Hi @stopbar, it's s difficult balance. Trial and error in large part. Every home is different.

Pure weather compensation control is good, but not perfect. It doesn't factor in some variables that can increase or decrease the heat loss of your property, such as; solar gain, changes in infiltration/exfiltration due to wind & weather, thermal lag - which is how long it takes for heat energy to pass through the fabric of your home, etc.

If you trim the WC curve down at one end, or both ends, it will impact the thermal energy delivered to house proportionate to external reference temperature for the whole heating season. My system is running on pure WC and drifts a couple of degC either side of my ideal set point, the largest factor being solar gain increasing the indoor temperatures a couple of degC.

Thank you pretty sure it's solar gain causing the increase I think I'll leave alone at mo

I have a Aerona3 13kW and my current settings are max FT 33c at -3 and a min FT of 23c at OAT of 16c house gets some serious solar gains (weird for a 70’s bungalow on a cliff top in Wales!) one side of the house gets no sun the other it is full on. The non solar gain side is constant 21-ish the other side is hotter than the sun! Really hard to control it, but I am determined to get it under control! 

@grantmethestrength thank you interesting numbers.  Did you find a flow temp of 33° at -3°C was sufficient to stay warm?  What was your electricity consumption during January if you don't mind me asking.  We were using a maximum of 31kwh during the day average around 20kwh plus 40kwh on night rate (Economy 7) Inc an electric AGA which uses a consistent 30kwh per night. My unit rate then was around 5.5p overnight and 37p day (ouch!!). Taking out the AGA and background useage of around 6Kwh that leaves between 24kwh to 44kwh for heat pump.  The high figures were for days with overnight temp of -7 and below zero in fog all day.  The heat pump  seems to spend quite a lot of time defrosting........These are all back of fag packet comparisons based on my historical usage logged over 20 odd years.  Hope you find this useful.  Thanks in advance.

To be honest our figures are all over the place as we only had it installed in Jan and we are still messing around with it. We started off with the installer leaving it at 50c constant and working off the indoor thermostat as an on/off gas type set up which resulted in 60kWh a day usage. We then out it on 45c at -2  WC but using the indoor them as on/off. We finally (in no short measure thanks to the folks on here) went to full WC and have been slowly turning the flow temp down and found that 33c@-3 works for us. I don’t have reliable energy monitoring on the HP as of yet but I guess we got it down to about 20kWh per day, but I won’t know true figures till we have gone through next winter.

At the minute I have also placed it in silent/night mode and it barely ever goes over 1kW power draw (except in DHW mode). This means it is cycling on and off but not rapid cycling, I think it is massively oversized for our house so I doubt I could get it to run at a constant low power unless I turned on fixed flow rate and messed around with that. 

I am currently trying to build an AI agent that will learn the house and control the HP via modbus (think a poor mans Homely).

With the solar we have massively reduced our usage and will soon be adding a battery into the mix so will again massively reduce our costs. 

@stopbar 

We have a 13kw R32 Grant and found running 30/-5 & 25/15 works (its a very efficient house).

Most of the time I just run it off flow temp of 25 / 27° for 6 to 10hrs a day and allow the temp stat to control the house. I understand why people use WC, to try and keep it constant. But before setting it up it helps to understand the house needs and then set the WC correctly.

You might also want to understand how the buffer works in your system (if you have one installed), as that can effect the flow rate / temp

I am currently learning as much as possible about our setup and thought I would try and get a grip on how much heat our rads emit at different flow temps, I have not yet tested this data yet but feeding the stellrad tech sheets into Claude AI it threw back this.

I am going to try and integrate this into my calculations, not sure how but I will try lol!

Regards

Si 

@grantmethestrength I'm afraid all of that is above me pay grade, I don't even have the heat loss calculation for the property (late 70's detached dormer bungalow) in East Anglia. I'm guessing even with Double Glazing, plenty of loft insulation and cavity wall filled it's not brilliant.  The installation was an ECO 4 grant with solar panels which are really doing their stuff now (no battery as yet sadly) but of course the heating is not actually required!  So for me I will continue to fine tune by 'feel'  as it were and keep my fingers crossed. I will probably lower the top flow temp by another degree but won't know until it gets cold again how effective that is?  Thanks for all your information. Mark B.