Venting a high efficient system

[Richard Roshak]

This is a 6 year old Ruud 120,000 btu gas forced air system. I'm looking for documentation why the venting pipe diameter should not be reduced. There was no manual with this unit and I cannot find anything from the manufacturer.

[David Bell]

It can leave the furnace at 2" and increase to 3" but increasing on a horizontal run requires eccentric reducers to facilitate proper condensate flow back to the furnace.

[Markus Keller]

I would suggest going to the Rheem/Ruud website and pulling pdf's off of there. Typically from my experience, anything over 100K is specified with 3" PVC.