View Full Version : Dishwasher Installations - Air Gaps

05-31-2011, 11:25 AM
I understand a dishwasher air gap, on the drain side, protects unsanitary water from getting back into the dishwasher and contaminating dishes.

My dishwasher is not next to the sink. It is in an island. It's about 12 years old and I will assume it has a high loop or built-in air gap to protect the water supply line side of things.

Could someone please explain what happens when the drain air gap needs to kick in? This happens when there's restricted flow or a clog in the drain line?

I'm concerned about where the drain water that it stops from backing into the dishwasher goes. Into the sink when directly attached? Onto the floor in my case if an air gap were to be installed? Without an air gap it would just back up into the dishwasher if there were a problem?



05-31-2011, 01:55 PM
First, the anatomy of an air gap is that the waste does not have a continuous, enclosed path - there is a gap it must fall through, so if the line gets clogged, that gap keeps the drain from the DW from getting the waste pushed back into it. Typically, the air gap is installed through a hole in the countertop. This means that if it does back up, it would go out onto the counter, and potentially the floor. Sometimes, it is mounted on the sink rim, so it tends to go into the sink, but if the drain is clogged, that could fill up. But, consider if there is a clog, it's likely the sink would fill up and then flow onto the floor anyway. There are other ways to install an air gap, but the one shown in the diagram Terry added is the most common for a home.

05-31-2011, 02:53 PM
It has nothing to do with the water supply lines. It is to prevent a siphon and ensure that you are immediately aware if the drain ever backs up. Without, it is possible that the drain and sewer could back up into your dishwasher and you might never be aware of it. It is to prevent a situation where you could be eating off dishes that were unknowingly soaking in sewer water the day before.

Some dishwasher installations drain into a standpipe, which if done properly, would also act as an air gap. This type of installation might or might not be allowed where you like.

05-31-2011, 03:58 PM
Thanks. I don't think I've ever seen them installed around here. I guess, then, a water alarm is needed on the counter in order to catch things in time, as wherever the d/w is located it doesn't seem to really make a difference. Would there be any advantage to moving it back to the sink? It works fine for me where it is, functionally, as far as the dishes go.

I think I'd rather the d/w itself fill up and just shut down on a backflow. Are floodstops any good for this situation? In speaking with a plumber about the washer, he said the floodstops can cause strange things to happen in the house so he doesn't use them.

So, without a gap, if the dishwasher gets contaminated water in it, it eventually drains out so you don't know that this has happened?

05-31-2011, 04:56 PM
I missed your post, Cacher. Thanks.

ensure that you are immediately aware if the drain ever backs up.

One is immediately aware how? Spewing on the counter? Sink full of water? Flooding on the floor? Is there an alarm? I never stay around the kitchen when the d/w or other appliances are running. This is what worries me. How this works from a user's point of view, when something is going wrong. If it's not going to fill up the d/w, it will dump into the living space. It doesn't sound like it shuts off the water or sounds an alarm in the emergency. It is so not fun to walk downstairs and find a flood in the living area, moreless sewage. So much damage.

I've never had the d/w overflow or backflow to my knowledge, but maybe this is where stinky dishes come from and we just don't know it? Haven't gotten sick. A relative's dishes smell sometimes too. Though they appear to be clean, no junk around.

Some dishwasher installations drain into a standpipe, which if done properly, would also act as an air gap. This type of installation might or might not be allowed where you like.

Very interesting. It currently drains into a standpipe (apparently not done properly though). I guess I don't understand why rising, backflowing water wouldn't enter and travel through whatever open tubing is in the vicinity. Unless there is some sort of a shutter door on the end for one-way water transmission out. Though I understand how a continuous tube, when the drain water changes direction, would have more force and just dump it all back in.

Would you not consider putting your d/w anywhere but next to the kitchen sink for plumbing reasons like this (other than convenience of install)? Dish-wise, I'm fine with it on the other wall.

Maybe handwashing is for me! ;)

Thanks for trying to help me figure out what I need to do with the d/w setup when the cabinets are replaced!

05-31-2011, 04:57 PM
It takes a special set of circumstances to get a backup into the DW. Doesn't happen often, but it could, and you may never know. Some places require an air gap. Some don't. It is the safest way to install it. All commercial installations use an air gap to protect the people that may eat there from (at least that) manner of contamination. The DW has a pump, and when that is running, it would take a pretty complete blockage to prevent it from spewing water out whatever points it can get to. The goal is to never get any of that, or other dirty water be able to get back into the DW. The air gap does that. A high loop doesn't.

05-31-2011, 05:01 PM
A dishwasher MAY have an integral air gap in the drain, although I have NEVER seen one and would not depend on it if it did, because ANY problem with the connection to the drain, such as a dip, would cause it to overflow. Normally, the drain line comes DIRECTLY off the pump, and YOU HAVE TO supply the air gap/backflow prevention. They DO HAVE air gaps on the water feed, however.

05-31-2011, 05:31 PM
This is what I read last night, which I thought was a good description of the two-fold contamination issues, with the water lines and the drain, and how each are addressed with separate air gaps and loops.

