posted 6 years ago
I second the suggestion to insulate the ground from the exterior. Otherwise the outermost two feet of the greenhouse soil will be at or close to freezing all winter.
Wrapping copper pipe around a hot pipe and running water through it is a bit tricky. If you have a pump running all the time it should work just fine. But if you're relying on a thermosiphon or gravity, the water can slow down enough to boil and then you have steam and other problems.
Might the boiler give off enough ambient heat to the greenhouse without any trickiness? Or does it have an extra outlet that you could tie another heating loop to for the greenhouse?