Whether or not you need an air gap depends on where you live. Some locales require it, some don't.

BTW, if you need an airgap, there are some companies that make a soap dispenser that goes over the airgap.

It also depends on what kind of dishwasher you get. Mine used some kind of high loop whatever. In California, you supposedly have to have an airgap, but
my inspector (Atherton, Ca.) told me to just point out in the installation guide that says the high loop method is sufficient. Of course when he came for the final, he never even turned on the lights, so I may have gotten lucky.

I'm in California and needed either an air gap or a high loop. I went with the air gap because I still have my old DW and didn't have a lot of faith in my GC's son-the-plumber. When I get a new DW, if they can do the high loop, I'll replace my air gap with a soap dispenser. I've always had one, so it really doesn't bother me.

We can use a high loop. The hole through the cabinet to the disposal was actually drilled near the top of the cabinet so when we replaced ours there was no way we could mess it up! It did also come with a tie cord to attach the hose to the cabinet at an appropriate height if needed.

Here's a method many here have used over the years. Install the faucet and the air gap. After inspection, remove the air gap and replace it with your soap dispenser. The need for an air gap has been debated here muchly, given that the code requiring them is seen as antiquated when compared to modern DWs.

My cleanup sink soap dispenser is sleeping under the sink awaiting final inspection. Of course that depends on my no-show electrician finally showing up for the 4-5 minute job that he's been putting off for months and which DH is capable of doing but refuses to do for fear he'll electrocute himself based on watching as a child his reckless father mess with live wires and shock himself. Not that I've thought at all about this at ALL for many, many frustrating weeks. ;-)

I will link an older thread about air gaps. Your subject title is "is it necessary?" Well, air gaps are required by Code in only a handful of states; according to the thread below, California, Texas, Washington, Nevada and maybe a couple others. So the answer to your question would be "no" unless you live in one of those states.

To be clear on a few things:
- The air gap's purpose is to prevent dirty water from your dishwasher from flowing back into the dishwasher to re-dirty the dishes. But it's considered an obsolete method to do so.

- The air gap has been replaced by what's been mentioned in this thread, the "high loop hose" installation on the dishwasher.

- High loop is more effective than the air gap at preventing backwash into the DW, which is why most states don't require the air gap any longer.

- New dishwashers today come with the high loop hose already installed. Is the OP buying a new dishwasher or using an older model?

See the thread linked below which also has a video link to show you what a high loop hose is and how to install it (if your DW doesn't already have it pre-installed).

Here is a link that might be useful: Thread about Air Gaps

I'd say the real reason for the code is not primarily backwash into the DW, but concern about spoiling the water supply for the local community. If your city water loses pressure, then clogs at the homes and draining of dirty water can cause backwash into the clean water supply lines into the home. When water pressure is restored, this contaminated water is delivered to local homes out of the faucet, possibly sickening many people. This is also why backflow preventers are required on some pools and some other connections between clean water supply and generation of non-potable water. Airgaps are more effective than high loops at preventing this very rare, but very serious, problem.

Penn State Extension discussion of air gaps prevention of siphoning non-potable water back to the clean water supply (including by dishwashers).

Posted by akchicago (My Page) on Sun, Jun 24, 12 at 21:57

"To be clear on a few things:
- The air gap's purpose is to prevent dirty water from your dishwasher from flowing back into the dishwasher to re-dirty the dishes. But it's considered an obsolete method to do so."

This is absolutly false.
The airgap rule was instituted to prevent waste water from being introduced into the potable water system via the siphoning effect when water pressure is lost from such occurances as earthquakes. which are a common occurance here on the truly left coast.
The first mfg to succesfuly address this was Miele by installing a discharge side backflow preventer. It worked so well that on many occasions it was a convincing argument to waive the requirement.
They subsequently developed a supply side backflow preventer, effectively isolating the appliance.

Interesting. The question about whether an air gap is necessary has come up on the Kitchens Forum many times over the years. But this thread is the first time I have heard about what Zartemis and Snoonyb posted. I am always glad to learn something new. But it made me have a lot of questions.

Why have 40+ states switched from air gap to high loop requirement?

