1. Heating Degree Days 2. Net heat delivered (BTU delivered * efficiency) 3. Calculate BTU used per heating degree day 4. Calculate your heat load (BTU/hour) 5.

1. Heating Degree Days 2. Net heat delivered (BTU delivered * efficiency) 3. Calculate BTU used per heating degree day 4. Calculate your heat load (BTU/hour) 5. Multiply by the ASHRAE sizing factor (1.4). That's enough to allow you to size/resize a gas/oil furnace, as well as see what sort of costs you'll run. To compare directly to a heat pump is difficult, as you also need to pull in your electricity cost, gas/oil cost, and if you have demand pricing, what sort of temp you keep your house at. I will say though, going from something like a 9 SEER AC/heatpump to 17 is huge. 17 -> 20? Not so much.