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Thread: Understanding Fixture Units

  1. #1
    DIY Junior Member bengal21's Avatar
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    Default Understanding Fixture Units

    Hello,
    If someone could answer the following question for me it would be appreciated. And the units I am using are from the UPC.

    If I have a 2" vent that can serve a total of 24 FU can I split the fixtures on opposite sides of the room into 1.5" vents (as long as each side doesn't exceed 8 FU) and tie them together into a 1.5" x 1.5" x 2" tee to run up to connect with the main stack? They would all run vertical until 6" above the flood elevation of the highest fixture.

    Am I correct in summing the FU for each vent branch? i.e.

    Vent one (1.5") has a total of 7 FU and Vent two (1.5") has a total of 5 FU so each vent alone is under the 8 FU for a 1.5" vent. So tying them together for a total of 12 FU in a 2" vent is acceptable because it is under 24 FU?

    Thanks for all of the expert advise received. It is tremendously helpful.

  2. #2
    Moderator & Master Plumber hj's Avatar
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    Default vents

    As a practical matter, I do not think I have EVER seen an inspector count the FU's for vents, and neither do I do it.

  3. #3
    Journeyman & Gas Fitter Doherty Plumbing's Avatar
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    Quote Originally Posted by bengal21 View Post
    Hello,
    If someone could answer the following question for me it would be appreciated. And the units I am using are from the UPC.

    If I have a 2" vent that can serve a total of 24 FU can I split the fixtures on opposite sides of the room into 1.5" vents (as long as each side doesn't exceed 8 FU) and tie them together into a 1.5" x 1.5" x 2" tee to run up to connect with the main stack? They would all run vertical until 6" above the flood elevation of the highest fixture.

    Am I correct in summing the FU for each vent branch? i.e.

    Vent one (1.5") has a total of 7 FU and Vent two (1.5") has a total of 5 FU so each vent alone is under the 8 FU for a 1.5" vent. So tying them together for a total of 12 FU in a 2" vent is acceptable because it is under 24 FU?

    Thanks for all of the expert advise received. It is tremendously helpful.
    You are absolutely correct....

  4. #4
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    Default

    I count them if I see something that just doesn't look quite right. Usually that would be every fixture in the house vented through an 1 1/2" vent.

  5. #5
    Plumber Winslow's Avatar
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    Default

    Quote Originally Posted by bengal21 View Post
    Hello,
    If someone could answer the following question for me it would be appreciated. And the units I am using are from the UPC.

    If I have a 2" vent that can serve a total of 24 FU can I split the fixtures on opposite sides of the room into 1.5" vents (as long as each side doesn't exceed 8 FU) and tie them together into a 1.5" x 1.5" x 2" tee to run up to connect with the main stack? They would all run vertical until 6" above the flood elevation of the highest fixture.

    Am I correct in summing the FU for each vent branch? i.e.

    Vent one (1.5") has a total of 7 FU and Vent two (1.5") has a total of 5 FU so each vent alone is under the 8 FU for a 1.5" vent. So tying them together for a total of 12 FU in a 2" vent is acceptable because it is under 24 FU?

    Thanks for all of the expert advise received. It is tremendously helpful.
    That is OK as long as there isn't a toilet being vented through 1 of the 11/2 vents. Toilets require a 2" vent under the UPC.

  6. #6
    Moderator & Master Plumber hj's Avatar
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    Default vent

    The question is somewhat simple for a set of local vents, but the subject becomes more complex if he is merging all of these vents into a single roof penetration, because then any code requirement for the main vent comes into play. Does your code require one FULL SIZED vent all the way to the roof? Does it require that the combined AREA of all roof penetrations equal the AREA of the incoming sewer line? There could also be other local requirements.

  7. #7
    Plumber Winslow's Avatar
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    Quote Originally Posted by hj View Post
    The question is somewhat simple for a set of local vents, but the subject becomes more complex if he is merging all of these vents into a single roof penetration, because then any code requirement for the main vent comes into play. Does your code require one FULL SIZED vent all the way to the roof? Does it require that the combined AREA of all roof penetrations equal the AREA of the incoming sewer line? There could also be other local requirements.
    The UPC requires the aggregate cross section of the vents to be equal to the cross section of the main sewer. To verify the local requirement would require access to ammendments by the local jurisdiction.

  8. #8
    Moderator & Master Plumber hj's Avatar
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    Default Upc

    That is the UPC, but the IPC may allow ANY sized penetration or use the diameters of the pipe to determine the quantity and sizes. (The IPC writers were not mathameticians").

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