Dave; I use an 8" in my 12x20 greenhouse, with a raised mass apx 2.5 w x 2.5 T x 15' long. I Use the mass to set the for starts on to keep them warm. There is a post just for rmh and greenhouses , that may answer your question. A copy of ianto evans book will also answer a lot of your questions.
Not all who wander are lost... J.R.R. Tolkien
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