>>>Bill used the iron inside tank of a hot water heater<<<
Wish they still made them that way... Now they are like a big tin can with a glass lining.
Years ago, I had a 2 barrel Sotz wood heater that put out 240,000 btu - Since propane got so expensive, I have not used the greenhouse, even removed the cover. Now, I am going to move half of it to the back of the house (South side) this fall - 24’ X 48’ instead of the 96’ it was. The one heater will heat that very nicely, even in the really cold weather. In fact, it will add heat to the main house on sunny days.
move half of it to the back of the house (South side) this fall - 24’ X 48’<<<
You will be amazed how much heat the sun will give you.
Be sure to either have a mass of water storage...LOL, fish tank or barrels of water to serve as passive collectors for the heat, it works, for I have used the method.
You should see how well plants do, sitting over an open tub of water, on a board, and I mean the regular potted plants, not some soil-less system.
I had placed buckets and tubs of open water on top of the barrels of water, and being me, soon put a board across them and plants on them, good for seedlings, orchids and every thing, that goes in a pot.
The attached greenhouse, even makes the evaporative coolers work better in the summer.
The first year we were here, the cooler did not do a good job.
The second year was great and the only change was adding a solar greenhouse and leaving the door open into it.