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Thread: connecting a 2 inch drain to a 1.5" drain. Bad idea?

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    DIY Junior Member bid19123's Avatar
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    Default connecting a 2 inch drain to a 1.5" drain. Bad idea?

    I am in the process of installing a washing machine in my apartment. I followed the info on a number of websites and connected a 2 inch stand pipe for the waste water. Problem is when I come to connect into the main drain I find its only 1.5 inches. I know it is against code in many areas but I cant see any other solution than to use a reducer to connect the larger drain into the smaller drain. The questions I have are:

    1/ How likely is it that the water will come back up the stand pipe and flood the floor?
    2/ How likely is it that the bathroom sink which shares part of the 2 inch drain will have water back up into it?
    3/ Is there any other solution apart from replacing the whole 1.5 inch main drain (don,t want to do this as its too big a job).

    Thanks in advance for any help or comments.

    Regards

    Alan

  2. #2
    Forum Admin, Expert Plumber Terry's Avatar
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    Adding the 2" standpipe may be the best you can do.

    About fifty years ago in the States, washers were run with 1-1/2" lines. It was kind of iffy though.
    Adding the 2" standpipe helps a bit though.

    Anytime you are installing a new line, of course using 2" all the way is better. In some places here, they now require a 3" line after the p-trap for suds control.
    I've seen cases where a 3" line in a crawlspace that wasn't graded properly caused soap suds to come out under a toilet.

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    In the Trades Gary Swart's Avatar
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    What some folks do when they can't enlarge the drain, is to empty the washer into a sink. The sink acts as a holding tank. It's not the best, but might be a solution for you.

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    DIY Junior Member bid19123's Avatar
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    Thanks for your help. I think I will take the see what happens approach and let you know the result!

  5. #5
    In the Trades Gary Swart's Avatar
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    Be sure you use a detergent that does not produce too many suds. The newer detergents don't require as large a dose to do the job and don't create a lot of suds so don't add more when you don't see the suds you may have become accustomed to with the older ones, just follow the label directions. Heavy suds not only don't clean any better, they could cause problems with the small drain size.

  6. #6
    Moderator & Master Plumber hj's Avatar
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    What kind of washer is it? Front loads hold a lot less water than top loads. If you have a drainage problem, you may have to use a 3" or 4" standpipe on the trap to get a bit more "storage" capacity.
    Licensed residential and commercial plumber

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