Hoping I can get some advice on a spec house we are considering buying that is about 50% finished. It is outside Nashville and is about 5057 sq ft--1200 in a finished basement, 2400 main floor and approx 1457 on second floor (in addition, another 410 sq ft on the second floor may be finished at a later date). The builder's plan is to have one gas furnace for the basement and first floor and a heat pump for the second floor (13 SEER). (Don't know any other info on the units but can find out if I know what to ask.) I do know that they plan on three AC units. My question is about whether a heat pump is adequate for that area for heating--a search listed the average low in the winter as 28 degrees.