Two Washing Machines - DWV Design (Double Fixture Fitting vs. Double Sanitary Tee)

[cshorett]

I am working on a double washing machine + utility sink in a Seattle basement. Blue shows DWV lines planned, going from two laundry drain boxes and utility sink to main stack (dark blue) w/ cleanout (green). See image below.

Two questions:
- Should the yellow connection be a double fixture fitting, double sanitary tee, or double wye w/ 45* elbows?
- Is an additional cleanout required in this install, or does the existing cleanout on the stack suffice?

Additional notes:
- Drain is 2" PVC
- Vent is 1.5" PVC (transitioning at yellow)
- Main stack is 4" cast iron

Thanks for any help!

 

[Tuttles Revenge]

The tee between clothes washers must be a fixture cross. 704.2. At minimum you need a cleanout that serves the sink directly. 707.4 exception (1). That cleanout would technically serve the clothes washers too, but I would suggest adding a cleanout to the vent of the clothes washers that more easily cleans the tee and the drain below it directly and easily. Cheap insurance against needing it.

When you cut into the cast stack, you Must support the weight of the stack! Its very important that it not shift. We recently went into a building and found a spot where the stack had been cut and not supported and the stack shifted in the unit above. Don't assume the original installation supported any of it. The connection to the stack can be a san tee or a wye.

Your drawing indicates a vent for the sink, but its drawn incorrectly. The fixture drain / trap arm can not raise vertically after its vent. Instead consider at the end of the horizontal drain, elbow vertically then installing a 2x1.5" san tee. That san tee horizontal branch is the sink trap arm and the vertical is the vent. The vent can not connect back to the stack if its a drain stack from fixtures on the floor above. But most seattle homes would have a 2" vent from the basement for an old floor drain. If not then you can combine the sink and clothes washers vents and use an AAV.

Another thing to consider in Seattle is that you likely need a backwater valve for your new fixtures since they're in the basement and likely under the flood level of the next upstream manhole cover. To determine your need, start with the Seattle Sewer Map.
If the Floor level of the basement is below the manhole cover upstream of your sewer connection, then any fixture on that floor needs to be protected by a backwater valve.

 

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[cshorett]

@Tuttles Revenge - this is exceptionally helpful, thanks for such a detailed reply.

A few follow up notes:
- This is replacing existing laundry plumbing so thankfully already have a 2" san tee in the stack I'll be reusing
- Similarly, you are correct on the vent - I have a 1.5" vent that was already tapped and I plan to reuse. That was poorly shown on my illustration, but it runs vertically next to the main stack.

For the backwater valve, I assume it would be installed between the main stack san tee and the washing machine wye?

 

[Tuttles Revenge]

@Tuttles Revenge - this is exceptionally helpful, thanks for such a detailed reply.

A few follow up notes:
- This is replacing existing laundry plumbing so thankfully already have a 2" san tee in the stack I'll be reusing.

Good, tying into a stack is not the easiest thing. Some are difficult to support due to their weight.

- Similarly, you are correct on the vent - I have a 1.5" vent that was already tapped and I plan to reuse. That was poorly shown on my illustration, but it runs vertically next to the main stack.

Usually they are 2", but 1.5 is fine for your fixtures since likely less than 1/3 of that vent will be horizontal. And likely impractical to upsize it by tracing down its connection upstairs. I guess it depends on the age of your home. Most homes in seattle were built in the 20's and while they're not cookie cutter, they do have a LOT of similarities in construction.

For the backwater valve, I assume it would be installed between the main stack san tee and the washing machine wye?

Yes, I should have put that in the drawing. But it might not be necessary too. It just depends on the elevation of the floor vs that next upstream manhole cover. If anything you can install it but remove the flapper if its not required. Its harder to install it after the fact. It does need to be in the horizontal line since the fitting has a built in drop it eats up about 2ft of grade. It also needs to be accessible since its a mechanical device and should be checked periodically for debris build up.

 

[cshorett]

One final wrinkle on this project - my existing stack connection is ~10" above the floor and the combination of the double fixture fitting + wye / 45* fitting adds another 11", bringing the weir height to 20+" above the floor. Seattle code 804.1 seems to indicate I need to have that trap set between 6" and 18". Any thoughts on how to make the above design work given those constraints?

 

[wwhitney]

Any thoughts on how to make the above design work given those constraints?

If you have sufficient depth (perpendicular to the plane of the elevation in #2), there are lots of options. But if you want everything to be in a single layer in the plane of the elevation with no pipes crossing, that's a bit more difficult.

You could obviously lower the connection at the stack with some trouble.

Or you could join each laundry drain to your 2" horizontal line coming off the stack separately. From left to right, going upstream, you'd have an upright wye (barrel horizontal), a 45 for the drain to go horizontal, an upright combo (barrel horizontal) for the vent takeoff, and then a trap for the standpipe. Repeat that behind each washer. The two laundry vents would need to rise to at least 6" above the flood rims of the standpipes, and then one or both could turn horizontal so they can combine into a common vent that rises up to connect to your existing vent.

Cheers, Wayne

 

[Tuttles Revenge]

I was thinking similarly to what Wayne was proposing.

Bit of a crude rendition.

 

[cshorett]

Very helpful ideas. Challenge I have is there is limited space to push to standpipes to the right. See photo with ideal drain box locations. Dryer box locations make it preferable to bracket drain tie-ins with standpipes on either side...

Assume the trap mockup in photo wouldn't work as it lead to a double trap w/ sink ~10' to the right of the main stack?

 

[wwhitney]

Seems to me you are going to either have to use the third dimension (possibly furring out the wall), use a common trap for both standpipes (not sure if that's legit), cut into the stack to lower the connection, violate the 18" maximum standpipe trap height (which does seem arbitrary, the IPC doesn't have it), or move one of the drain boxes.

Cheers, Wayne

 

[wwhitney]

You could use (going downstream) trap outlet pointed to the right, upright combo for vent-takeoff, street quarter bend, into the top of an upright combo on your ~10"AFF horizontal drain. That puts the trap arm 8-5/16" above the horizontal drain, better than the 11-1/4" stack up from a double fixture fitting above an upright combo. But it sounds like you'd still be above 18" AFF on the trap. Maybe close enough?

How far below the next downstream (?) manhole cover is your backwater valve going to be?

Cheers, Wayne