I have a 12 x 12 block building that I am thinking about turning into a greenhouse. One side will be windows. The front is all glass. this room housed a hot tub for 14 years but is not used anymore. I am planning on removing the roof and replacing it with clear corrugated roofing material. I live in Pittsburgh so my biggest concern is heat over the winter. Does anyone have any suggestions for me about the best way to go about heating it? I was thinking about looking on craigslist for an old small furnace. I am figuring I need to keep it in the 70 degree range since we are really big into tropical plants. Planning on tropicals, vegetables, perennials, and annuals. Any help or suggestions will be appreciated.