@novemberromeo, I know you already pulled the trigger and had your heat pump installed, but we did discuss your case with Emma about the sizing of your heat pump. Jump to around 17:49 of this interview.

thanks @Mars - just watched it, and very interesting!

Two weeks have passed since everything was commissioned.  The ASHP has used 520 kWh of electricity so far, averaging 35 kWh per day (excluding the first day when we used 64 whilst everything was getting up to temp).

I've kept the DHW set to 52 degrees and the heating flow rate set to 40 degrees.  Everything seems to be working ok.  Most rooms are warm all the time.  The larger / less insulated rooms struggle to maintain temperature overnight when it's cold outside, but we had the same problem with the oil boiler.   We use the Air To Air heat pumps in those rooms to give a quick blast of heat to get it back up to temperature first thing in the morning and that works fine...

I'm wondering whether it's worth experimenting with the weather compensation curve or not, as 35 kWh per day in Winter seems fairly reasonable. 

I've seen comments in the forum along the lines of "your heat pump is driving down the motorway at 90mph in third gear" and am wondering how I can find out how mine is performing, other than simply looking at the number of kWh units used each day.  What are the metrics I should be looking at to determine if:

a) heat pump is adequately sized (remember I settled on the 12kw pump after being recommended an 8, 12 and 20) - is there any way to know if I made the right choice?

b) heat pump is running efficiently

@novemberromeo I would move to weather compensation as its pretty much bound to save you money. It will lower and raise your flow rate temperatures as you need them lowered/raised. At the moment I assume given the cold snap that 40 easily meets your heat needs? Or did it get a bit chilly? On warmer days, you probably don't need 40 at all, so why bother heating to 40?

@novemberromeo 

Are you controlling room temps with a thermostat?  if so I would agree with @batalto that it's definitely worth trying weather compensation. If (like me) you're trying to heat the house using flow temps only, if the ASHP isn't heating your whole house at 40C, it definitely won't do it at 35C. 

Try Graham's article here for a dscussion on ASHP sizing. I've asked him a follow on question.

https://renewableheatinghub.co.uk/when-is-a-kw-not-a-kw#comment-923

52C is quite hot for HW but it's a personal preference. 

35 kWh per day isn't bad at all (is that for heating and HW?); I've averaged 29 just for heating.  

Just a little update since my last post...

I've since discovered that our UFH has a weather compensation unit (this was there before the ASHP was installed), which is responsible for mixing the boiler water with cold water... and thus completely ruining the efficiency of the ASHP because it doesn't matter what flow temp I set in the ASHP, the UFH weather compensation unit is then mixing it with cold water to lower the temp!

I've asked my installer to come back and remove this, as well as the other UFH mixers, which should hopefully simplify the system and improve efficiency.

In the meantime, I've reduced the flow temperature from 40 to 35.  I am now averaging 19 kWh per day for heat and hot water, although the last week has been much milder outside than before the switch, so some of this reduction is going to be due to that.