Hi,
This is a good writeup on using air collectors to heat water:
http://www.builditsolar.com/Projects...aterHeater.pdf
While I like the EPDM lined tanks, maybe a metal tank is better for air heating collectors.
Al did a hybrid air and water heating collector:
DIY Solar Air Heating Collector with Water Heating
I think that the car radiator scheme that was mentioned above would also work.
Gary