This is for RobVG, or anyone else that is interested. While not the typical geek topic, I think it is still rather geeky. While all the specifics are not perfect, I've uploaded a SketchUp file to the HZ gallery that illustrates the basic concept of heating the ground in a greenhouse using the hot air in the structure. This acts as a thermal mass and can help cut heating costs during the winter and act as a heat sink in the summer.

The basic concept is to pull hot air down from the top of the greenhouse and pass it through a series of ducts buried below the surface. The air then returns to the greenhouse. This has been developed by the Rocky Mountain Institute and I have seen it in action in a greenhouse at 8000 feet in the Colorado Rockies.

The file is here: (file no. 51 from the old board's gallery, not restored). If that link doesn't work, you can find it in the Community File Gallery. (let me know how it works out)

Here's a screen shot:

(picture no. 176 from the old board's gallery, not restored)