We're in the process of cleaning out an old gravity system in preparation for installation of a new mod/con and controls. The usual 3" mains leaving and returning to the basement above where the old boiler (entombed in 1992) used to live. What's odd here is that each of the 3" mains makes a loop around the crawlspace, returning to the boiler. Radiator supply and return lines are tied into the same section of main, in what looks to me like piping from a counterflow steam system. The radiator trim is all hot water, and old. I'd guess 1915, when a second story was added to the original 1880-something house.
I'm having trouble seeing how gravity hot water circulation would actually work in this configuration -- especially now that the mains are uninsulated. Re-piping all the supply lines for 18 radiators would not be easy, and (at this point) converting back to steam is not a viable option.