Improve efficiency: Heat the hot water when it's cold

My flow controller has a mode that tells it to heat the hot water if the temperature (measured at the bottom of the tank) drops by 5C. You can extend that all the way to a drop of 15C if you look in the menus.

Sadly that means a shower at 20:00 causes the heat pump to kick in when the outside temperature may have dropped.

As discussed elsewhere, it’s better to wait for it to be warm outside before you use the heat pump.

However, another factor is that the default behaviour means the water in the tank is already warm when the heat pump kicks in.

My hot water is at 42C so a 5C drop = 37C and the theoretical CoP is 2.9. If we use a drop of 15C then it begins heating when the water is at 27C and a CoP of 3.7. However, if we leave it alone and wait until shower is completely done then for example yesterday the water in the tank dropped to 11C and the CoP was 4.

In this graph looking at the efficiency of the hot water based on the average outside temperature for the day, we can see there was a bit of an improvement. The blue and green lines represent some of the experimental outcomes and the red line was the old behaviour.