This tweak is going to look a bit odd.
It’s benefit is more pronounced if you are turning the heat pump on and off quite often.
Following a burst of heating the hot water in the cylinder the flow temperature will be high. For example, my hot water is set to 42C which gives a flow of 58C when my heat pump has finished. The trick is then to shut off the heating energy but pump the toastie water around the radiators.
This doesn’t sound like it’ll do a lot, but it’s one of those “every little helps” tweaks.
Looking at a run from yesterday (2020-02-19) I can see I got 5.9kWh of space heating with a CoP of 3.61.
If we look at the same event but cut off the tweak at the end we can see we got 5.4kWh of space heating with a CoP of 3.36.
So that’s 0.5kWh of space heating for “free”.
As a side-benefit it also helps the algorithm that is watching the return temperature to know when to turn the heat pump on again because it’s brought the return back down the same temperature as the water in the radiators. Otherwise I’d need to have a sensor on a radiator to know how hot they were because the return would be up at 52C at the end of the hot water heating.
A few of you will be eyeing that “Efficiency” chart up with scepticism. I know I did. However, it’s because we are only paying for pump energy and we’re getting all that useful heating. It’s a figure of fun in our house because sometimes the efficiency is something like “7200%” Clearly that’s not a sensible CoP, but it does accurately represent the work / effort. On my home-grown web-app I’ve smoothed the CoP over 5 minutes to make it more sane in the graphs.