hi all I have decided to go with a geo system and had 3 units to decide on Water Furnace series 5 5 ton, Hydron Module revolution 4 ton and Climatemaster Tanquility 22 5ton.
My question is about a split unit where I would have a gas furnace running as aux heat with the geo unit. The furnace in theory would run instead of the strip heat, the ecm variable speed motor in the furnace would be shared between the two units. the real reason for this setup is the furnace would run when power is lost as a back up heat source and I would not have to use a very expensive backup generator to run the whole geo system and could get away with a portable generator to run the furnace and other important stuff.
Does this make sense? has anyone seen such a configuration? Is there too much that can go wrong to many things going on? If the geo system works right the furnace never fires, is it good for a furnace to sit and fire every once in a while?

Hi Frank, I don't know how I didn't see this sooner.
There is a lot more to your question than you realize i.e. btu loss and design temps.
The geo can not run simultaneously with your gas furnace in most situations meaning it generally costs more to operate a system as you describe. We do have aging grids in some of my AO where folks will have frequent lengthy outages so folks tend to suffer the higher operating cost to avoid that expensive generator you mentioned.
In these cases we will generally design for a lower balance point which may add $2,500 or so to the install but keep auxiliary heat use to a minimum.

I'll assume that you mean natural gas. Once you throw in the generator issue, I expect that simply using gas alone is your best ROI. Perhaps with some mini-splits for cooling and heating when the weather is mild.

After doing some research on the 30% from the government they would not cover the price of the whole system (geo plus new furnace) for that reason we will go with the whole geo package.
But my wife wants a heat backup, so we will leave our existing furnace in place which will require more sheet metal work and dampers to be able to switch from one system to the other in case of power outage. we can still use a portable generator at that point.
Yeah natural gas but the cost to run it plus new furnace and ac is only a couple grand difference between that and doing geo.

Which loop would be better: for a 5 ton system
Four horizontal bored loops, consisting of 4,400', 1", DR11C polyethylene pipe manufactured by Charter Plastics networked together for burial horizontally 9'-15' deep depending on the soil condition. or
Four horizontal slinky loops, consisting of 800' of .75" DR11C polyethylene pipe manufactured by Charter Plastics networked together for burial horizontally 6-8' deep depending on the soil condition. or
Four horizontal loops consisting of 3200' of .75 DR11C polyethylene pipe buried 6-8' deep 10' apart.

Frank, you can google "geodesigner" and download the software to run your own reports, especially since you are considering Climatemaster equipment. (unless your contractor has provided you with geodesigner reports, then maybe you don't want to go to the trouble). It's a good way to see all the variables that go into ground loop design decisions.

Sounds as though all will have similar performance.

will check that out! Thanks

is the horizontal loop better than a slinky in that the slinky has more pipe in a smaller area and that there is only so much heat to be drawn from a certain amount of earth?

If the horizontal loops you mentioned are 1 pipe trenches (down and back) then yes they would collect more btu's. If they are 4 pipe trednches then maybe not. Btu's collected are based on depth and foot (of pipe) per foot of trench (as well as pipe diameter).
Are the bored loops 400' u tubes or 400' total?
Why are we focused on the loops as all will likely work? Let's look at design. Someone suggested a 4 ton and 2 others suggested a 5. What is the building loss/gain?