View Full Version : Advice for disposal install..

12-28-2008, 10:28 AM
I am installing a new disposal under my double kitchen sink. I have a dishwasher but no disposal previously therefore there is no existing infrastructure for the new disposal. For what it's worth the disposal is an Insinkerator Evolution Septic Assist.

Below are a couple of pictures of my existing plumbing under the sink. You can see the dishwasher outlet coming from the right in to a P-trap, the drain sticking up from the near center of the bottom of the cabinet with a vent cap on the top, and another P-trap below the left sink drain pipe.

I have seen Terry's drawing showing a good disposal plumbing install but I have a couple of questions:

1. How should I modify things with the drain being in the center (or will I need to modify them at all?)? Can I just install the disposal on the left side? I will probably have to lower the horizontal pipe between the two sinks a little in order for it to fit in to the disposer (not sure though).

2. Can I purchase a longer dishwasher discharge hose and plug it in to the disposal? Do I need to use an airgap as well (apparently one was not installed originally). Once I hook up the dishwasher to the disposal can I just cut off the other P-trap for the dishwasher and cap it off?

Thanks for any advice / suggestions you can offer!

12-28-2008, 12:02 PM
Ayy, Chihuahua!

You should cut off near the floor ( right below the first Tee) and start over.

That dishwasher connection is totally illegal and dangerous. Even if your code ( which apparently is quite lax allowing the studor vent) allows no air gap, you do not have an adquate HiLoop.

Plan on running the dw drain line into a HiLoop and then into the side of the disposer.

Best way to do this is get a large basketfull of fittings...baffle tee, trap, couplings, 1/8 and 1/16 bend, etc. Put the disposer on and work upfrom the floor fitting things in as you go. Take all the unused fittings back to bib blue or big orange.

12-28-2008, 12:14 PM
I suppose you are located somewhere where that studor vent/AAV is kosher. I imagine this installation is in an island, is it?

(1) I think so. If you want minimal fuss, you will find that the tee under the righthand sink could be 3/4 inch higher, or perhaps up to 3 inches lower without changing any of the rest of the PVC, except maybe a new tailpiece for the RH sink. You should be able to get this to the right level to meet the disposal outlet, if you mount the disposal under the LH sink.

(2) The DW drain is wrong as-is. Although it may not be required where you are, virtually everyone on this board will insist that you install an air gap for your dishwasher. The line from your air gap could run into the disposer. Make sure you remember to remove the knock-out.

If you start cutting into the PVC, then there are problems here that aught to be addressed. The studor vent should be level, which yours is not, and it could be a bit higher in this space. Capping the existing DW trap arm might be allowed (I don't know) but replacing the whole vertical section would let you eliminate that trap arm entirely, get the AAV level and raise it up a little higher.

Personally I'm a complete hack, so if it were me, I might leave these items for another day. But that's not advice.

12-28-2008, 12:31 PM
I would start out with using one of these...


12-28-2008, 01:59 PM
Absolutely as the others have pointed out, cut at the base of the first Tee at the bottom of the cabinet. Start fresh from there.

The Studor AAV is allowed under the IPC / IRC if installed properly. Your whole setup there needs to go and your plumbing will thank you when it is done right.

One item of advice on a related subject: Find out what circuit the dishwasher is on and see what the total load is on that circuit before you just plug in the disposal. You may be running a new line depending on the size of the disposal and the existing load on that circuit.

12-28-2008, 02:23 PM
Just a comment. On my disposal the plumber who did the work said not to run the dishwasher drain to the disposal as it eats them alive. I am at 24 years on that disposal. He connected the dishwasher drain to the left side of a two bowl sink with the disposal on the right. This seems like a simpler setup. I guess you get the air gap for the dishwasher from the open drain, but I guess you could muck this up by closing the drain. But we have never closed the left side of the sink as it is used for draining dishes. Maybe this is not to spec now, but it was back then.


12-28-2008, 05:29 PM
How does this happen┐ Do gremlins sneak in at nite and wreak havoc under sinks┐ Or is this a tenant and they are modifying their setup┐ Seriously, does one pay for this and say, "good job!" Enquiring minds want to know. :confused:

12-28-2008, 08:55 PM
He might be referring to the nipple on the disposer, where the DW drain is connected. That nipple does tend to be the first thing to "rot away" on a G/D.

12-28-2008, 08:57 PM
Not on the new Insinkerators!
Plastic! We'll all die waiting for that to rot off.

12-28-2008, 08:59 PM
I've seen plenty of plastic ones rot out. Unless you are talking about some new space-age polymer.

12-28-2008, 09:04 PM
Maybe the old blue gray die cast ones.... But then again that was corroding metal...
Please tell me how plastic disposer nipples rot off...
I require enlightenment!:cool:

12-28-2008, 09:13 PM
Maybe the old blue gray die cast ones.... But then again that was corroding metal... Please tell me how plastic disposer nipples rot off...I require enlightenment!:cool:

Abrasive soaps, bacteria...


12-28-2008, 09:22 PM
I fail to see the enlightenment!

12-29-2008, 03:36 AM
Have not run into corroded plastic stubs just those "ole blue" insinkerators.

12-29-2008, 05:35 AM
That is the factory end of the hose, so unless the dishwasher is not next to the sink there is plenty of hose somewhere that you can pull out and connect to the disposer. Terminate that dishwasher drain trap. As far as the height of the tee, that will become apparent as soon as you install the disposer. If it has to be lowered that is a simple thing.

12-29-2008, 06:20 AM
I was sitting here trying to remember just how many "rotted out" disposal nipples I have seen over the years. I know I may be going a bit senile but I'm pretty sure the number is 1. I don't think I'd worry too much about that particular aspect of the installation. More to the point would be that since the drain is all but centered under the sink waste piping, when the disposer is installed things are going to get very very tight indeed. I suspect avoiding the inadvertant S trap may be all but impossible.

12-29-2008, 07:06 AM
I have seen the nipples corroded...all the time, but I don't remember one being the cause of a leak. The pot metal upper housing is thinner metal than the nipple, and of course we know that they corroded through.

12-29-2008, 07:22 AM
I believe Verdeboys reference was to plastic corroding...:D

12-29-2008, 07:40 AM
Well here in Illinois you are not allowed to hook up a dishwasher discharge to a disposal. As well the disposal needs to have its own trap separate from the other side of the sink.

Section 890.710 Food Waste Disposal Units

Installation. Food waste disposal units shall be trapped separately from any other fixture or compartment, shall be connected directly to the sanitary drainage system, and shall be properly vented. Dishwashers shall not discharge into food waste disposal units. Units may have either automatic or hand-operated water supply control. (See Section 890.1130(a), (b) and (c).)

Commercial-Type Grinders. Commercial-type food grinders shall be provided with a waste line at least 2 inches in diameter. (See Appendix F: Illustration D.)

Appendix F: Illustration D.