As long as heat is not being lost I am sure the pipes would eventually heat up the greenhouse. I was getting the air to increase about 9 to 10 degrees F when the intake was around freezing. I don't know much about how trench size would effect the heating and cooling properties. I really came up with this idea out of my head and just went for it. I found very little information about it on the internet until after I had built it. Curtis Stone who is in Canada built a really good passive solar greenhouse but I think it cost him around $30,000 CAD.