GPM Capacity of 2" Shower Drain

Discussion in 'Shower & Bathtub Forum & Blog' started by GAtkins, Aug 3, 2009.

  1. GAtkins

    GAtkins New Member

    Messages:
    2
    Greetings gang and thanks in advance for any help:

    we are remodelling our shower and are looking to put in multiple shower heads and some body sprays.

    our current drain is 2".

    My question is what is the maximum GPM flow that this drain will handle? Also, is there somewhere on line where I can look at the calculations, etc?

    Thanks again for any help.
  2. Thatguy

    Thatguy Homeowner

    Messages:
    1,459
    Location:
    MD
    http://www.engineeringtoolbox.com/pressure-loss-plastic-pipes-d_404.html

    http://www.engineeringtoolbox.com/darcy-weisbach-equation-d_646.html

    You need to know pipe length and height above the opening into the next larger pipe, pipe materials, # of elbows, etc.
    Last edited: Aug 3, 2009
  3. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,128
    Location:
    New England
    Assuming you don't have anything greater than say 1" pipes supplying water, it should not be a problem. Now, if it is a really large area of shower, you may want a second drain to limit how far the water has to go to get to the drain.
  4. TedL

    TedL New Member

    Messages:
    604
    Location:
    NY Capital District
    I'm not sure that the links above are an answer, as they deal with pressurized flow, and you're asking about gravity flow.

    I think you can get a workable answer, though not exactly to the question you asked, by looking at your code to see how many fixture units it assigns to the shower heads and how many it allows for the drain pipe size. You're going to have to work within that code if you plan to pass inspection.

    Here's the IPC as an example, not intending to say it controls for you.

    http://publicecodes.citation.com/st/index.htm
  5. GAtkins

    GAtkins New Member

    Messages:
    2
    Thanks for the help so far, but I remain confused.

    Arkansas has adopted the codes in the link. Our house was built circa 2003.

    our master bath shower has a 2 inch drain, 1 shower, 1 toliet, 2 sinks, 1 bath tub separate from the shower.

    the remainder of our house has 2 additional toliets, 3 additional bath sinks, 1 kitchen sink, and 1 washer.

    If I total this up correct with what I have, it comes to 22 units. The rating for a two inch drain if I am reading the table correctly calls for a maximum unit value of 21 for a 2 inch drain with only one quarter inch of slope per foot.

    for a slope of one half inch per foot, it apparently allows up to 26 units.

    I want to add to my existing master show 1 additional shower head (total of 2), and 4 body sprays. The gpm rating of the two shower heads will be 2.5 gpm each for a total of 5 and for each of the body sprays 1.75 gpm for a total of 7 for the sprays and a grand total of 12 gpm for the entire shower.

    When I consulted my plumber this afternoon he said that the maximum GPM for a 2" drain is 12gpm, so according to him this will work.

    I would like to get some independent confirmation of this.

    Will the above work with a 2" drain?

    Thanks again in advance for any help/replies.

    I don't want to remodel the entire shower and then not have it drain.

    Glenn
  6. Thatguy

    Thatguy Homeowner

    Messages:
    1,459
    Location:
    MD
    That's what I get from my 2000 IRC, assuming no water closets: 21.
  7. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,128
    Location:
    New England
    I think you're missing something...your toilet drain is at least a 3" pipe, and the whole house may go into a 4" pipe. The outlet of the house drain system is at least 3".
  8. johnfrwhipple

    johnfrwhipple Bathroom Design & Build - North Vancouver, B.C.

    Messages:
    4,393
    Location:
    North Vancouver, BC
    Flow Rates of a 2" Shower Drain

    I found this old post and thought I could add in my own two bits.

    I have done a load of private testing on the subject of flow testing and have found many of these 2" drains can handle easily between 8-16 GPM (Gallons Per Minute). In my tests most drains see no build up of water (above grate) at 8-10 GPM or less. When the flow rates creep up higher so does the water level above the grate.

    At 18 GPM I have measure 3/8" of water above the drains grate.

    We have also noticed that flow rates drop when the drop to the P-Trp is not vertical. We also noticed that the higher the drop the better the flow rate. We have also noticed that any mis installed fitting can greatly lower the flow rates.

    Hope this helps.

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