That's a 'cool' project you're in
I read a book on home GSHP, mostly water-water. It had a few pictures of homemade contraptions with a word on what
not to do when doing it yourself. I can't tell you more at the moment b/c a friend borrowed the book.
I'm wondering if installing a window A/C unit backwards would work as an air-air heat pump? It'd heat only one room, so you'd want it only where you spend most of your time, keeping the rest of the house cooler. There is also the question of efficiency - it might gobble too much electricity.
AC_Hacker: Maybe this thread should move to the
Saving@Home subforum? There's more on heat pumps there. There is also EcoModder's evil twin,
EcoRenovator, with a forum and everything. You can find most of us there