High Converse, can you clarify your question a bit more? I am so confused by hot air thru a hot water tank, etc. What is a "popcan heater? I heat my AP GH system air and water with my home made wood stove but I haven't a clue what you're talking about. Sounds interesting. Here's a pic of my GH wood stove in our unfinished GH:
I have thought of making a solar collector by enclosing a flat coil of black tubing in a frame covered with clear plastic. Water will absorb heat better than air. A pump in the FT and the collector should be all you need.