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  1. #1
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    Default Converting 2" PVC to 1.5" before or after p trap

    I'm installing a shower and can't seem to get the waste pipe right. I have 2 inch PVC attached to the drain and I need to convert it to 1.5 inch to attach it to the waste line. I 1st tried it with about 6 inches of 2" PVC coming out the drain from the shower, then coverted to 1.5" P trap and ran it to the waste line about 1.5 feet of length with a 90d elbow. No leaks, looks great, but won't drain. What am I doing wrong? Do I need to use all 2" PVC and P trap until I reach the 1.5" waste line?

  2. #2
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    Default Re: Converting 2" PVC to 1.5" before or after p trap

    Sounds to me most of the problem is the waste pipe should be 2 inch all the way.

    If you have a sink that will be sharing the same drain then it would be 1 1/2 inch for the sink with an adaptor to tie that into the 2 inch drain.

    The other consideration would be the vent .... do you have air behind water for the shower drain ?


    Just a thought.

  3. #3
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    Thumbs up Re: Converting 2" PVC to 1.5" before or after p trap

    Quote Originally Posted by B Hickey View Post
    I'm installing a shower and can't seem to get the waste pipe right. I have 2 inch PVC attached to the drain and I need to convert it to 1.5 inch to attach it to the waste line. I 1st tried it with about 6 inches of 2" PVC coming out the drain from the shower, then coverted to 1.5" P trap and ran it to the waste line about 1.5 feet of length with a 90d elbow. No leaks, looks great, but won't drain. What am I doing wrong? Do I need to use all 2" PVC and P trap until I reach the 1.5" waste line?
    What do you mean, air behind water? The whole bathroom is vented from the main stacked vent. I never had a problem with the tub/shower that was there before. When I remodled I had to move the drain hole over about 6" to center it for the shower. I wouldn't think that would have effected the venting at all.

  4. #4
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    Default Re: Converting 2" PVC to 1.5" before or after p trap

    Quote Originally Posted by B Hickey View Post
    What do you mean, air behind water? The whole bathroom is vented from the main stacked vent. I never had a problem with the tub/shower that was there before. When I remodled I had to move the drain hole over about 6" to center it for the shower. I wouldn't think that would have effected the venting at all.
    All righty then .... air behind water is the basic principal of "venting".
    So .... this is a retrofit and you have the existing vent in place.

    However .... if the shower is equiped with a 2 inch drain then that has changed the "fixture unit rating" from the existing tub that is being replaced that had a 1 1/2 inch drain which would have had approximately 7 1/2 gallons per minute to over 10 gallons per minute with the 2 inch.

    In other words the the 1 1/2 inch main drain is now too small and should be replaced with the 2 inch to accomodate the shower flow capacity.

  5. #5
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    Default Re: Converting 2" PVC to 1.5" before or after p trap

    Two things I see that may or may not make a difference. First is the 90* elbow. Is this a regular 90 or is it a sweep 90? The difference is a regular 90 is a square corner which requires the water to stop and change direction, slowing the flow. The sweep 90 is a long round corner, water will change direction without having to stop. All changes in direction should utilize sweep elbows, including the T into the main waste line. The second thing is the location of the drain size reduction. To do it mid way between the pan and the main waste line will cause turbulence, slowing the rate of flow. Doing the reduction at the main waste will minimize turbulence and increase flow.
    I suffer from CDO ... Its like OCD, but in alphabetical order, LIKE IT SHOULD BE!!!

  6. #6
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    Default Re: Converting 2" PVC to 1.5" before or after p trap

    Good point Sprucey regarding the 90 .... but I also don't think it's a good idea to reduce down in size of drain piping.

  7. #7
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    Default Re: Converting 2" PVC to 1.5" before or after p trap

    Quote Originally Posted by canuk View Post
    Good point Sprucey regarding the 90 .... but I also don't think it's a good idea to reduce down in size of drain piping.
    Agreed, however if working with what you've got ...

    If this is work is permitted (i.e. sanctioned by the local governing body and paid for ), then it is likely that the waste main will require upgrading to 2" all the way out to the 4" main that exits the house, eliminating the need for the reduction and probably the slow drain issue.
    I suffer from CDO ... Its like OCD, but in alphabetical order, LIKE IT SHOULD BE!!!

  8. #8
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    Default Re: Converting 2" PVC to 1.5" before or after p trap

    What U guys are saying makes a lot of sense, I'm not sure I like the idea of converting the whole waste line to 2", it is about 12 ft of pipe with sink and upstairs bath attached, not to mention the T fitting on the 4" main waste would have to be replaced.
    I am going to give it a shot with 2" PVC, P trap, and sweep elbow. With my fingers crossed. Thanks

  9. #9
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    Default Re: Converting 2" PVC to 1.5" before or after p trap

    Let us know how things go.
    I suffer from CDO ... Its like OCD, but in alphabetical order, LIKE IT SHOULD BE!!!

  10. #10
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    Default Re: Converting 2" PVC to 1.5" before or after p trap

    Quote Originally Posted by canuk View Post
    Good point Sprucey regarding the 90 .... but I also don't think it's a good idea to reduce down in size of drain piping.
    Choking down a pipe size is considered a code violation, correct?

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