With temperatures in the low 20s last night, we are running through our first real test of how our house deals with extreme cold. I’ve mentioned before that I’m building a spreadsheet based energy model (not unlike a simplified PHPP), and that figures our peak heat loss at a little under 2KW (7,700 BTU/h) with an outside temperature of 20°. Our house is set up for zonal heating though with each floor being about half of that heating need. So in practice, on the nights that we have run the heat we have tended to run it a bit on the first floor, then run it on the second before going to sleep, then shut everything off and leave the heat off until maybe the next evening. And this worked pretty well last night as well. We warmed the house to about 68° inside before turning off the heat, and with a low of 21° on our ecoroof and no active heating inside, we only lost a few degrees over night keeping the house comfortable by this morning.