Furnace vent

[Jeff Eastman]

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[Eric Barker]

Very often around here the vents are over-sized which leads to condensation forming. Can't think of a reason to step up the size and I'd question it.

[Bob Harper]

Possibly they re-used an old vent for older equipment, which required the larger vent. Since the newer equipment requires a much smaller vent, they re-usd it.

Then again, it could be they had some larger pipe lying around and decided to get rid of it.

Needs bracing vertically and in two horizontal axes. What does it serve? What is the total BTU input rating?

[Byron Lentz]

The vent should NEVER be upsized. In the manfacture's installation guides that I have read (been doing HVAC stuff 20+ years) never have I seen where it is OK to upsize. The venting properties of the furnace will be comprised. And yes the products of combustion will cool below the dew point and water will condense and run down the vent into the combustion chamber. I have made a lot of money replacing furnaces damaged this way.
Byron