I have a Trane brand heat pump with electric furnace as back up. My HVAC guy said he turns his to emergency heat ( furnace only) when the outside temp drops into the 20s.or lower. Everything I read on the internet says never do that , that the lowest electric bill will be if you leave it on heat pump mode always, as it will automatically turn on the furnace when it can't keep up. I can see that being true if when the furnace turns on that the heat pump turned off. But since both run even if the furnace is running, how can that be more efficient??