If common ducts or floor radiation can service both, then you can use 1 system that might not cost as much as two. The real difference is in the loops, however, so one big system may only be 2 or 3 thosand more than 2 smaller ones. While I understand that 2 or 3K is real money, look at cost projections without geo.
Good Luck,
Joe

Unless you are absolutely dead set on going geosource take a look at air to air split system heat pumps. Daikin is one and Mitsubishi is another. With those systems one outdoor unit can serve several indoor units. If the two houses are close together you might be able to have one outdoor unit serve indoor units in both houses. There is a limit to the length of refrigerant piping that can be installed. I'm installing the Daikin system in my ICF 2000 sf house in Idaho. It will produce heat down to 10F outdoor temperature.

Also consider installing one geosource unit that heats or cools water in a storage tank. Pump that water to air handlers in both houses.

Has anyone installed one of these Daikin Heat pumps and verified how they perform or are we just taking the manufacturers word on it?

Based on my personal experience, I would encourage you to spend a little money on a good load calc for both structures and then run the data through an energy audit application such as the Elite (www.elitesoft.com) and compare geothermal to anything else you think might be a good solution for you.  One of the big problems with geothermal is first cost.  People have sticker shock when they see the quote.  If you put on your life cycle cost hat, though, folks who walk away from geothermal end up leaving money on the table in the long run and this energy crisis is probably not a short term deal.

I'm guessing your heating and cooling season in SC is similar to what we experience in North Texas except our summers tend to get a little hotter so your experience should not be that different from mine.  I took my blue prints to a reputable HVAC person here in Texas and worked with him on reducing the energy footprint of the home during the design phase using the Elite Rhvac load calc application.  We went back and forth a few times until I was able to reduce the size of my geothermal system by 1/2 (i.e. spend your first dollar on insulating and sealing the house).  Then he took the load calc for the home and ran it through the Elite Energy Audit and we compared the Earthlinked geothermal to 3 different systems (an 18 SEER air sourced heat pump with a natural gas backup unit, an 18 SEER AC with natural gas heat and a 14 SEER AC with natural gas heat).  He loaded bids for all 4 systems, loaded the cost of electricity and natural gas and hit go.  The energy audit said the geothermal would average around $35/month and actuals is averaging a little of $40 for the past 12 months so he pretty well nailed the load calc.  The next best equipment was the air sourced heat pump and the energy audit said it would run about $100/month.  The geothermal also produces about 70% of my hot water which averages out to about $30/month.  The additional amount I borrowed for the Earthlinked system added about $25/month to the 15 year mortgage so, from my perspective, the hot water savings and the additonal mortgage amount for the geothermal canceled on another out.  When I net it all out, geothermal improved my monthly cash flow by $100/month and will get better as the cost of electricity and natural gas goes up.  We're expecting a 20% increase over the next 12 months in Texas and we're all sitting on the front pew wondering what the preacher says it will be the year after that.

Having said all this, I would recommend looking closely at the Earthlinked system and incorporate it into your comparison.  There is a good rep up in NC somewhere around Raleigh.  You can find him on the Earthlinked website at www.earthlinked.com.

Good luck and it you want, I can send you a copy of the load calc, energy audit and the performance history.  Just send me a private message and I'll get back with you.

Good luck.

Setting aside for the moment the excellent comments already contributed above, from simply a design/implementation point of view, with respect specifically to water loop GSHP units, yes, you can have 2 units running off of a common, single water loop.  This is indeed the case for me - 2 units, one closed (vertical) water loop.

In fact, if you were to proceed with water loop GSHP units, I would think this would be much more economical, versus installing 2 water loops, one each for each unit.

Best regards,

Bill

Thank you for all the replies! I haven't heard of Daikin before, so I'll look into that too. There's so much to learn!

Dale, I'm glad your system is working out for you, and I'm interested in those calculations (will send email).

Bill, thanks for the info! Someone told me that we would need 2 loops, but it seems like there should be a way to run both buildings from one. I'm planning to meet with an HVAC specialist here, and I'll keep my fingers crossed that he'll say he can do it. Do you cool water in a storage tank as suggested above, or do you split the line to each unit? If you could provide a quick description of your set-up, it would help me a lot in speaking with the HVAC person.

Thanks again!
Trish

 

Trish, I split the line to each unit.

See the first image at http://welserver.com/WEL0043/ for a layout of my set-up.  There's no reason my second unit couldn't be located in another nearby building.  Obviously the loop heat exchanger has to be sized to accommodate both units.

Best regards,

Bill

Trish,

Dan Bandimere uses a common geothermal loop for a split system for Earthlinked equipment and I don't see how this would be any different unless the buildings are too far apart.  In some cases, he has combined vertical and diagonal wells to increase the capacity of a single well field and avoid well field conflicts at the same time.  Dan's web page is www.comfortmadesimple.com and he has a place on his web page where you can contact him. 

You can run as many units as you want off a common loop. The easy mis understanding however is that this changes footprint. It may not as the groundloop will have to be big enough to transfer heat from all units or capacity will be reduced.
J

Joe, would that mean it would be possible to over-size my loop for a bit more in costs and have the extra piping there for putting in the second unit when it's more in the budget? It seems to be doing the loops all at once is probably a fair bit less than doing two separate digs.

By all means put all of your ground loop at the same time.