

Drew

Ive opened all but the bedroom radiator valves, which they want to be cooler. Im running my Samsung at about 42C flow at the moment and still getting a COP of 3.8-4.0 so the Vaillant should really be doing a lot better than it is even at a FT of 38C.

Ive attached the COP as reported by the app and the Arotherm COP figures.