fixture units

Discussion in 'Plumbing Forum, Professional & DIY Advice, Tips & ' started by ilya, Jan 13, 2010.

  1. ilya

    ilya In the Trades

    Messages:
    105
    Location:
    Akron Ohio
    Twinpeaksr's post and Terry's reply brought something to a head for me. I don't understand how to use fixture units for supply or DWV. I only know they help calculate pipe sizing requirements based on fixtures served. Can someone here help please? Thanks
  2. FloridaOrange

    FloridaOrange Plumbing Designer

    Messages:
    1,317
    Location:
    SW Florida
    You get drainage fixture units and that relates to the pipe size (somewhat) and more to the venting pie sizes.
    Water supply fixture units relates to water pipe sizes.

    What are you trying to understand?
  3. ilya

    ilya In the Trades

    Messages:
    105
    Location:
    Akron Ohio
    I'm starting with the assumption that there are charts for these units, and that x number of units requires a certain size pipe, be it supply or DWV. A lav sink would seem to have less Fixture Units than a shower, for example. But that's the extent of what I think I know.
  4. iconicplumber

    iconicplumber JMan plumber/gasfitter

    :D i wish i could help but if it was easy everyone would be doing it.

    the dialogue would fill this page and more on how to properly size dwv(drain waste and vent), that being said if you had a more specific question i might be able to help you out.
  5. jimbo

    jimbo Plumber

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    Last edited: Jan 14, 2010
  6. hj

    hj Moderator & Master Plumber Staff Member

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    Dfu

    Drainage sizes are dependent on the number of fixtures connected to the pipe and the pipe lengths. Water fixture units are also based on the number of faucets, BUT, there is also a factor applied because they know that not every faucet and toilet is going to be used at the same time. Therefore, water supply tables vary according to the building's usage and number of occupants.
  7. gator37

    gator37 Retired prof. engr.

    Messages:
    108
    Location:
    Alabama
    Good answer Jimbo

    Good answer Jimbo.
    A fixture unit is a fictitious number (from a code which is based on years of collective experience) assigned to a fixture (just a number assigned to a fixture), be it waste, vent, water supply, etc.
    It provides a diversity to sizing, in other words if you had two 2" lines from a fixture, you would not size the collector pipe for twice the area of one 2" pipe. This is done because it not highly likely that all the fixtures would be turned on at the same time.
    Some codes like the NPC or the IPC have examples on sizing in their appendix.
  8. gator37

    gator37 Retired prof. engr.

    Messages:
    108
    Location:
    Alabama
    another Good answer HJ

    another Good answer HJ
  9. Twinpeaksr

    Twinpeaksr Engineer

    Messages:
    26
    Location:
    Denver, IA
    On the DWV side, is there a down side to oversizing the vent or drain? I can't think of any but thought I would put it out there.

    Thanks!
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