We’ve come a long way since the days of gravity-hot-water heating. The physics haven’t changed a bit, but the systems sure have. Trapped air was a challenge back in the day before we had effective air separators and circulators. The Dead Men would slope their big horizontal pipes slightly upward to encourage the air to rise out of solution when the water got hot. It usually floated up into an open expansion tank in the attic. Have you ever seen one of those? They were often riveted at the top and bottom; most had a gauge glass. Some of those tanks were made of copper. I’ve known a few contractors who licked their lips at that sort of scrap history.
On these jobs, the Dead Men would usually pipe their cast-iron radiators with the inlet and the outlet both at the bottom and on opposite ends because they didn’t have circulators. The water would rise slowly to the top of each radiator as the colder water fell out of the radiator and into the return pipe. This becomes a challenge when you add a circulator to the system because the hot water now finds it easier to just scoot across the bottom of the radiator. It looks like an air problem from the outside, but it’s not. You solve it by throttling the flow into the radiator.
