Geothermal energy as a heat source

Tom,
Several years ago before building a house on MDI, I suggested using
geothermal energy as a heat source. The builder rejected my idea because of the island being granite. Was he correct?
Ted

Hi Ted,
No. If you were using wells as a source, there is no problem.
If you were trying to bury coils, there might be an issue, but I would
assume there could be enough soil on a given site to bury a loop.
Tom

Tom
A lot of TNTing was done to make room for the basement and then
backfilled the area around it. What minimum depth is necessary for the
coils: 6 ft, 7 ft, 8 ft to still do an effecient job?
Also, what would be an est. cost for installing a geo-unit to replace just
our oil furnace ?
Ted

Hi Ted,
I would expect a ground coupled unit would cost about $15-20,000.
I am not sure how deep it should be, since I am sure there are variables of
the ground water, soils and the heat load of the building.
Generally, I think deeper is better (and more expensive!!).
Tom