Where are you building? Geothermal HVAC runs exactly the same as a reverse cycle AC system except the condenser/evaporator is in the ground instead of the air. It will cost almost exactly the same as a normal reverse cycle AC except with 20 times the start up cost. Unless your in a very, very cold spot in tasmania where a normal reverse cycle system might struggle to heat on the coldest of nights, you are really flushing A LOT of money down the toilet. The split system in my bedroom has a COP of almost 5 which means it creates 2.5kw of cool or warm air using only 0.5kw of power, although this number changes with outside temperature (the advantage of geothermal).
From what i have seen geothermal HVAC costs the same if not more than putting your house off grid. Better hope you don't have rock too far down. How far is town gas? Gas and solar hydronic is great!
