(My search skills must suck, because I know I've seen several threads to this effect over the last few months)
I live in Upstate SC
I'm trying to figure out what the most efficient changeover temperature should be from HP to the gas furnace. Because of a location issue with the outdoor remote temperature sensor for the tstat, last week we had some evenings that were less than the 40° F changeover setting, but the remote thermo only registered in the upper 40s... thus heating never switched to gas furnace. The house was still plenty warm and comfy on HP, but if it's more efficient ($/BTU added) to run the gas furnace at those temperatures, I'd like to. I know this value changes as the cost of electricity and natural gas change. I was just trying to find a way to determine, based on a set price for each and respective efficiencies of each heating unit, which the best changeover temp would be.
Trane 3 ton XL16i HP
Trane 80k BTU XV90 furnace
I don't have model numbers in front of me but can provide them later if needed (crumbsnatchers gained unlawful access to the study and wrecked havoc with my files).