Hey Hoss
I remember heating an old single glazed glass greenhouse back in the 60's. I only had a 40000 but furnace and it would run constantly on those -20 nights.
My idea for the small gh size is just something temporary for this winter. A bigger one is in the thinking stages but it will have to be a freestanding affair. Right now I'm maxed out on garage/shop size of 720 sqft. Per the zoning people I can build a larger gh but it can't be attached to the garage. Zoning also allows an outbuilding (shed) of up to 168 sqft. The bigger gh could substitute for my shed. One or the other, but not both.
Heating is a concern and I'm going to do some calculations to see what it would take for my little 4x6 idea.
I was thinking even going to extent of adding some styrofoam panels over the glazing at night just to retain heat.
I did some computer sketches to see what this thing might look like.
The red frame would be the existing garage door trim.
An inside and outside view.
Ron