Interesting you mention that you’re surprised most installers don’t set up their curve like this, any heat pump supplied from freedom over the years that GH was in charge, would have been setup with a pretty reasonable curve of 37@15, 50@0 and with external thermostat control.

I always thought that was a bit of a cop out from a design perspective, but having looked at most heat pump controllers on the market I’ve come to the same conclusion as GH did, they’re awful things to use as a thermostat 🤣.

It is a cop out and as I say upthread will incur a penalty of 3-6% or more if it's more than a a couple of degrees out.   However installers don't wast to return to site and setting up the perfect wc curve is iterative, so if running cost isn't critical it's an approach that has attractions.  I'm not advocating it, just recognising it.

Why 37 at 15 incidentally,  that's almost obviously too high in most cases, leading to a further penalty.  Is this so that radiators always feel warm to avoid callouts?

@jamespa yes… it’s what I run at too. Keeps the wife happy.

I just ran the numbers through my WC simulator.  This calculates, based on Ecodan R32 data, the efficiency gain from various WC curves relative to fixed temperature operation, based on an OAT pattern representative of southern England.  If we take your 50;0, 37;15 case and assume that this is 5C higher above the ideal' WC curve until it reaches the floor at 37C, then the curve gives a 10% improvement on fixed temperature operation, as opposed to 20% if the WC curve is set up properly.  So you can either look at that as a 12.5% penalty relative to optimum, or as a halving of the benefit of WC.

Most of the penalty is due to the curve being above the optimum value not the ('lizzie ') floor.  So one good approach would be to set it up initially this way and (during the coldest season) gradually reduce the cold end until it didn't quite heat the house, then turn it up by a degree.

As I say I'm not advocating this method, only recognising it.

I should have added more flesh to my post but I was on my phone - We actually set the top end of the curve to the design flow temp, 45 degrees in my case (so 47.5 flow). The bottom end of the curve was also chosen as we have fan coils in some rooms/areas (kitchen, hallway, bathroom), and no-one wants to stand under cold air...

I also figured that lifting from 15 degrees ambient to 37.5 is hardly much work that I wasn't going to sweat the details. Also handy to have the rads deliver some heat on the odd occasion that it's 18 degrees outside and raining and the wife wants the heat on. Handy to be able to deliver some heat into the property in that case. I almost thought about switching to a fixed flow temp of 45 during the summer for this precise reason, but haven't needed to yet.

I did a couple of days testing running weather comp and a fixed flow temperature with thermostat control on similar average temperature days, one being 8.7 and the other 9.0. Very little difference in target flow temp. 

Fixed flow temp (to simulate a slightly elevated flow temperature), compressor cutoff was 30.8, WC obviously varied between 29.1 and 29.2. But the starting temp was the biggest variable. WC never started the compressor until return temp had dropped to 21.6 degs, while the fixed with thermostat could be as high as 25 degs.

Fixed and thermostat

Space heating CoP 4.8

kWh used over 24hrs 6.8kWh

WC

Space heat CoP 5.4

kWh used over 24 hrs 4.6

You will see from the image the difference in run duration and frequency of starts. The one with plenty of starts is the thermostat.

CoP is generally made up of two components at a fixed outside temperature. Flow temperature and dT, the wider the dT the better the CoP.

The rest was not definitive as the temp varied over the day. But a 30% reduced kWh used was quite a suprise, when comparing WC to using the thermostat. Think that must be down to the the increased shorter duration cycling and less higher quality heat being delivered

The image below is the day I implemented the change in strategy to the left straight WC to the right thermostat control. The off period is some solar gain switching off the heat pump.