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PVC

BFlanzbaum
BFlanzbaum Member Posts: 26
in Chimneys & Flues

On a 90+ furnace is is possible that the flue pipe is TOO big? The charts call for 2" but the intake and exhaust are 3". Any issues with that? Is it never too big???

Comments

  • mattmia2
    mattmia2 Member Posts: 12,137

    As long as it is arranged so that the condensate can drain.

  • ChrisJ
    ChrisJ Member Posts: 16,537
    edited May 2024

    I'd recommend changing the title to "Direct vent piping question" or similar.

    Many may ignore this thread as they think it's a question about PVC piping rather than venting.


    As far as I know, as long as the pipe fits where it needs to, and as Matt said can drain in the proper directions then no it cannot be too big. Of course 3" fittings cost a bit more than 2" and 4" even more so, but the furnace won't care.

    I also believe many tell you to go up a size, such as 3" if the piping is too long or if there's too many elbows. I'm betting that's exactly what happened in your situation. There will be charts in the installation instructions telling you what size you can use for so many feet etc.

    Single pipe 392sqft system with an EG-40 rated for 325sqft and it's silent and balanced at all times.

    Intplm.
  • GroundUp
    GroundUp Member Posts: 2,322

    Yes. Most manufacturers have a minimum equivalent length which is directly relative to backpressure. If it's designed for the backpressure of 2" and the flue is upsized to 3", there is a reduction in backpressure which can cause issues with the combustion. I've run into this quite a few times when the flue is short in equivalent length.

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