Solar Furnace for under $50

That might be good for only one room. What would be even better is if a black wall of solid concrete were behind it. The wall would absorb solar heat during the day and then release it to the tubing for a couple of hours after sunset. In any case, I hope it uses double-wall glass panes to retain the heat inside.

That's pretty creative. I would think that he would need a bigger one though for the kind of house he might have.