Does anyone have experience with sizing a geothermal system for an old house? Arriving at a realistic heat loss figure seems to be a problem, because most people seem to want to oversize systems on a "better safe than sorry"--or maybe that should be CYA--principle.

Our take is that we could install a reasonably large system--probably 4 tons--in the cellar with ductwork to the first floor, then go through an actual year with our existing oil-fired boiler and steam radiators as a backup and see how the house actually performs after adding insulation in the attic and cellar and doing something about the windows. Then we could add an additional 1 or 2 ton system in the attic with ductwork the the second floor if we think it is necessary.