How Dishwasher Air Gaps Work & Why You Need One

What is a Dishwasher/Sink Air Gap?

If you are installing a dishwasher or already have one but without an air gap, you need to install one. Not only is a dishwasher air gap required by code, it is the most effective way of preventing backflow and contamination of your dishwasher when the drain line clogs.

Before air gaps became common, dishwashers were plumbed by looping the drain hose high underneath the countertop. That method has little success in preventing wastewater backflow to the dishwasher. More on this later on in this post.

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If water from your sink is going into the dishwasher, you have a clogged sink drain line or garbage disposal. To fix this problem, you will need to unclog the sink drain and install an air gap to prevent the same from happening in future.

But what is a dishwasher air gap and how does it work? Do you even need one?

A sink/dishwasher air gap is a fitting installed between the dishwasher drain hose and the sink drain line/garbage disposal to prevent backflow, contamination and an overflowing dishwasher. The top of the air gap has a finished cap and pops 2 inches above the sink’s rim. In case of a clogged drain line, the air gap prevents the wastewater from being siphoned back to the dishwasher.

One thing to remember is that the dishwasher, garbage disposal and sink share the same drain line. This means that if there is a problem with one of the 3, all the others will be affected. That is the reason you will see water backing up to your sink when running a dishwasher.

An alternative to a dishwasher air gap is looping the dishwasher drain hose high underneath the sink counter or a dishwasher standpipe. Both methods less effective than an actual air gap.  Since an air gap is a requirement by many building codes, plumbers will insist on installing one during a dishwasher installation.

A good dishwasher air gap should be higher than the rim of the kitchen sink. This allows water from the air gap to flow down to the sink when the drain hose is clogged. Dishwasher air gap under the sink will flood your floor in case of a clogged drain line.

Dishwasher Air Gap Plumbing Diagram

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Image credit: Code Check

How Does a Dishwasher Air Gap Work?

As we have seen, a dishwasher air gap works just like a backflow preventer. But how does this simple fitting work differently from a high loop?

To help you understand the idea behind a dishwasher air gap, I need you to look at your kitchen sink and the faucet, especially the spout.  As you will notice, there is a few inches between the spout and the top of the sink’s rim.

What would happen if the faucet spout was lower than the rim of the sink and the sink drain happened to clog? Wastewater will be siphoned from the sink and up the faucet spout contaminating your fresh water.

This is exactly how a dishwasher air gap works. It prevents the backflow of water to the dishwasher which causes flooding and contamination of the fresh water supply.  

A dishwasher air gap looks like an inverted Y. the top of the air gap (the portion you see next to the faucet) is the actual air gap.  The air gap then branches into 2 under the kitchen sink. One branch is connected to the dishwasher drain house while the other one is connected to the garbage disposal or sink drain line.

Wastewater from the dishwasher goes up through the first branch of the air gap and after reaching the junction it descends through the other branch and enters the garbage disposal and out into the drain line.

To understand how a dishwasher air gap works, let us imagine 2 scenarios:

1. Sink Drain or Garbage Disposal is Clogged

When the garbage disposal or kitchen sink drain is clogged, it means wastewater from the sink cannot flow out into the sewer lines. Where will the wastewater go to?

If you do not have an air gap, water will flow back to the dishwasher via the drain hose. Actually, if the drain hose is looped high, as more water enters the loop the stronger a siphon is created.

When enough water has filled the loop, the dishwasher drain hose will siphon water out of the sink and dump it inside the dishwasher. This is why people with no air gaps have water backing up into the dishwasher when draining a sink.

With a dishwasher air gap already installed, things are a little different. Why? Because there is a gap of air between the sink drain line/garbage disposal and the dishwasher.

The air gap obeys the law of science which states that Water cannot siphon back through an unpressurized gap of air. This means that wastewater inside your sink or garbage disposal cannot be siphoned /sucked into the dishwasher drain hose.

2. Air Gap is Clogged

Sometimes the air gap itself can get clogged. When that is the case, 2 things will happens.

  • Dishwasher will not drain.
  • Water will leak from the top of the air gap.

If the air gap branch connected to the dishwasher drain hose is clogged, you will notice that the dishwasher will not drain.  The only thing you can do in that case is to disconnect the hose from the air gap and try to snake it.

