Why is the outside temperature important? Well, in winter, the outside temperature 1st thing in the morning can be -5°C on one day and +12°C the next, so HeatingSave calculates the "heat loss profile" of your specific building using the current outside and inside temperature to work out the absolute latest time to switch on the boiler for the rooms to be warm for when you want them. Equally, HeatingSave switches off the boiler early on warm days at the end of the heating period.