I am studying installing an earth tube heat exchange system that will run on solar energy. The big hurdle is designing the system to exchange the air in a 3500 square foot house hat was built in 1889. I have seen it done a few different ways, in different regions, so I am incorporating what I have learned, and adapting it ti my region. If you would like to talk more about it, let me know, and I will be happy to share my thoughts, and listen to yours.