PVC Flue venting size

[Trent Tarter]

This 90+ furnace has flue pipe that increases in size. I would think there's potential for condensation to develop and accumulated near the joints/reducers. No sure when this was done? 

[Bill Kibbel]

The manufacturers' instructions usually contain very specific details about if, where, and how the intake and exhaust piping can increase or decrease dimension. Same with direct exhaust water heaters.

After looking at just a few different manuals, you'll see there's no rule of thumb. More often than not though, a diameter increase for the exhaust is not permitted outside of the furnace cabinet.

[Chad Fabry]

The instructions I've read always specify the diameter change in the vertical portion so that condensate does not accumulate.