When the average temperature in the occupied rooms drops below 19C (16C at night).
23:13:30 <kim> heating status
23:13:31 <sian> Heating is on (thermostatic, threshold = 19.0). Average temperatures are: upstairs 17.8, downstairs 16.4, occupied 17.5, whole house 17.1, max 19.7, min 15.4