What's the purpose of the Flue pipe section?

I am curious why the flue in this picture has a section of 6" piping and reduced back to 3". What’s the point?

I believe that is simply a muffler and the venting that extends to the floor is for condensate to drop out.

I’d go with Scott. He is more versed in HVAC.

I would at minimum have a heating contractor review/clean the furnace, especially with the age.
If I remember, those were manufactured late 80’s early 90’s

Yes, it is a muffler.

I would not take responsibility for the inspection of this unit. Too many problems and re-calls to get caught up in. Google it.

I worked for a mechanical company that was a Lennox Dealer. Lots of head ache!

Good heads up David. This was new to me,Thanks

That was a new one on me too. Still don’t know how that set-up worked though. I don’t see where that is a vent.

Thank You everyone for your input! This is my first time using the forum and I am very pleased with the quick responses! I will for sure utilize this forum more often and hopefully have more answers and less questions in the future.

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Off the top of my head, do not quote me on this yet, Condensate pipe for high efficiency NG condensing forced air furnace. Be back with a link. I have inspected several.

If my memory serves me well the large condensate manifold/filter neutralizes acidity from condensate produced by the condensing furnace produces as not to damage the homes and city drain pipes. Not in the office. I will be back with some info.

Manufacture, Model & Serial number would help.

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Download the manual and read it. Many HE units specify doing that when the exhaust run is short.


I actually did not get the information off this unit because it was not an inspection I was doing. I stumbled upon it during an open house visit (in Northern Colorado). I took a quick picture. Next time I see it I will not be surprised! Thanks for your help!

Morning, Bryan.
At one time or another we all come across an item, system or component we have not yet encountered. It’s the willingness to self inform and share that becomes a privilege, honor and/or duty.

Happy if I helped. My pleasure, Bryan.
Best regards.
Robert Young

The pipe at the bottom of the unit is the exhaust. It vents upwards with the combustion air intake (top of the unit).

The exhaust pipe must be installed with a slope back to the furnace. We do not want it to drain to the exterior as it will freeze and stop the unit operation.


Combustion gas condenses in the exhaust pipe and flows to the trap where it should drain to a proper discharge point (per code).

Good pic, and information David.
Indeed - any high efficiency equipment heating manufacturer indicates to pitch exhaust back to furnace.