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Thread: Pump or gravity

  1. #1

    Default Pump or gravity

    Hello y'all,
    I am doing a complete remodel of the existing house and a large addition. I am cutting out the old cast pipe in the house and replacing all with PVC. I was on a septic, I have taken that out and want to use the city sewage system. What I am trying to determine is if I should use gravity or get a grinder pump. When the pipe exits out from under the slab, I have to put in a couple of 45s or a 90 and shoot to the road which is about 300' away from the house. Should I put 4" pipe into a tank with a grinder pump to 2" to the road. Or, 4 inch all the way to the road and let gravity do its thing? I am thinking the city sewage is at 5 or 6 feet in depth at the road so the grade should be right on over that distance. Any ideas / opinions are appreciated.

    Thank y'all,
    George

  2. #2
    In the Trades Bob NH's Avatar
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    Default

    Gravity is a much more reliable force than electricity and requires little maintenance. It still works when the power lines are down. If you can meet the grade requirements, gravity is the way to go. Grades can often be flatter for outdoor piping, not the standard 1/4" per foot used inside buildings.

    I would work with 45 degree turns rather than 90s if possible, and don't forget cleanouts. It is possible to turn pipe in a long sweep so in 300 feet you could make quite a change in direction without elbow fittings. There is usually a limit on how many degrees you can turn each joint.

    There are grades of plastic sewer pipe that meet codes and require much less labor than concrete, vitrified clay, or cast iron.

  3. #3

    Default

    If doing gravity, it's obvious that it's critical to have enough slope. What's not obvious is that it's also critical that you not have too much slope. Solids don't 'fall' down the pipe because of sheer gravity (unless the pipe is vertical). Solids are carried down in a 'flume' of water. If the slope is too steep, the water will flow too fast, and leave some solid waste behind. The pipe will clog over time.

    That being said, grinder pumps are not as reliable as gravity. In fact, a gravity 4" drain will pass a lot of things a grinder has trouble with like sanitary napkins.

    Go gravity.

  4. #4

    Default Thanks

    Thanks to you both. I feel better about the gravity way. I was leaning that way from other advice already.

  5. #5
    Moderator & Master Plumber hj's Avatar
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    What's not obvious is that it's also critical that you not have too much slope. Solids don't 'fall' down the pipe because of sheer gravity (unless the pipe is vertical). Solids are carried down in a 'flume' of water. If the slope is too steep, the water will flow too fast, and leave some solid waste behind. The pipe will clog over time.

    Old wive's tale that has been disproved countless times.

  6. #6

    Default

    Then the inspector in my town is an old wife, bkz he's the one who warned me.

  7. #7
    In the Trades Bob NH's Avatar
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    Default Sewer Erosion

    The limitiation on high slope in a sewer is related to erosion caused by high velocity. That can be a problem with sewers carrying large and sustained flows, and especially with concrete pipe. It is not a factor with a drain from an individual residence. Greater slope cleans the pipe more effectively.

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