Double Sink + Disposal Drain Routing

burgess40April 22, 2007

What are the rules of thumb for undersink drain routing from the sinkdrain and disposal to the trap in this configuration?

Left basin has a disposal with DW connected to DW inlet via a high loop (no airgap).

The trap is not directly underneath either sink, necessitating a center-outlet configuration.

I am trying to route around various undersink mounted trashcans, shelves, and such without 1)violating code or 2)causing slow draining or backing up. With enough elbows, an infinite number of paths are possible :^)

This is what I've got so far:

1) slope all horizontal pipes slightly downward

2) use a baffled T, not a Y

3) 1 1/2" pipe at minimum

4) horizontal section coming from (downward) disposal outlet must be below manufacturer specified level on disposal.

5) Minimize bends

6) flow may only be downwards or horizontal (obviously)

While I'd appreciate any input, I've got two primary questions:

Will I run into slow draining if I drain the right sink immediately into an elbow (then run horizontally before descending via another elbow and pipe into the baffled T) versus longer vertical drop first and then horizontal?

Does it matter how the T is oriented? (ie sink drain routed into "top" of T and disposal outlet into side of T versus sink into side and disposal outlet into top). The allowable relative heights of the horizontal segments in a center-outlet configuration are my question: should the sink horizontal outflow be above the disposal horizontal outflow or visa versus or doesn't it matter?


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The Plumbing Codes will permit up to 3 sinks of a common type and size in the same room to be connected to one common P-trap permitting that the maximum horizontal distance from the center of the sink drain opening to the P-trap is not more than 30".

When a disposal is attached to one sink the sink drains are required to be connected by means of a baffled Tee.

Under ordinary circumstances if the "waste arm" from the wall is midway between the sinks we would use a "Center Waste Kit" which has a horizontal baffled tee and a short horizontal line to each sink. With the common center waste kit both of the horizontal lines have a 90-degree bend on the end of the line turning upwards to attach to the sink tailpiece.

When we attempt to connect a disposal with a common "Center Waste Kit" or "End Waste Kit" we run into a problem because the discharge port from the disposal is approximately 3" below the sink drain and the disposal drain port is a horizontal port. If we then attach the manufacturers supplied ¼ bend to the disposal port that will place the waste kit to low to properly connect to the waste arm from the wall.

When connecting a double sink with a disposal you should first select a "Disposal Type Center Waste Kit". The disposal type kit has one horizontal line with the preformed 90-degree bend to attach to the sink opposite the disposal. The cross tube to the disposal is a straight tube with a flange on the end.

You begin by discarding the formed ¼ bend that comes with your disposal unit and attach the flanged cross tube to the disposal using the rubber gasket and steel lock-ring supplied with your disposal. This will give you a cross tube coming straight off the disposal at the same level as the disposal discharge port.

You then connect the baffled tee with the side opening pointing downward, which will then input into the P-trap. From the opposite side of the Tee you attach the horizontal cross tube with the preformed ¼ bend on the end, which will then connect to the sink tailpiece.

To answer your second question, will it matter which way the TeeÂs are oriented? The answer is yes. An "End Waste Kit" has a vertical Tee; which only has one baffle to direct the flow from the side input to the vertical drop position. A Center Waste Kit has a Horizontal Baffled Tee which two baffles, one in each direction to direct the flow from the ends of the Tee to the side outlet. If the TeeÂs are used in the wrong configuration the baffles will cause an obstruction to flow.

    Bookmark   April 23, 2007 at 5:54PM
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This is a question about installation of the garbage disposal on a double offset sink. I'm in the middle of a kitchen remodel.

The left bowl will be the large bowl, with the dishwasher to the left of it. I asked my contractor to install the garbage disposal under the large bowl (because that's where we do the majority of washing and where all the food scraps accumulate).

He insisted that the disposal has to be installed on the small bowl. Otherwise it will both cause loud gurgling noises and cause the disposal to not function properly. This doesn't make any functional sense to have the disposal under the small bowl. But I've only ever had single-bowl sinks so I've never had to think about this before.

Is he right? Is there some configuration of traps and tees that would get around his concern? And even if he is right, what's the actual risk of installing it on the large bowl?

Another consideration - and this one is rather unconventional: I would eventually like to pipe the small bowl drain to a water collection system in the backyard, which is directly outside the kitchen. Basically, collect the water from rinsing produce and other relatively clean washables for reuse in the garden. This may sound kind of way out there and I'm sure it doesn't comply with code, but I do live in California and the specter of drought increases every year. It just seems like such a waste of perfectly good, clean water to have it drain away as wastewater.

I don't expect the contractor to agree to do this last part, but at the very least it would help if he'd install the disposal on the large bowl. Any advice appreciated!

    Bookmark   June 6, 2008 at 12:29AM
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I recently remodelled my kitchen. I have a double sink with a disposal and dishwasher on the right. When I use the disposal the waste comes up in the other sink.
Did my carpenter mess up the plumbing?
Can you please send me a drawing of the configuration of proper plumbing in this instance?

    Bookmark   December 10, 2008 at 4:54PM
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