In this question I am asking bout what is marked with #3 in the picture. In my case that pipe goes to the ceiling ..partially open in the furnace room and from there through the finished ceiling it traverses a hallway and I am not sure if it joins the drain of my bathroom downstairs or the drain of my sink or bathube upstairs, they are all represented in the picture. I need to verify the integrity of this pipe. How can I do it?
Edit: the drain tube was installed about 30 years ago when the old furnace was installed (an Olsen, first models of condensing furnaces)
Edit2: looking closer I discovered that there is a pipe that brings water to my humidifier (old an and not in use anymore. Most probably the tube for the pump follows this pipe to the source of water. The worst part is that I also discovered that just above the furnace the tube diameter it reduced by coupling the initial tube with a smaller diameter tube which is the one that actually goes through the floor joists, have no idea where. So now it is out of question to use fishtape to see where this tube goes.