So I'm trying to take at least a rough look at what GSHP might save me vs. ASHP+ 97% propane furnace and my numbers aren't as encouraging as I was expecting. I'm wondering if there is some flaw in my logic. Please critique. 1) Get heating and cooling loads from Manual J 2) Divide the heating/cooling loads by the difference from setpoints to design temps. This gives BTUH/degree. 3) Multiply by 24 to get BTU-day/degree. 4) Look up degree days on degreedays dot net. Base temp for HDD is 60 (figure decent envelope and internal loads to get to a set point of 70), base temp for CDD is 65. 5) Also look up HDD to a base of 20 degrees. This is to figure how many degree days a high efficiency ASHP will need the propane furnace to take over. For figuring running cost of a high efficiency ASHP+propane furnace, Cooling - (BTU-day/degree * CDD) / 18 SEER / 1000 w/kW * $0.105/kWh Heating (ASHP) - (BTU-day/degree * (HDD - HDD20degree)) / 10 HSPF / 1000 w/kW * $0.105/kWh Heating (Propane) - (BTU-day/degree * HDD20degree) / 88756 BTU/gal * $1.99/gal For figuring running cost of a high efficiency GSHP, Cooling - (BTU-day/degree * CDD) / ave. 30 EER / 1000 w/kW * $0.105/kWh Heating - (BTU-day/degree * HDD) / ave. 4.3 COP * 0.000293071 BTU/kWh * $0.105/kWh For a 4097 sq. ft. house I'm only coming up with $570/year in savings for geo. What am I doing wrong?