I've noticed that if I let the heat drop too low at night, 20 day, 16 night type range, I appear to be using more energy then if I maintain at 18 overnight.
The reason being the 4 degree jump I ask for in the morning kicks the back up heater for the heat pump on and that is a large electric furnace. It will run for 30-40 minutes to get things back up to temp then coast along at 20. If I leave it near 18 just the heat pump runs for a few minutes every hour or so.
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