I have a 3 year old furnace (forced hot air) and live in Northern Coos, in a pocket that is well known for high winds and heavy snow fall. My house is 50 years old and I was sitting around in shorts on Friday night. It was -16 when I got home Friday evening and -22 (plus the wind) when I started my pickup for work on Saturday morning. I've lived in that house for 13 years and never have had a problem keeping it above 65.