I ran across some pretty cheap pool heaters:
4x8 Enersol solar collector with 15 year warranty?? $190 from solarcovers.com
FAFCO 2x20 solar collector with 10 year warranty $135 from poolproducts.com
Smarthome.com has a similar 2x20 collector with 5 year warranty for $130. They say it has a 40,000 BTU rating, and they also offer a 4x20 for $250. They have a good online pdf collector installation manual that has some good diagrams and info.
Interestingly enough while browsing through the manual I....
Do It Yourself Solar
