I first want to thank all those that post to these forums.  I have learned a lot from the links that have been posted as well as the posts themselves!

I am 2 or 3 years away from stating a home in the Texas Hill Country north of San Antonio.  There is very little soil and a lot of limestone. 
Our lot has a 20% slope so it will be a walkout and we are investigating the tradeoffs and costs of cut/fill/elevated construction.

As you can imagine, cooling is a much bigger issue than heating.

Is geothermal a viable alternative in a solid rock environment?

Thanks,

ICF Curious

Yes it's viable. It will require more loop than a heating dominant area. If using an EarthLinked DX system, you'd most likely use a V1.5 or V2 loop. This would be a vertical bore hole loop, with 1.5 loops per ton, 100' deep (so a 4 ton would be six-100' deep holes), or 2 loops per ton, 75' deep (so a 4 ton would be eight-75' deep holes).

I have been following this forum with great interest, especially the threads relating to Texas installations.

To follow up on my original viability question, it seems that a DX system will have an advantage here because it will require less drilling.  Do you a think water cooled system will competitive in this environment?

We are thinking about 4000 air conditioned sf on 3 levels (1400sf walkout, 2000sf main floor, 600sf loft).  I hope to convince my wife to downsize a bit.  It woud be easy to lose the loft, but hate to give up the views.  The views are to the southwest, so afternoon solar will be a problem.

ICF Curious

Keep the loft - we built something similar and like it. Make sure the space has a frequent purpose that will have you daily using it.

Work on the size of every room - check out Susanka "not so big house" series. Think long and hard about how you and all other occupants will actually live in and use every square foot in every room. Figure out what you need and build only that.

Work even harder on that southwest side. Optimize window size - install what you need to get the views, but position and proportion the windows to provide only what is necessary - good view while sitting or standing as appropriate to the room. Your toes and knees don't look outside - they don't need glass to look thru (minimize height of windows)

Spec windows with low SHGC (Solar Heat Gain Coeefficient) this makes a HUGE difference on west walls.

Consider exterior overhangs and Bahama shutters and any other viable exterior shading. Retain any existing trees on that side if at all possible. Protect those trees and roots from construction impacts. Interior shading is a poor substitute as much of the heat gain still enters the building envelope even if light is blocked.

Be darned certain that the load calc reflects all of the above...

We have the windows figured pretty much as you describe.  The higher the window, the better the light penetration.  The bottom 2 or 3' don't contribute much as you correctly point out.  The challenge is external shading.  So far, we have not hit on any schemes that are both effective and aesthnetically pleasing.  We may end up with motorized Hunter Douglas type externals.  Maybe by the time we build the "switchable privacy" glass will be cost effective. 

We will continue to play with the floor plan.  The walk-out creates some nice recreational space and an "outdoor room", but also contributes to our excess space.  Our deed restrictions require 3500sf and we hope to reduce to that (keeping the loft).

Still courions about DX vs. watercooled units with vertical drilling in limestone, though;-).

ICF Curious

A DX system will work wonderfully for you. Limestone will conduct the heat transfer very well. Plus, you should need less drilling with DX than water source. Of course, it will have to be left up to your local installers and the bids you compare.

One thing that comes to mind is: if you elevate the site with fill to avoid cutting too much rock for your foundation, 5 feet of fill over the top of a horizontal loop works as good as a 5 foot deep trench (dx or water source).
Good Luck,
Joe

Searching this forum with a similar question and wanted to know how this project turned out- 2 years from July 08 would be more or less today!  What type of system did you end up with?