Basement Laundry Tub & Washing Machine question

I've searched online for the past 2 nights and still can't quite figure this out for myself so I'm hoping someone here can.

In my basement I have a laundry sink and washing machine connected with 1 1/2" pipe into a 3" drain. The washing machine ties into the sink's drain before the P-trap. I tried my best to capture it in the attached drawing - the washer sits next to the laundry tub and the dryer between the washer and the 3" drain pipe.

Every time the washing machine empties, it backs up into the sink. I'm not sure why this is happening but its been this way since I moved in. I snaked the drain from the laundry tub to the 3" pipe and it didn't fix it so I was left thinking it was something to do with the way it was plumbed.

I just bought a new laundry tub because the old one is filthy from paint, crud, and 28 years of wear & tear. Instead of just simply replacing the tub and living with the back-up problem I thought I might as well fix the whole thing properly so I don't get the washing machine water backing up into the sink anymore.

I thought that installing a new 2" drain for the washer and tying it into the 3" drain might solve the problem but I wasn't sure how that should be vented.

What's the proper way for this to be hooked up? Is it correct already? Any help/advice would be appreciated.

It's a T into a 1.5" drain and you're washing machine is trying to discharge a ton of water into it... I'm not at all surprised.

Can you not just dump your washing machine into the sink itself? Or do you prefer a separate drain?

Newer washers require a 2" trapped and vented drain. An 1-1/2" will not handle the volume, so it will backup into your sink. Now, you could just drain into the sink. The sink will hold the excess water until it can drain out the sink's drain. Nothing really wrong with doing that other than it precludes the use of the sink while you are washing clothes.

the concept of a standpipe is - I think - that it acts as a temporary tank, a little cylindrical one, holding the washer's discharge for a second or two while it runs through the P trap. Once someone plumbs that pipe to a sink you lose that advantage since the height of the sink floor is way below the level the water would be at in the standpipe.

A 1.5" drain line is the right size under all codes in both US and Canada when there's a laundry tub (as far as I know). A laundry sink may be connected to a 1-1/2" line, but if you pull the sink out and make it just a laundry standpipe the drain line must then be a 2" line -- under most, many but not all codes. I think where you are, Ontario, you may have a 1.5" drain size standpipe, or it may be grandfathered so you wouldn't need to redo the whole drain line.

The first thing to consider is the drain line size allowable and whether you want to be up to code.

Another good thing to know is that a front loading washing machine uses a lot less water and also discharges much less water per discharge too.

hope this helps.

The Uniform Plumbing Code requires 2" drains for sinks (except lavatories and bar sinks) and has
for quite some time (decades).

Run 2" to the 3" stack, then a 2" trap and standpipe for the washer, and 2" until you vent and trap arm for the sink. The sink will have a 1.5" trap.
The vents can be 1.5"

The 1 1/2" horizontal drain is too small. The only reason the washing machine does not overflow the standpipe is because it can overflow into the sink. In addition, connecting the washer to the sink drain above the trap, only made the situation worse, but did prevent the overflow from the standpipe.

Terry said:

Run 2" to the 3" stack, then a 2" trap and standpipe for the washer, and 2" until you vent and trap arm for the sink. The sink will have a 1.5" trap.
The vents can be 1.5"

Click to expand...

Are there guidelines or code that dictates the height that all of these parts can be above the floor?

With the current cleanout and 3x3x1.5" tee on the main stack, another tee is going to bring the 2" drain for the washing machine off the stack at roughly 20" above the floor. Will that leave me enough room to install a trap and new standpipe? (I hope that made sense). Basically, what's the maximum height above the floor that the top of the standpipe can be?

Terry said:

Run 2" to the 3" stack, then a 2" trap and standpipe for the washer, and 2" until you vent and trap arm for the sink. The sink will have a 1.5" trap.
The vents can be 1.5"

Click to expand...

So how's this for a better solution? I attached a diagram.

If I run a new 2" for the washer to the existing 3" that should allow for the washer to drain more efficeintly. What about the venting for that 2" line? Do I have to tie it into the small existing vest or is it allowed to be tied back into the 3"?

There is for the height of the standpipe above the p-trap weir. I believe it is 18"-30" under UPC. Where I am (under IPC), the standpipe can go to 42" tall.

As for everything else, you just need enough height to maintain 1/4" per foot slope.

The laundry tub should be 2" until the vent.

You could combine the washer and the tub into the same 2" line out from the stack, and use a combo up for the washer similar to part of your drawing.

You cant vent to a wet stack. B no
You can revent to the vent. A yes