The Purpose of an Air Gap on a Dishwasher

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Many American kitchens have a small stainless steel or chrome cylinder installed on one side of the kitchen faucet. Many people don't know anything about this kitchen sink fixture other than the fact that it has something to do with their dishwasher. This is known as a dishwasher air gap, and it is critical in keeping the water in your dishwasher sanitary so your dishes come out clean.

What Is a Dishwasher Air Gap?

From the outside, the dishwasher air gap seems to be a 2-inch-tall metal cylinder on one side of the kitchen sink, but the internal plumbing of the air gap is the important part since it prevents contaminated water from draining back into your dishwasher due to a stopped-up sink, a plugged tube or a clogged garbage disposal. This unwanted reversal of water flow is known as "backflow."

As long as the air gap works, backflow is prevented, and you will never wash a load of dishes only to have them come out covered in rotting food particles. The air gap also ensures that the potable water supply never intersects with your contaminated water on the rare occasion that there is some negative pressure in the water line.

While there are other methods to do this, the dishwasher air gap is the most effective, which is why so many plumbing codes require that every new dishwasher be installed with an air gap. More and more cities and states add these codes every year, so homeowners are often surprised to find that the plumber they hired to install a new dishwasher won't install the appliance without first adding a kitchen air gap to the sink. Although it may be possible to bypass these codes by doing a DIY install, it's generally best to use an air gap because they are the only 100-percent effective backflow prevention method.

How Does an Air Gap Work?

A dishwasher air gap is made from two hoses that have a gap of air between them, ensuring that cross-contamination is impossible. A pressurized dishwasher hose brings up dirty water from the dishwasher into the air gap. The other hose brings the water down into the garbage disposal (or, if no garbage disposal is installed, directly into the tailpiece under the sink drain). The water flows through the dishwasher drain hose and into the air gap and the tube before flowing down into the garbage disposal or tailpiece.

The fresh air in the air gap ensures there is no way to cause negative pressure in the dishwasher drain hose that would suction dirty water back into the dishwasher. Instead, if the garbage disposal, sink drain or air gap drain line is plugged up, the water will flow out of the air gap and into the sink through the hole in the decorative head cover. If you notice water is flowing out of your cover and into the sink, then it means the drainage tubes are blocked and need to be cleaned.

It's worth mentioning that some air gaps actually have two inlets. This is because they either connect a dishwasher with two drain lines (one for each compartment of the machine) or because they connect both a dishwasher drain line and a wastewater line from another appliance, such as a water softener or reverse osmosis water filtration system.

Image Credit: Stephen Paul for Hunker

Installing a Dishwasher Air Gap

While many people hire a plumber to install a dishwasher air gap, it's a simple enough DIY project for most homeowners. The first step is to locate the air gap hole in your counter (just to the side of the faucet right above your sink if you have an undermounted sink) or in your sink.

Many sinks have a predrilled hole in the sink deck precisely for this purpose. These are fitted with a flat, disc-like cover if there is not already an air gap in place. If you have a hole like this already, you just need to remove the disc or existing air gap to install a new air gap. If you don't have a hole already, you'll have to use an electric drill and hole saw to cut a 1 3/8-inch hole.

Once you have a hole ready, it's time to connect the hoses. The dishwasher should come equipped with a 5/8-inch hose, and garbage disposals are sold with a 7/8-inch hose. If you are using a previously owned dishwasher or garbage disposal or if you are connecting directly to the sink drain, you'll need to buy hose from a home improvement or plumbing store. The hose can be cut to size with a hose cutter if necessary, leaving a little slack in the line.

Connect the 5/8-inch dishwasher drain hose to the 5/8-inch leg of the air gap using a stainless steel hose clamp. Then, connect the larger, 7/8-inch leg of the air gap to either your garbage disposal or a Y-branch tailpiece between your sink drain and the drain pipe using a length of 7/8-inch tubing. Use a hose clamp to secure the tubing on each end.

While it's possible to install an air gap without a garbage disposal, it is always preferable to connect the dishwasher to a garbage disposal when possible. That's because this allows larger pieces of debris to go into the garbage disposal, where they will be ground up when the garbage disposal is turned on.

Push the air gap through the hole on your sink or counter and then tighten it by threading the nut along the threads on the air gap. Put the metallic cover on the top of the air gap. Run the dishwasher on the rinse cycle to test the tubing for leaks, and if you see any, stop the dishwasher and resecure the hoses where leaks occur.

Troubleshooting Common Air Gap Problems

Water leaks can either be fixed by replacing the hoses or resecuring them. Other than that, the majority of problems with air gaps occur when water flows out of the air gap and directly into the sink. This is usually a sign that there is a blockage in the lines. For most blocks, you can remove the air gap cover and use a long-handled bottle brush to clear out any food particles or other things that could be clogging your drain hose.

If you only recently installed either a dishwasher or garbage disposal, American Home Shield says it's possible that the person who did the installation forgot to remove the metal or plastic knockout plug in the garbage disposal. To fix this, simply remove the hose between the garbage disposal and air gap and then remove the plug with a hammer and screwdriver. Reconnect the hose, unplug the disposal (or shut off the breaker in your home's breaker box) and use your hand to remove the knockout plug from inside the garbage disposal.

The High Loop Alternative

Not all homes with dishwashers have air gaps. Some plumbing codes only require something known as a high loop, which is simply a high loop fitted under the sink with a bracket. Just like in an air gap, gravity causes the water to flow into the garbage disposal. Fresh Water Systems says that the peak of high loops must be installed at least 32 inches above the kitchen floor in order to create enough of a slope to prevent dirty water from re-entering your dishwasher. In theory, this should be high enough that if the drain pipe backs up, it should flow into the garbage disposal and/or the sink drain.

However, if pressure drastically drops on the dishwasher side of the line, this can cause a suction effect, and dirty water can get sucked back into the dishwasher. Air gaps are still preferable for this reason, as they are the only way to guarantee that when the drain line becomes plugged, the contaminated water will flow into the sink rather than back into the dishwasher drainage line.

It's worth noting that while some people install high loops because they think air gaps are unsightly, you can buy disguised air gap fixtures, like soap dispensers that also house the dishwasher air gap. This is a preferable alternative to using a high loop method because it still guarantees that no wastewater will backflow into the dishwasher.

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Jill Harness is a blogger with experience covering architecture, design and decor trends from around the globe. As she lives in what would politely be called a "fixer upper," she is particularly interested in writing about DIY projects and repairs. Most of her home design writing can be found at www.homesandhues.com. You can find out more about Jill's experience and learn how to contact her through her website, www.jillharness.com.

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