Hi,
I don't know much about this, so take these comments with a grain of salt.
I did try to assemble all of the actual design and build information I could find on earth tube cooling, and I put it here:
http://www.builditsolar.com/Projects/Cooling/passive_cooling.htm#Other
This stuff deals mostly with air tubes, but some of the data might still be helpful to you.
My tentative conclusions after looking through this material are in the last link in the section.
Some things to think about:
- The Hait book states that the ground temperature is nearly constant year round at an average depth of 20 ft. Above that it varies some with season -- the closer to the ground the more it varies.
He further says that this 20 ft down temperature is about equal to the year round average temperature of the air in the same location. So, 1) the ground temp in FL is going to be considerably warmer than the ground temp in MT, and 2) you are going to need to go down a ways to find some worthwhile coolth.
- I think you are right to be concerned a humidity. With the scheme you mention, you won't have to worry about condensation in the tubes, but you still may need dehumidification to make the air comfortable after running it through the radiator?
This is just my prejudice, but I wonder if you would not be better off putting the earth tube budget into other things that would tend to keep the house cool? Some of the links above seem to indicate that peoples expectations for earth tube cooling are not usually met. You have already taken a good step by keeping the square footage of the house down. I'd think about good insulation, good windows, good overhangs to shade the windows, good passive solar design, white metal roof, ... and, a really efficient and slightly undersized AC for when you really need it.
A good article on optimizing a house for hot/humid climates:
http://www.builditsolar.com/Projects/SolarHomes/crumearticle.htm
Again, just my 2 cents
Gary