I work in an old building. Not that old, really, only about 70 years, but old enough.
And in our hallway is what I thought was some kind of pressure equalization conduit up to the roof. At least that's what I think M told me when we moved in.
Today I found out that it's an overflow drain. See, in the ceiling, above the ceiling tiles but below all the ductwork and plumbing, are these plastic bag sorts of things. Well, sheets, maybe, rather than bags, but I'm not sure. These sheets are, I guess, somehow fastened / sealed to the walls. And in a couple of places, there are valves, and these valves connect to overflow drains.
Thus, when the air handlers freeze up, or there's too much condensation, or it rains really hard and the roof drains overflow, the water is caught by these plastic sheets and funneled into the overflow drain tubes.
All of which makes sense, I guess.
The part that leaves me completely fleeping boggled is, why do the drain tubes empty into our hallway, rather than outside? (One explanation that was offered is that this part of the building used to be a loading dock, and that the drains might have been on the outside at one time.)