A pro or semi-pro here on the board will hopefully
be able to better explain it for you. There is a
narrow rubber hose (a condensate or condensation
line) which runs from my furnace to my crawl space
drain. Apparently it is a good idea to have the
line flushed/cleaned out now and then to avoid it
developing any kind of blockage. It's apparently
also possible for the line to freeze if the
crawlspace isn't insulated, and the line spans a
long distance (like mine does). The technician
who came to my place disconnected it from the
furnace and *blew* into it to clear the blockage.
Don't know if that's any help to you!