Holohan's Hydronic Forum: A Special Section Sharing Exchanges Between Dan And His Fans.
Dan replies: There were many gravity systems with no tanks. I've seen them, and I have to admit, they will make you scratch your head at first. The Dead Men fed the radiators on these jobs bottom-to-bottom, and they kept the tops of the radiators filled with air. Keep this in mind when you're refilling the system. As for Pumping Away, this type of system has multiple compression tanks (each radiator becomes one), so the point of no pressure change will locate itself somewhere in between all of them. Eventually, the air will be absorbed from some of the radiators and vented from the system. Just pump toward the radiators. And don't worry, these gravity systems play by different rules.