I am constructing a pvc greenhouse. It's a lean to desgin that is 10x10. It will be 7 feet high where it is attached to my house and 6 feet high on the other end. My ultimate goal is to produce warm weather veggies in the winter. My plan is to run poly tubing up and down all of the walls, across the ceiling, and under the sand floor. The tubing will be hooked to an 80gph fountain pump which costs about $12. The pump will be inside of a 5 gallon bucket of water. The tubing will leave the bucket and twist through a drum of hot compost which I believe gets to about 140 degrees. My thought is that a constant recirculation of this water will be enough to keep the green house around 80 degrees during the winter. The greenhouse will have two layers of 6 mil plastic with bubble wrap inbetween. I will also have a barrel of water inside to help hold the heat in. There will be a 10x1 potato bed that will be 2 feet deep. And an L shaped bed that will be a total of 19 feet long. It will be 16 inches deep and 2 feet wide. If I can actually get all of this to work, I would like to add in a temperature controller that turns the fountain pump on and off accordingly. Does anyone have a similar system? Or any thoughts on the subject?