At what temp does a heat pump become inefficient? I have a Carrier Infinity 13 duel fuel package unit w/the thermidistat. I am trying to figure out the ballance setpoint when to shut off the H/P and switchover to gas. The guy that installed it set it @ 40*. I thought that was too high and spoke to someone else @ the dealership that installed it and he said set it @ 20*. That sounds too low to me. I know there are formulas & calculations that I could do with energy costs & factors, but all I want is a good ballpark #. Thanks.