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Old 03-20-2006, 11:54 AM
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Is an air-gap required with a disposal?

Hi folks,

Thanks to some electrical help I received here, I installed a new In Sink Erator garbage disposer in our kitchen. Iím left with the question of whether or not to install an air gap in the dishwasher drain line. I called both Maytag and In Sink Erator, but the advice I received was conflicting, vague, and sounded somewhat off the cuff. So perhaps someone can help shed some light?

Hereís the conundrum: The dishwasher was installed when the house was built (2002). Weíre assuming (!) that the installation was done properly. I downloaded the installation instructions, and found two relevant passages:

Quote:
Originally posted by Maytag
5. Drain Hose Preparation
Dishwasher drain must have a 32Ē loop ... If a loop cannot be put in the installation, an air gap may be used.

9. Drain Hose Attachment
If draining into the disposer, check to make sure disposal plug has been removed.

OK, so they mention an air gap, but suggest it only if a loop is not possible. Since weíve been here (~4 months) we have yet to have any sort of problem with the dishwasher draining, so we feel comfortable assuming that there currently is a loop in the drain hose. Furthermore, when it directly addresses draining into a disposer, it does not say that an air gap is necessary.

This question actually arose from reading the disposalís instructions. They say:
Quote:
Originally posted by In Sink Erator
12. Connect Dishwasher Drain
Connect the dishwasher to the disposer through an air gap. Most dishwasher manufactures recommend that the discharge water runs through an air gap to prevent backflow and/or dishwasher contamination.
So... is an air gapís function sufficiently different from a loop such that it is necessary to install one when adding a disposal to an existing dishwasher? Or if the loop is already there, would adding an air gap be redundant? Again, I received conflicting answers from the two companies, so they were not much help (at least as far as this issue is concerned).


Thanks for any help/insight you can offer!

Rhythm
Old 03-20-2006, 12:25 PM
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Posts: 12,684
The air gap must be in the dishwasher drain line. This prevents a direct connection from the dishwasher to the sewer. In my houses, which meet code, the garbage disposal goes directly to the waste line.
Old 03-20-2006, 12:52 PM
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Location: San Francisco area
Posts: 16,107
If you have the space (eg: hole in the sink deck) to mount the airgap, I strongly recommend you put one in.

If your sink drain backs up, the gap will prevent the dishwasher from resembling a septic tank and filling up with sewage. Ive seen dishwashers fill up with sewage / drain water so badly that the muck poured out onto the floor when someone opened the door in a situation where the drain backed up and the sink was filled almost completely with water.

Also, IIRC, airgaps are called for in the uniform building code, which is a national reference - not all areas adhere fully to it, and some will go above and beyond UBC in their requirements, but it's a good starting point.

For the definitive answer, check with your local building/plumbing inspector's office.
Old 03-20-2006, 12:53 PM
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Location: Shakedown Street
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Quote:
Originally Posted by David Simmons
The air gap must be in the dishwasher drain line. This prevents a direct connection from the dishwasher to the sewer. In my houses, which meet code, the garbage disposal goes directly to the waste line.

Right. The dishwasher used to drain directly to the sinkís waste pipe (presumably after going through a loop). Now it goes directly into the disposerís dishwasher inlet, drains into the disposer, which in turn drains down the sinkís waste pipe. Iím wondering if the loop is enough, or if thereís something special about a disposer and I now have to install an air gap.
Old 03-20-2006, 03:11 PM
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Join Date: Aug 1999
Posts: 16,451
I just want to add that if you do go with the airgap, be aware that not all air gaps are created equal.
When I redid my kitchen 10 years ago I bought an airgap from the local orange big box home store. At the same time, I converted my portable dishwasher to built in (using the factory kit) Anyway my older dishwasher started having drain issues. I chalked it up to my older dishwasher and didn't give it much thought.
Fast forward 6 months. The white paint is peeling off the air gap. I will be damned if I go though this again in 6 months so I go to a real plumbing supply store to buy a replacement.. After the installation of the new air gap, all my drain issues disappeared.
As in many things YMMV.
Old 03-20-2006, 07:24 PM
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Location: Las Cruces
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Quote:
Originally Posted by Rhythmdvl
Right. The dishwasher used to drain directly to the sinkís waste pipe (presumably after going through a loop). Now it goes directly into the disposerís dishwasher inlet, drains into the disposer, which in turn drains down the sinkís waste pipe. Iím wondering if the loop is enough, or if thereís something special about a disposer and I now have to install an air gap.
There is nothing special about the disposal that creates the requirement for the air gap. Straight into the waste pipe (as you had it before) or into the disposal, the drain hose from the dishwasher is connected directly to the sewer pipe. Think about that for a minute.

What the air gap does is prevent backflow from the sewer pipe into the dishwasher via siphoning action. If for any reason the drain pump fails in the dishwasher and the water in the hose starts to run back, the air gap will break the siphon. A loop CANNOT do this, and it will allow waste from the sewer pipe to be drawn back into the dishwasher.

In order to work properly, the air gap must be positioned higher than the flood rim of the sink that is served by the drain pipe you are using. That requires a hole in the sink. The loop is easier to install since it doesn't require the hole, but it's not equal to an air gap.

Install an air gap.
Old 03-20-2006, 07:32 PM
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Quote:
Originally Posted by Dag Otto
Straight into the waste pipe (as you had it before) or into the disposal, the drain hose from the dishwasher is connected directly to the sewer pipe.


I mean that the sewer pipe and the disposal are the same in the context of a cross connection. In plumbing terms, a cross connection is a connection between a potable water supply and a drain line. Such connections are a health hazard. There are many ways this can happen, and differing means of controlling it. An air gap is a simple method and is adequate for the dishwasher drain.
Old 03-20-2006, 08:28 PM
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If/when/while the drain is blocked by the load of garbage placed into the disposer, the impeller becomes essentially a pump that pumps garbage laden water into your dishwasher. The air gap prevents this. If you, your so, your kids, your stupid cousin you had house sit, etc. never feed anything or any more into the disposer than it can instantly maserate, then maybe you can get by without one.
Old 03-21-2006, 10:01 AM
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Join Date: Oct 1999
Location: Shakedown Street
Posts: 12,950
Cheating?

The sink is a composite material, so cutting a hole for an air gap will be problematic. * The dishwasher is flush with the left side of the sink, so there isnít counter room there, and running it to the right side of the sink has its own issues. Iíll figure something out if I have to, but...

I encountered a similar issue when I set up a drip irrigation system for our gardensóthe possibility of water being siphoned back into the main lines. This required the installation of a check valve between the filter and the valve manifold.

Would this work on a drain line? Or would there be some problems such as the relative lack of pressure or the possibility of clogging?

Rhythm





*I feel like I should put in a plug for American StandardóIím a sucker for great customer service. The rep said cutting into one of their composite sinks was not recommended, but since they know people will do so anyway, she told me the best way of going about it. Thatís good service in my book.
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