Laundry drain line, sizing

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After reviewing some posts here I wonder if a recently installed laundry closet that ties into an existing bath tub drain line was done correctly. Wondering if this was done correctly and won't introduce any draining issues. Ugh. I am not that knowledgeable about plumbing but gather the laundry drain line should have been 2"...

The specs used to design the system are:
  1. The washer box is 30" off the floor and has a 1-1/2" diameter, 18" tall stand pipe w/a p-trap at the bottom
  2. The drain line then runs about 4ft horizontally with about 1/4" drop per foot to a vertical drain used by the tub (it is vented)
  3. The tub drain also has a p-trap and is about a foot below the point where the laundry line connects.
  4. At the point where the tub drain heads down into the ground, the pipe is 2" in diameter.
A compact washer (low water usage) will be installed; the line has only been tested using a bucket of water poured into the washer drain (no issues observed but not the best test). Conflicting opinions were offered on the proper diameter of the washer drain line.

Having second thoughts on this. Thanks for any helpful feedback.


The Plumbing Wizard
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Bothell, Washington
The washer drain is 2" with a vented p-trap.
The tub is normally run with 2" and can have a 1-1/2" p-trap that is vented.
The washer drain should come in behind the venting for the tub, or it will siphon the tub p-trap.
Hey, wait a minute.

This is awkward, but...

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