So far, I don't believe anyone has answered your question about running two houses and a machine shed on a geothermal field.

I have a pretty good sized field with one house, and I don't believe I would want to hook another house to that field.  I'm sure it would be possible to design a field to operate all the heating and cooling you describe, but make sure a professional designs it, knowing the requirements of a system.  At first, it seems logical that if you have some coils in the ground, you could keep hooking on more houses, but you have to remember that you have so much heat to get rid of in the summer, and so much heat to pump into the house in the winter.  There is some sort of formula used to determine the amount of feet of buried pipe to handle so many tons of heating/cooling, but I don't know what that formula is, and I am sure it would vary according to where you live.

As to whether geothermal is a good idea, I am convincied it was a good idea when I installed our unit three years ago, and even more sure it is a great idea as prices of fuel keep getting higher.  We did not go cheap with our unit, but we sure did lower our monthly bills.  It is hard to determine just when the payback is, as prices for gas and electricity have gone up so much since we installed our unit, and we don't know what we would have been paying with the old system.  I do know that we still haven't exceeded our old utility bills, and electricity and natural gas have really increased in our area.  I'm not even sure how much the electricity and natural gas has gone up, but I believe they are close to double what they were before we installed our geothermal system.

I believe it is now almost a no-brainer for installing geothermal in a new house, or a house with no heating/cooling system presently.  Don't worry about the payback, it will come quickly.  Ed Winkle mentions at 58 he is wondering if he is too old for a payback, but I am older than that, and know that if the payback does not occur for me in my lifetime (which I think it will) the payback will still be there for whoever might sell my house or live in it after I am gone.  The increase in the value of your house alone will be a payback for installing a geothermal system.

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