In the fall of 2008 I converted my south-facing front porch into a 72 sq ft solar air heater, and the results have been excellent. Upstairs wood consumption dropped by about 1/3 and I no longer use ANY nat gas for heat. I posted some in-process photos in this thread: https://www.hearth.com/econtent/index.php/forums/viewthread/24643/ I estimate that, on sunny days in winter, I get 50k-60k btus per day from this heater. It's working so well, I thought I'd add another one right next to it. So I'll share my process of building the second heater, which I started on July 5th. The first photo shows the space where it will go. The walls of this house are thick (16 in.) masonry, so I won't be putting any new holes in. The idea for this heater is to bring cold air in through the basement window (bottom right), use a sub-flooring with partitions to distribute it evenly across the front glass, take the heated air into the house through the top 1/4 of the upper window with fan assist, and install a floor-level vent in a hallway 25 feet into the house to complete the circulation loop. When operating, it will heat upper and lower rooms at once. As I discovered while building the first one, construction is painfully slow when building triangular spaces, probably 5 to 10 times slower. So this indeed is a labor of love. The second photo shows the concrete slab poured, giving a better idea of what will be happening.