What is the
greenhouse construction? Single pane glass? Plastic? How warm do you need to keep it? Are you trying to just prevent freezing, or have it warm
enough that something keeps actively growing through the winter months?
As a data point, I run a 4" J-tube system with a small
cob bench in ours. This is enough for me to run a fire for 4-6 hours in the late afternoon and evening, and keep the inside temperature about 5-7 degrees C higher than outside. The glasshouse is 6x3 m, and the floor is dug into the ground to a depth of 75 cm. Single layer 3mm glass, so the thing leaks heat like nobody's business. On the coldest nights we ever get here (-3C) the inside is still a few degrees above freezing, but too cold for tropical plants, so I tend to let those go dormant for the winter by keeping them on the dry side.
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