On the other hand, if the air gap branch connected to the garbage disposal or sink drain line is clogged, wastewater from the dishwasher will start to shoot from the top of the air gap and flow down to the sink.

If water is leaking from the air gap but the sink drain line and garbage disposal are not clogged, you have a blockage on the descending branch of the air gap. Disconnect its hose from the garbage disposal and snake it.

Dual Inlet Dishwasher Air Gaps

As you already know, garbage disposals or kitchen sink drain lines have one opening for connecting a dishwasher. What then can you do if you want to connect 2 dishwashers or a dishwasher and a water filtration system?

That is where a dual inlet air gap comes in. It has 3 branches whereby the middle branch is where the 2 side braches drain to and it is then connected to the garbage disposal.

Dual inlet air gaps are properly designed to prevent cross-connection. In plumbing, cross-connection happens when 2 water pipes that shouldn’t communicate ends up communicating. Like when your shower balancing valve fails and cold water starts to flow to the hot water lines.

In this case, the air gap is designed in such a way that wastewater from the dishwasher will not find its way to the water filtration system.

Dishwasher Air Gap Alternatives

If you are one of those people who do like the idea of an air gap sticking from your sink, there are other options to explore and prevent backflow of wastewater back to the dishwasher.

You must however be in an area where air gap installation is not a code requirement.  Let us look at the 2 alternatives to dishwasher air gaps:

1. High Loop

A high loop refers to a type of installation whereby the mid-section of the dishwasher drain is lifted high and fixed underneath the sink counter to form a loop. Wastewater from the dishwasher goes up then down through the loop.

The high loop works like an air gap but it is limited in some ways. A high loops makes it hard for wastewater to flow back to the dishwasher if there is a clog somewhere in the drain line. Instead, wastewater backs up into the garbage disposal and up into the kitchen sink.

For a high loop to work effectively, the top of the loop must be about 32 inches from the floor of the kitchen.  If you have less clearance than that, you will need to install an air gap. There needs to be a nice slope on the loop to prevent backflow of water to the garbage disposal.

If the loop is rather small, it will act like a toilet drain trap. A clogged garbage disposal or sink drain will be enough to cause a siphon and have water sucked into the dishwasher.

In most cases, high loops will work without any issues but the fact remain that they are not foolproof.

If the water pressure in the ascending side of the loops falls relative to that of the clogged garbage disposal or sink drain, the wastewater will be sucked into the dishwasher.

High loops can also fail, especially due to sagging and poor installation which will in essence reduce their effectiveness. It is however the best alternative to a dishwasher air gap.

2. Dishwasher Standpipe

A dishwasher standpipe is a vertical section of pipe which looks like a plumbing vent. They are commonly used to drain washing machines.

As a standard, standpipes must run higher that the P-trap (the U-shaped bend of the drain line) and needs to be vented. When used to plumb a dishwasher, standpipe needs to run as high as possible.

That vertical section of pipe works like an air gap but it is limited just like the high loop method. For a standpipe to work effectively, it needs to be higher than the sink’s overflow/rim.

Since that is not possible, water will spill from the top of the standpipe and down to the floor in case of a clogged drain line or garbage disposal. I do not recommend this option.

 A high loop is far more effective and simpler than a dishwasher standpipe. The best method to prevent wastewater backflow to the dishwasher however remains to be the air gap.

Dishwasher Air Gap Under Counter/Sink

To gain maximum benefits of a dishwasher air gap, it needs to stick about 2 inches above the sink’s rim. A dishwasher air gap not only prevents backflow of wastewater to the dishwasher but it also ensures that in case the drain hose is clogged, the wastewater will be channeled down inside the sink.

The air gap also needs to be at a higher elevation than the dishwasher. Even if you manage to install the air gap under the kitchen counter, what will happen when it clogs?

Wastewater from the dishwasher will spill and flood your kitchen floor. Just have it installed properly.

The main reason some people opt for a dishwasher air gap under the counter is purely for cosmetic reasons. Most don’t like how conspicuous the air gap is adjacent to the faucet.

You can always think of creative ways to conceal the air gap head if you don’t like how it looks.

How to Install a Dishwasher Air Gap

It is not that hard to install a dishwasher air gap. Of course you can pay a plumber to install one for you but it will cost.

If you want to install a dishwasher air gap on your own, the video below will be of help instead of me writing 1000 words on the same.

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