My furnace, a high-efficiency gas model from Rheem, is about 5 years old. I recently had to replace one of the pressure switches and so I was looking around the inside of the furnace.
There is a condensate trap which is fed by two tubes, it then discharges outside of the furnace into a condensate pump (because I don't have a convenient drain in the area).
One of the tubes comes from the exhaust vent pipe of the combustion air system. I don't know the purpose of the other (smaller) one, marked "???" on this picture:
So, to my question: See how the "???" hose actually rises about 3/4" from the drain port before it turns downward into the trap? It's like an inverted P-trap :) Is this intentional or should I shorten that tube so that it doesn't require a certain depth of liquid before it drains? Or should I just leave it alone?
(Edit - This is a question about installation technique and not factory configuration. I have found an answer in the installation manual and will post it below. Please don't close the question!)