Additional info:

Has a 10 seer carrier AC 5 ton

Infinity thermostat single zone, can handle 2 HP and 2 gas furnace heat stages?

Frank,

Your just north of me by about 20 miles.  I'm In Howell.  I'm heating 2270 sq ft plus 1450 sq ft basement with an open loop 3.5 ton unit.  My home is 2 x 6 walls a little over r19.  With a closed loop system a 5 ton would most likely be all you need.  I kept my propane 90 plus furnace as you suggested you are planning, I thought I might needed if it really go cold.  Even with -5 deg temps outside the 3.5 ton kept us warm.   I would suggest against trying to do a split system with the propane system.  Make sure the duct work that connects the geothermal unit to your current ductwork is sound insolated on both supply and return.  The added air flow can make the system very noisey without the insolation.

Keep us posted on how things are going,

Regards,

Thanks for the reply Eric.

I was leaning to keeping the Carrier system just because it is nearly new and that contractors price is much lower (much less work for him to install?).  There is an electrostatic air filter and humidifier on the system, contractor C might not have to move those, whereas the other 2 would

Do you think you have lost any efficiancy by keeping your propane system?

Did you put the geothermal after the propane burner or on the return side?

Thanks again.9:47:37 AM

My system is quite different from the norm. The propane system and the geothermal unit are in parallel to the main feed plenum. I have ducting dampers that isolates the unit not in use from the duct work. The design has powered dampers being controlled through relays that only allow one unit at a time to function. The heating unit switch is made by selecting emergency heat from the thermostat.  This shuts down the geothermal unit, switches duct dampers and activates the propane furnace only after the dampers are fully open. 

After going through this last winter I have found that the geothermal unit keeps up just fine with the heating demands. The current plan is to leave the propane unit though it really isn't needed.