When I turned on the gas heat a couple of weeks ago in cool, damp weather, the humidity in the house rose.
Warm, muggy weather returned. The draft from the main drain was smaller than before, and not much water was dripping. The hose of my Bissell carpet machine, a wet-or-dry vac, fit nicely over the drain pipe. It sucked out water and a lot of mulched leaves. Now it's dripping 3 liters an hour, and the draft is better.
I wonder how all those bits of leaves got in there. I guess the condensate was seeping out before the level reached the overflow drain.