And something else might happen when you add a circulator to one of these old gravity jobs. As soon as it starts, the circulator will look for the compression tank (hydraulically speaking) because the circulator needs to know what to do with its differential pressure. The tank tells lets it know by acting like a hydronic bookmark, a point of no pressure change. The circulator responds to this point by either raising its discharge pressure (if you pump away from the tank) or lowering its suction pressure (if you pump toward the tank).
When you lower the system pressure, air comes out of solution, makes noises in the pipes, and often gets trapped in the radiators. When you have a lot of air trapped in radiators, you wind up with a lot of points of no pressure change. This confuses the circulator because it doesn’t know what to do with its pressure differential. It sometimes lowers the pressure at places where it shouldn’t, and that can leave you with a lot of annoying gurgling.
Another friend ran into this problem once. The Dead Man had connected his freestanding, cast-iron radiators in a down-feed system, with the inlet at the top, and the outlet at the bottom of each radiator. Both connections were on the same side of the radiator. Now, my friend’s a very bright engineer, and he figured out what was happening when he installed pressure gauges on either side of the circulator. He was pumping away from the compression tank, but he was still getting a wicked drop in pressure at the circulator’s suction flange. This told him the circulator was seeing more than one “compression tank” out there in the system. He knew the air in the radiators was the problem, but since the Dead Man had gravity circulation in mind, and not pumped flow, my friend was having a very hard time getting the air out of those radiators.
He finally solved the problem by installing a good air separator at the boiler, and then raising the system pressure. When you raise the pressure, the water absorbs more air and brings it around to the air separator. It took him a few days, but he finally got rid of the air. He could see the result of his efforts on the gauges, which now showed a positive pressure at the circulator’s discharge.
It’s nice that we get to work with both the new stuff and the old stuff, isn’t it? America is a heating museum. Enjoy it!
Respectfully,
Dan Holohan
"Plumbers Protect The Health Of The World."
