I have a 4 ton air source heat pump. It has both an internal heater and will run a defrost cycle by using the furnace in winter. I put in the full system a cou

I have a 4 ton air source heat pump. It has both an internal heater and will run a defrost cycle by using the furnace in winter. I put in the full system a couple years ago. The entire system (100k BTU high efficiency furnace with variable output and 48k BTU, 22 SEER2 and 10.5 HSPF2) and the entire system was less than $17k. I also got over 4k in rebates (fed, state and manufacturer). I'm well north of Chicago. The heat pump is rated to -22F but have the lockout set to 15F to cut over to the furnace. Mostly for wear and tear reasons. But it has a COP rating of 1.75 at 5F. So still more efficient than any furnace well below where I run it.