Here is another spreadsheet, this time comparing the Goodman 4 ton and a 96% AFUE gas furnace to my existing 80% gas furnace. It's quite clear this time that this unit would be cheaper to run than the 80% furnace even down to 0°F, although heat load requirement would be higher than what the unit could put out at that temp. The changeover to nat. gas would be the outdoor temp. below which the heat loss of the house exceeds the heating capacity of the HP unit. I estimate that would be around 15°F in most cases.
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