Under what circumstances would a dishwasher in someone's home contaminate the potable water system, and why wouldn't that issue also be solved by the high loop installation?

I think that any other line blockage in the home (toilet) would be more of a risk than the dishwasher. Also, what is described in the article from Penn State above is also solved with a high loop installation. So is the waste water issue that needs an air gap instead of a high loop hose only for areas where earthquakes occur? Sorry for all the questions, but I am still not clear on why an air gap would work better than a high loop.

no one likes to talk about death and dying.

No one likes to talk about their installation being the one that poisons the community water supply.

What zartemis posted is correct.

I once posted similar information about it, too. After that, the few oldtime members who seemed to be involved in THAT one thread then stopped posting that high loops were perfect.

Zartemis added that it's a rare occurrence. I cannot add more information about the topic of risk assessment. I did post that it is unlikely (duh, of course it's unlikely). No one I know is willing to discuss this infinitesimal risk. There are a few plumbers' forums but they are not statisticians in public health and epidemiology.

I do know (or think I know) that the Miele DW is not a perfect solution to the air gap. For this topic, there are discussions on line (in Gardenweb's Appliance forum, among other forums).

So, air gaps DO serve a purpose. But it is an ongoing debate whether or not they should be mandatory.

It is important to know that your clean drinking water depends on a large number of safeguards that all work. If one fails, then contamination can spread through the water system.

A high loop is protective in most cases. It is very protective against preventing dirty water in the sink from spilling into the dishwasher when the sink drain is clogged. An airgap does the same thing -- they are equally effective at preventing this relatively common problem (i.e. the high loop is not better, it can't be: an airgap IS a high loop plus an opening).

Where it can fail is if the dishwasher is continually trying to drain water against a clog in its drainline. With an airgap, the water has someplace to go (into the sink, and possibly onto the floor it is a small sink and the clog affects it as well. This normally won't foul the clean water supply. Why? You'd have to pump water against water pressure. But if you've also lost city water pressure? Ah, that's the problem.

It's rare because it requires 3 coincidences: loss of city water pressure, clog in the home, and running the dishwasher. But with 100s of thousands of people in a large city, the risk of this combo of events rises.

In a common toilet or sink, the water inlet is above the drain and water free falls through what is, functionally, an airgap -- there is no way for water to siphon back up the clean water inlet (look in your toilet tank while it flushes to see). A dishwasher needs to spray the water, and these sprays come from the bottom of the dishwasher (it may also spray from the top). If a dishwasher only sprayed water from the top, it wouldn't need a high loop or an airgap, one would be there by design, like a toilet or sink.

BTW, many places are now requiring backflow preventers on handheld shower fixtures if they are over a tub -- if the fixture dangles into a full tub there could be a risk of siphoning back up the shower hose. This I think is exceptionally unlikely, but we had to put one in to pass inspection (it was the only thing that kept us from passing on the day of the last scheduled inspection).

My CA city doesn't care if you have a Miele. Still need an air gap.

Didn't someone on this forum put the air gap on the outside of the house? That could I guess be an option in many parts of CA, but the things are so common and expected I wouldn't go to great pains to avoid an air gap.

i would like to see more options for DW drain plumbing. It is true that the amount of information out there is next-to-none. (!) I think that we need to resolve this concern for a clean break against siphoning, AND a few other concerns. One is noise. Noise is not life-threatening it is just a permanent irritant. I have found the drain gurgle noise irritates me, in any kitchen whether open concept or closed behind doors. Especially when I think of the X hundred thousand hours invested in R&D to make DW's quieter and quieter. All that effort for nought, when you hear the drain gurgling. Some DW make a big noise in the drain. Strangely, no DW manufacturer brags about their drain hose making little noise in the sink drain or the GD connection. Doesn't anyone else care about this? It would make sense to me, to have a multipurpose air-gap-to-outside-wall which makes DW drains silent (!) Whatever little gurgle did occur would be audible to those who would go stand outside near the outdoors portion of this big new hose to outside air gap thingie.

The gap to outside was a 'Johnson' tee or johnson gap. When air gets sucked in or out, it is taken from an outside wall so the gurgle noise is outside. Or, so I believe.

