It took several incarnations to come up with a satisfactory design for the bottom heated greenhouse bench. In the final version we used two 55 gallon drums welded end-to-end for the firebox and a salvaged piece of 12” stainless steel chimney for the horizontal flue. We learned the hard way that a large firebox and flue are necessary to dissipate the intense heat into the surrounding air chamber and to minimize heat stress on these components.
Any claim about winter production of fresh vegetables, with minimal or no heating or heat storage systems, seems highly improbable. The weather is too cold and the days are too short. Low winter temperatures, however, are not an insurmountable barrier. Nor is winter day-length the barrier it may appear to be. In fact most of the continental US has far more winter sunshine than parts of the world where, due to milder temperatures, fresh winter vegetable production has a long tradition.