Pipe dreams or practicality
I can enthuse with the original poster about how NG air conditioning *might* be desirable, a couple years ago I went through the same thinking. My conclusion is that in the real world, you will be better served by a heat pump. Geothermal if you wish to get radical about efficiency.
Let me offer another direction of thought while it's all imaginary... have you ever thought about lithium bromide (LiBR) absorption system instead of ammonia? It has been used in commercial installations, it operates at far lower temperatures and thus might be usable with solar collectors providing some of the energy. And when required, a shot of NG to make up the BTU shortfall. There was one Texas man who did adapt a LiBr commercial unit to his house, and he discussed using solar energy to provide much or all the heat input, but as far as I know he never actually built it. I would describe him with respect and some affection, as probably an eccentric old coot.
Whatever you do, I would consider it prudent to keep a Trane around for reliability purposes<g>. Any brand actually, so long as it was a designed install by someone who thinks like the leaders on this board.
Best of luck -- P.Student
P.S. Don't forget to engineer a decent ventilation system when you plan your ICF home!
[Edited by perpetual_student on 04-12-2005 at 11:07 AM]