I agree with Bob, I grew up with a hand fired gravity feed boiler system. Radiators got hot and stayed hot. When my Mother couldn't handle the coal and ashes and longer I replace the boiler with natural gas. A fraction of the water volume in the boiler with a single circulator pump. Both worked well. I wish I would have saved the Burnham coal boiler. It was smashed and sold as scrap.
Today I wouldn't know how to design a gravity circulation system. I know of no textbooks on hydronic design for gravity flow. Anyone know how to design gravity flow systems? Pipe sizing, flow rates, temperature differentials, etc.