Calculating cost-effectiveness of geothermal vs. propane boiler
We are adding on about 600 sq. ft. to a 150 year old farmhouse in Michigan, resulting in a total of about 2,400 sq. ft. I am having a hard time trying to calculate the cost effectiveness of expanding the current hot water and air conditioning systems ($9,400) vs. installing new, forced-air geothermal ($33,700).
The house currently has no insulation in the walls and roughly R-9 of fiberglass in the attic. The addition will be built with an ICF foundation, 2 x 6 construction and Icynene. The Icynene will also be sprayed into the walls of the existing structure, and the roof deck.
We currently have a roughly 10 year old, 82% efficient propane boiler with baseboard heat, a standard central air conditioning system (the previous owner fitted it using closets and the like to run ducts), and electric hot water.
I am estimating that our heating costs would be cut in half following the elimination of our existing kitchen (built aboveground with no real foundation and minimal insulation) and the addition of insulation, despite the added square footage, with little change in the costs of air conditioning and hot water. Based on this estimate, and the estimate provided by the geothermal contractor, the annual cost of the existing system would be $2,100/yr. vs. $1,000 for geothermal. Including the 30% tax credit, this would take about 13 years to pay off (not too good!).
How do I know these estimates are accurate, though? And what about maintenance costs? I have just learned that we need about $1,300 in repairs to the boiler (in addition to the cost of expanding the system) -- will it keep needing substantial repairs? And the condensor and air handler for the air conditioner are also about 10 years old -- will they need to be repaired or replaced? Will we be kicking ourselves if we keep the existing, aging system? It does work great, and the baseboard heat is so responsive, comfortable and clean....
Your input would be most appreciated!