12-02-2007, 09:16 PM
Andrew Buckwell

Re: air gap in newer dishwashers

The issue as I see it is that the dishwasher cannot drain if the garbage disposal dishwasher intake pipe becomes blocked without an air gap device. This is from Mike Casey:

"All dishwashers built during approximately the last 20 years have a built-in water supply air gap to prevent cross-connection and possible backflow to the water supply. If you look at a dishwasher out of the box prior to installation, it is the hose looped above and into a funnel-like device on the side of the dishwasher. This is the built-in supply air gap.

Most dishwashers do not have built-in backflow prevention from the drain hose side. This is where the drain air gap device functions; to prevent contamination of the inside of the dishwasher, not the water supply. This usually would occur if there was a kitchen sink drain backup. The UPC (western code) requires that an air gap device be installed to prevent backflow into the dishwasher (see illustration). This device prevents backflow by keeping the connection above the fixture flood level rim and having an opening to atmosphere (prevents a siphon). Most IRC and other codes allow the high loop method (fasten drain hose high at underside of countertop). This installation will work in most cases unless a siphon occurs.

(see previous picture of air gap device)

As you can see, the countertop air gap device for the drain prevents only contamination of the dishwasher, a disgusting event, rather than contamination of water supply, a potentially hazardous event".

Mike Casey
Kaplan Professional Schools

05-31-2011, 05:40 PM
Why do I read so many people having trouble with these things? Spewing with no apparent drain problem. There seems to be such controversy over them. It's to Code, so there shouldn't be any question what to do, but honestly, it sounds like they create a mess and aren't enforced across the board. I just don't want water damage as a result of installing one.

Curious, do you people babysit your water appliances as they run in the event there's a problem? Most people I know, set something to run and go about their business elsewhere in the house.

05-31-2011, 06:31 PM
Installed on the counter, if it gets clogged or the drain gets clogged, the pumped water has to go somewhere. If you can mount it on the sink, there's a chance it would run into the sink, but if the drain is plugged, the sink may not be big enough to contain it all. Now, they don't fail very often. While I run mine on its timer function at night, it's best to only run any appliance like that when you're around.

06-01-2011, 06:58 AM
I missed your post, Cacher. Thanks.

ensure that you are immediately aware if the drain ever backs up.

.... not fun to walk downstairs and find a flood in the living area, moreless sewage. So much damage.

Some dishwasher installations drain into a standpipe, which if done properly, would also act as an air gap. This type of installation might or might not be allowed where you like.

Imagine a city sewer backup.
Just to get you to think of something.
There is no way to prevent property damage if drains back up.

Imagine your street sewer being stoppered, and neighbors only notice it when black water (fecal matter) begins showing up in low points in their house. Imagine that this lasts for a few hours until emergency crews open it up.

Imagine your particular connection to the city sewer being stoppered, so that nobody spots it except you. Again, it's feces coming back up the drain pipes. There is no way to prevent property damage if your drain blocks, and water backs up.

Imagine some part of your drain piping is stoppered, and it blocks the kitchen drain, and only the kitchen drain. All of the kitchen. What happens? Gray water will come back out the LOWEST point, which would be a "high looped" hose if you had a high loop under the countertop. The dishwasher installation known as the high loop Is Therefore the Weak Link in the Chain, in terms of handling health concerns (and disaster prevention concerns).

Imagine this setup allows your dishwasher to become contaminated with E. Coli. and other killers.
On top of that, imagine you hand wash a few things and let them dry in the dishwasher.
Or that you simply decide to take an old dish out of the dishwasher.
People do this all the time.
This has already happened before, in other places. It is a possibility.
It has also happened that people came home from an absence, used the dishes in the dishwasher, and died because of contamination which they did not see.

So, seeing a problem is better than not seeing it.

The advantage of an air gap is that you get to see the damage, instead of "Not Seeing" it.

If the worse came to pass, what might happen?
(Answer: you eat E. Coli, salmonella, or whatever other exotic killer is out there.)
This has already happened before, in other places.
It is a possibility.

All the other problems that you would have are still just as bad:
water damage somewhere in the house.
But you are still alive and healthy.
So, the air gap or standpipe is a means to keep you healthy.

It is inevitable that there will be damage to the house, when drains block and spew contaminated water back into the house.
Nothing can prevent it.
Think long and hard about this and you will stop asking most of your questions.
You've asked too many questions to answer them individually.

Does this help you?
I hope it helps.


(now, don't worry and don't overthink this.)

06-01-2011, 07:32 PM
Thanks Genie, for the bottom line. I think I like John Lennon's version better ; )

Yes, I am a person of many questions, like to understand things and their pros and cons. Simply my nature, and an occupational hazard. Didn't seem like that many. But hey. Cutting to the chase with the bottom line is always the best answer!

This evening I was discussing this with a family member. 85, says he doesn't want anything to do with one, fine as is, period. And I was telling him, for how sick and nauseous I've been for the past few weeks over this flood, I'd rather get sick with the dirty dishes and be done with it. Did not know it could be fatal though!

I would think one would see the the dishes were not clean if stuff from the drain had gotten to them. Or the d/w would smell from residue. But who knows. Apparently not always. Code is there for a reason.

I see your point, though, it might not be obvious on the plates, but the mess on the floor is. Not a very user friendly system! I was hoping for something a bit more refined ; )

Thanks for your help, everyone, in trying to understand what this thing is and how it works from a user standpoint. I am interested to hear what the township building inspector and local plumbers say about it, as I don't think I've ever seen one around here.

Back to your regular programming, as it doesn't seem like this is it!