My grandparents home in NYC built about 1900 had a gravity hot air system. The pipes were huge, I am guessing 2' in diameter and went from the furnace to the rooms on the first floor, and one big pipe went up to the center of the second floor. Return air was down the open stairs all the way to the basement. Circulation was strictly on the basis of the heated air rising.. A fan to help circulation would have made a big difference. Coal was the fuel.