Here is a link that might be useful: Buell site describing outside air gap

Hmmmmmmmmm I never hear water gurgling when my dishwasher drains (and it isn't an especially quiet one). Maybe that explains the mysterious lush spot of garden outside my kitchen window. Seriously--ferns, hydrangeas, under a roof so no rain (not that we get much) and nobody ever waters it. Looks like Forks in that tiny spot in my parched yard.

Davidro and Fori--I feel like home! Davidro is talking about the Johnson Tee, "invented" by a Washington state man, and Fori is talking about Forks. Have you ever been there? It's not exactly like the movie, btw. :)

Yep, spent the night there once. Riding my bike, nearly froze.

Every dishwasher already has an air gap on the supply side.

There is simply no way for water from inside a DW to be siphoned into the supply line.

None.

Follow the supply line from the entry valve and you will see an air gap before the water supply enters the side of the DW u well above the water level the DW is capable of holding.

The whole door is NOT water tight.

The only thing an air gap prev4nts is a sink full of dirty water from flowing back past the drain valve, through the DW pump, and back into the DW.

The dishes are above the holding level in the DW, so the water would pour out from the DW door before getting to them.

The reason a few places still require air gaps (instead of a simple high loop) is to sell air gaps and install them.

Every time a claim is made about a residential DW being contaminated by dirty water from a sink through a high loop it never pans out as real.

If you can point to a documented example please post it.

I have nee searching for on for over 20 years.

My city does not sell or install air gaps.

Forks is lovely but for some reason lately it's become full of vampire crap?

Thanks for all the interesting feedback! I just found out from my plumber it is code in my state so I guess I have to go with it. Not the prettiest thing to look at but it was it is:)

It's true that good dishwasher design will have the bottom spray outlets above the flood level of the dishwasher to further limit risk of siphoning (which is another reason I cited this as a rare problem). Of course, in drawer dishwashers, this can't be done (since they are only open from the top and could completely fill with water), so they can't mitigate the risk in this way (though, like door dishwashers they can use flood switches to detect some flooding conditions -- though for them I think the only switch is for when water leaks from the drawer bin into the outer shell. Floats and flood switches in dishwashers also help prevent this risk.).

Sometimes there is little margin for error -- if the inflow of water is greater than the rate it leaks from the door, you still risk siphoning. In this photo, the jets are just barely above the flood line in the DW:

Since the dishes sit in a rack even higher than the spray jets, they will only be at risk if the drain water gushes up , splashes up, or if the water level reaches them -- and if the water level reaches the dishes, it's also high enough to cover the spray jets and be a siphoning risk.

One non-safety advantage of an on counter air gap is that your sink will take overflow (up to its capacity) instead of your kitchen floor in the event of a drain clog.

FWIW, if I were setting the code, I'd consider a high loop sufficient. The alternate risks, though serious, just seem so remote. Not to mention if your city water loses pressure, you should just assume that the initial water when pressure is restored is bad. There are so many non-code installs out there of risky siphoning situations other than DWs (garden irrigation, fountains, fish ponds, pools, etc. Or even with a pool installed to code, if it is being filled from a garden hose dangling in the pool, loss of water pressure could result in siphoning). So it seems like overprotection of one small risk when there are so many others.

I'll also point out that the most likely place for a drain clog is in the sink p-trap, in which case the water will also fill the sink (because it will have entered the sink drain before encountering the clog). I'm not sure why it would be any more likely to have a clog between the air gap and where it enters the sink drain than between the dishwasher and the air gap.

gregincal that question of yours leads to nowhere. It is not that one spot that is the problem in the eventual scenario described further above in this thread.

Our dishwasher is on an outside wall and we have the exterior air gap (a little tube that sticks out of the house a couple of inches).

Even a dish drawer likely has an air gap on the water supply.

You are NOT going to siphon dirty water into supply lines with any appliance listed for use by the ratings agencies.

Even with a fill inlet well above the fill level in a front loading DW, there is STILL an small air gap in the water supply line on the outside of the unit.

It is often just a small plastic piece with water entering at the top, then spraying down past some openings to prevent ANY chance of contamination of the clean water supply.