Understanding Fixture Units

Discussion in 'Plumbing Forum, Professional & DIY Advice' started by bengal21, Dec 30, 2009.

  1. bengal21

    bengal21 New Member

    Messages:
    21
    Location:
    Des Moines
    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. :D
  2. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,843
    Location:
    Cave Creek, Arizona
    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. Doherty Plumbing

    Doherty Plumbing Journeyman & Gas Fitter

    Messages:
    810
    Location:
    Penticton, BC
    You are absolutely correct....
  4. nhmaster

    nhmaster Master Plumber

    Messages:
    3,189
    Location:
    S. Maine
    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. Winslow

    Winslow Plumber

    Messages:
    450
    Location:
    Hawaii
    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. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,843
    Location:
    Cave Creek, Arizona
    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. Winslow

    Winslow Plumber

    Messages:
    450
    Location:
    Hawaii
    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. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,843
    Location:
    Cave Creek, Arizona
    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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