My uncle has a 20,000 sq. ft. pallet shop he heats with one of these. No insulation and the doors are opened and closed all day. http://www.biomasscombustion.com/ They burn about 1/3 cord of thin pallet boards during an 8 hour shift. It's warm enough in the building I've seen the guys working in sweatshirts at 0 degrees outside. The blowers on his move air a long way across his building which may work well in long greenhouses. If you can source some cheap/free scrapwood it may be an option.