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IFGC question

Timco Member Posts: 3,026
What section covers venting a cat I waterheater in the same arm as a cat II fan assist furnace? I am missing it...found this to be the case on a job I just looked at and need to fix it before heating season....

Just a guy running some pipes.


  • Bob Harper
    Bob Harper Member Posts: 882
    CAT II ???

    Don't you mean CAT I furnace? I'm not aware of any appliances that are actually listed as CAT II. This is largely a theoretical category. There are a few vents out there listed for CAT II such as Protech's AL29-4C.
  • Timco
    Timco Member Posts: 3,026

    I walked right to my mechanical code book, but the gas book must be out in the truck. Regardless, as I recall, catII is for fan assisted 80% units. If I am wrong, so be it as I do not often use the categorys. I have a fan assisted furnace vented in the same horiz arm as a standard natural draft water heater and would like to quote the section pertaining to this. Did not mean to come across as lame...

    Just a guy running some pipes.
  • Joannie_15
    Joannie_15 Member Posts: 115
    Vent Categories

    Cat I is non-positive, "dry" vent

    Cat II is non-positive, "wet" vent

    Cat III is positive, "dry" vent

    Cat IV is positive, "wet" vent

    I believe the National Fuel Gas Code states that Category I appliances may never be vented in the same pipe as an appliance with any other vent category certification.

    There are some units out there with Cat II listed on the rating plate, but I've haven't heard of many.

    If what you have is actually a Cat I and a Cat III, I would not vent those together (unless there is an external mechanical draft system that is properly designed by the fan company), as the Cat III positive vent pressure may push those flue gases back through the Cat I negative pressure system.....especially when the Cat I is not operating.
  • Bob Harper
    Bob Harper Member Posts: 882
    common venting

    The IRC Ch. 24 and the full IFGC allow for common venting of CAT I WHs with fan assisted CAT.I furnaces if the vent is properly sized and it works. The problem with this setup is, it defeats three primary safety controls on the furnace: pressure sw, flame rollout and high limit. If there is a blockage in the vent/ chimney, the furnace will gleefully continue to vent out the WH draft hood without tripping any of these safeties. You could remove that draft hood and install a bullhead tee & x2 baro damper with spillswitches to the furnace and WH if your AHJ allows it.

    Otherwise, the rules for vent connectors, venting and chimney are the same.

    Joannie is correct about the CATs of venting. You don't see CAT II because it is virtually impossible to get a unit to vent atmospherically at that low stack temp. not to mention the amt. of condensation. The fan assist was a stop gap measure to tweak as much efficiency out of the unit without a secondary HX to capture the latent heat (ala' CAT. III & IV). IRC 2009 G2427.3.3 requires all positive vent pressure appliances and their venting to be listed for positive pressure. CAT. I is NOT listed for pos. pressure (UL 441, e.g. B-vent) or chimneys as defined in the building code. Chimneys are NOT approved for positive vent pressure.
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