To get round the issue of wasting heat from the hot water tank i had to abandon my initial control strategy of switching to hot water as soon as the buffer reached temp.
The Arduino mega already had a RTC (real time clock) attached, i was using it to timestamp the data logs.
I used the RTC to switch over from heating to hot water for 4 hours a day.
1H early morning to make sure there was water for a shower
2H from midday, air temp should be highest to help efficiency
1H in the evening to top up in case a bath is desired
This gave me the issue of how to stop the heatpump when the buffer was at temp and no more heat is required.
I used the circuit below to convince the heatpump it had achieved 60C.
On the left is the original wiring to the heatpumps internal water temp thermistor, to the right is what i changed it to.
From memory the potentiometer is 10K, adjust until the heatpump controller displays 60C. The changeover relay is driven by the Arduino when in heating mode and the buffer has reached temp.
Steve