Here’s a great DIY on how to make a solar heater. Although the one he made is not functional you can probably easily make it functional by adding an inlet pipe and outlet pipe plus a small PC fan. But the theory is very impressive and this thing could be made more efficient with a little bit more work.
Using the same equipment and methods, I determined the outlet temperature to be about 95 degrees – thus a 15 degree temperature differential. Not 110 degrees, but not bad , considering I didn’t even break $50 in materials – most of that being the plexiglas window.
Obviously don’t have the inlet and outlet attached to the garage – figures that the day I finish the furnace, it’s 80 degrees and sunny and it looks like we’re finally done with winter. Dad recommends wiring a pusher fan at the end of the inlet tube to keep the air circulating through the furnace.