as we find on Dunham, Webster, some Hoffman and other Vapor systems: it's a Float Trap/Air Eliminator. It serves as the only air vent for the entire system, so it MUST work well. The little device in the 45° elbow is a vacuum check, which should be removed since vacuum doesn't work well with oil or gas firing.
We often find these are stuck shut or only partially open. You can take them apart if you have the right tools, but watch out for bolts that snap off.
The Gorton #2 vent is a good replacement for this unit. If the system is large you may need more than one such vent.
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