After 8 years in the house I’m still amazed. We have had the heat off for a week or so, don’t pay a lot of attention anymore. Last night we left some windows in vent for the night. It went down into the 30 outside overnight and in the AM the house was at 65 degrees. We didn’t turn on the heat. Now, at 2PM it’s in the upper 50s outside and the house is up to 69. No extra heat, just heating itself.