26 Sept: Trigger Temperature - this is the temperature in the Sunbox, which is enough to start the sunbox pump going.
On the programmable thermostat. I reduced the trigger air temperature to 19ºC a week ago, and this week, I adjusted it to 18ºC. As the winter progresses and the ground chills, the delta-T should remain at 5-7 degrees - I can review this every week. During the summer, when the ground is static at just over 13º I used 20º in the Sunbox as the trigger. A week ago with the ground at 12.6º, I reduced the trigger to 19ºC. This week, I adjusted it to 18º and the ground was 12.1º. As long as there is a difference of more than 5 degs, it is worth adjusting the trigger weekly. To be consistent when I take the ground temperature, the heat pump is off from 10pm onwards, and the ground temperature measurement is taken at between midnight and 1am - having had time for the ground to rest.