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  1. #1
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    Default Use 3 or 4" pvc waste line?

    Howdy I'm adding a 1/2 bath wonder if i should use 3" or 4" waste pvc pipe. Advantage less $ 3" what draw backs?

  2. #2
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    Default Re: Use 3 or 4" pvc waste line?

    Howdy baghdadplumber, and thanks, i was leaning tword the less cost but will go with the 4".
    I added a popst to another question about pvc ... What DR# PE 160 psi pipe one could use to be a supply main under street, 30' wide and 60' to home any suggestions? Just cannot afford that much soft copper.

  3. #3
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    Default Re: Use 3 or 4" pvc waste line?

    Howdy i found out additional information on the net about waste drainage pipe sizing anf flow.
    "The codes provide us a table that lists all the different types of fixtures that can be attached to the drainage system and each fixture is assigned a DFU (Drainage Fixture Unit) value.
    We begin listing all the fixtures and their corresponding DFU value to find the Total DFU load for the structure. Once we have the total DFU load we then consult another table that lists the maximum number of DFU's that can be conveyed by each size of pipe. By example let us consider a few fixtures:
    Lavatory................... 1DFU
    Tub/shower................. 3DFU
    Watercloset <1.6gpf........ 3DFU
    Watercloset >1.6gpf........ 4DFU
    Shower stall............... 2DFU
    Kitchen sink............... 2DFU
    Kitchen sink w/disposal.... 2DFU
    Kitchen sink w/Dishwasher.. 2DFU
    Clothes washer standpipe... 2DFU
    Laundry Tub.................2DFU
    Laundry Group.............. 3DFU
    Floor drain................ 0DFU

    As you can see if you have a standard bathroom group, lavatory, tub/shower & watercloset we can use the Bathroom Group value but if you have extras such as two lavatories, watercloset, tub and separate shower or perhaps a bidet we must then use the value of each individual fixture to compute the bathroom group.

    We would then use the total DFU value for the whole structure to determine the size of the "House Sewer line" (line from the structure to the septic tank or municipal main.

    WE must then go back to each bathroom, Kitchen and laundry ad compute the total DFU value for that room to determine the proper size of the branch lines. As these branch lines connect together we must compute the combined load of the lines connecting to determine the size of the line downstream from that intersection.

    The codes then give us a table listing the Maximum DFU load that can be conveyed by each size of pipe. Example;

    ..........................Horizontal........Vertic al
    1-1/4".................... 1DFU ............. 1DFU
    1-1/2".................... 3 .................4
    2"........................ 6 ............... 10
    2.5" .................... 12 ................20
    3" ...................... 20 ............... 48
    4" ...................... 160 ............. 240

    The code has some exceptions to this rule by example, while a watercloset is rated at 3 or 4DFU which could be conveyed by a 2" line however we must also consider that a watercloset is expected to convey solid fecal matter therefore the code requires a watercloset to be set on a 3" or 4" line.

    Contrary to popular belief an oversize line will clog much faster than an undersized line.

    When a drain line is properly sized in this manner the line will only be 1/2 full under full load which provides a sufficient liquid level to convey any solids that may be present in the waste. In order to insure proper velocity of flow this keeps the upper half of the line open for the free transfer of vent air.

    If a line is oversized it results in the level of liquid being much lower. In many cases the level of liquid is then not sufficient to properly suspend the solids and the solids will rub and scuff on the bottom of the pipe dramatically reducing the velocity of flow. This also allows the liquids to then seep past the solids and continue down the line leaving deposits of the solids in the line. Once the liquid has passed the solids will then begin to dry in place creating a small dam for the next cycle until the solids build up and totally clog the line."
    I sent the man a thank you that posted this on ablog.
    Hope this may help others too.

  4. #4
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    Default Re: Use 3 or 4" pvc waste line?

    3" is perfect for what you are doing. Any plumber knows this.
    3" will carry up to 3 toilets on a horizontal line. If you are only adding a 1/2 bath then go 3". Dont waste your money.

  5. #5
    Join Date
    Dec 2008
    Location
    Midwest
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    159

    Default Re: Use 3 or 4" pvc waste line?

    Quote Originally Posted by Timothy Miller View Post
    Howdy I'm adding a 1/2 bath wonder if i should use 3" or 4" waste pvc pipe. Advantage less $ 3" what draw backs?
    It mostly depnds on what you will be tying on to. If you are tying on to 4" no problem. If you are tying on to 3" and want to run 4" to new bathromm. Make sure you make reduction in a VERTICAL position

  6. #6
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    Default Re: Use 3 or 4" pvc waste line?

    Howdy thanks again for the insightful information.

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