The SFB-3 came out in the 1970s during the back-to-the-land era. It is rated for about 100000 BTU. I have it connected in a loop with one radiator in my stairway that is 6 feet long, 18 inches high, and 8 inches deep. The whole system holds about 30 gallons of water at the moment. It needs more radiators in the house but that will be a next year project.