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!)

  • 2
    Would contact Honeywell. They will know if there is a design reason for that pipe to be like that. Changing it just because it looks funny is usually not a good idea.
    – crip659
    Commented Oct 10, 2022 at 12:45
  • 2
    I’m voting to close this question because we don't know what that tube is for or why the manufacturer decided to route it that way. The only way to answer this question is to ask the manufacturer.
    – FreeMan
    Commented Oct 10, 2022 at 15:36
  • Oh, I assumed that the installer put that in, and that it was some standard configuration. If it was set up in the factory like that, I totally agree.
    – bitsmack
    Commented Oct 10, 2022 at 17:05
  • Turns out it is an installation thing. I'll post an answer below in a few minutes...
    – bitsmack
    Commented Oct 10, 2022 at 17:25

1 Answer 1


I dug into the installation manual and the original configuration was incorrect. There are many different field conversions that can be made to the unit, some of which require repositioning the condensate trap. They all have the following in common:

Original configuration:


Modified configuration:


And the following note:


So, yes, I should shorten the tube.

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