Like this post? Share it!

air gapA dishwasher air gap is a device that is mounted on the kitchen countertop near the sink that is designed to prevent dirty/contaminated water from the sink or garbage disposal from flowing backward through the dishwasher drain line and into the dishwasher if the sink drain or disposal become clogged.

When water flows in the opposite direction of its intended path, it is called backflow. An air gap is a backflow preventer. An air gap is a simple, low-cost device that helps to prevent your dishwasher and the clean dishes that may be in it from being contaminated by dirty water that otherwise might backflow into your dishwasher. For example, if your garbage disposal gets clogged and causes dirty water to essentially be pumped back through the drain line and into your dishwasher. 

faucet to flood rim- air gapThis backflow of dirty water is caused by a phenomenon called a cross connection. (See our other blog for a discussion of cross connections.) Air gaps are designed to prevent a cross-connection at the dishwasher. The air gap keeps the hose that carries dirty water from the dishwasher separate from the hose that takes the dirty water to the drain underneath the sink. Since the two hoses are completely separate, the danger of wastewater flowing back into your dishwasher is eliminated.

The distance between your kitchen faucet and the rim of your kitchen sink is a great example of an air gap. On most sinks, you will see a distance of three to four inches between the faucet and the flood rim of your sink (see photo on the left).

A flood rim is the highest level that water can fill a container such as a sink or tub without overflowing.

If there is a sink full of dirty water and the drain is clogged and starts to overflow, the water will spill over the rim of the sink and onto the countertop and/or the floor.

dishwasher air gapDue to the gap between the top of the sink and the faucet, the dirty water will never reach the faucet. In other words, the dirty sink water will have no chance of contaminating the potable water system. This gap of air thus protects the potable water system from contamination.   

A dishwasher air gap serves essentially the same purpose as the air gap described in the previous paragraphs. In the photo to the right, there is no air gap. Now imagine that the disposal gets clogged. It is easy to see how water can flow from the garbage disposal and into the dishwasher drain line (the white hose attached to the left side of the disposal at the red arrow.) Once it enters the drain hose, it can easily continue flowing through the drain hose all the way to the dishwasher. Let’s look at how an air gap prevents this from happening.

How Does an Air Gap Work?

air gap

The photo above shows how the air gap is connected to the sink and to the dishwasher. There are two separate hoses. The two hoses are never connected to each other. Inside the air gap, you can see two tubes: one comes from the dishwasher, and the other goes to the sink’s drain.

The photos below show cut-away views of the same air gap. On the right side of the air gap, you can see a small tube. (It is easier to see in the top view photo, but you can also see it in the side view photo.)

air gap side view

Side view

air gap top view
airgap top view

Top view

The dishwasher pump discharges the dirty water from the dishwasher through the drain line and up through the small tube (blue in the second side view photo). The water from this tube sprays against the cap (shown in side view photo) which is located just above the end of the tube. From there, it flows down into the larger tube (red in the second photo), to the second drain hose, and to the drain line under the sink.

Since the water is being pumped into the smaller tube, it is under pressure and is able to flow quickly through the small tube. However, as it drains out of the air gap through the larger tube, it is not under pressure and is only draining by gravity. Due to this, the outlet line must be larger than the inlet line in order to be able to keep up with the amount of water that is being pumped to the small tube by the dishwasher drain pump. In the side view photo above, you can see that the outlet line on the left is significantly larger than the inlet line on the right.

how does an air gap work

Now, let’s suppose that your garbage disposal is clogged, but still running. This can cause dirty wastewater to be pumped up the drain hose that runs from the disposal drainpipe up to the air gap. Once it reaches the air gap, the dirty water will flow out of the slot in the air gap cover and spray back into the sink. (The slot is colored yellow in the large drawing near the beginning of this post.)

Since the two hoses are physically separated by the air gap, water will not flow backward from one hose to the other hose but will flow out of the air gap. In this way, the air gap prevents this dirty water from continuing to flow through the hose to the dishwasher, contaminating it.

Without an air gap, the clogged disposal could easily pump the dirty water through the hose and all the way back into the dishwasher. Also, without an air gap, a simple clogged sink can result in water flowing backward through the dishwasher drain hose and into the dishwasher.

Why is Water Spraying out of My Air Gap?

If you have water flowing out of your air gap when your dishwasher is draining, then it is likely that there is a clog or blockage somewhere. Here are some things to look for.

  1. The most common cause of an air gap leaking water is a blockage or kink somewhere in the drain line. First, look for a kink and fix that if there is one. If there is no kink, then there is likely a blockage caused by bits of food and grease accumulating. These kinds of blockages are most likely to be found in the hose between the air gap and drain/disposal because there is less pressure to move this stuff through the line. To correct this, you can try running something through the drain line to clear it. If this doesn’t work, you can disconnect the line and flush it out.
  2. If there does not appear to be a blockage in the hose, the air gap itself could be clogged. To check this, you can unscrew to top of the air gap and remove the plastic cap in order to look inside for a blockage. Running a small piece of wire into the air gap tubes should clear any blockage.
  3. If neither of the first two steps above solves the problem, then the blockage is either in the disposal itself, in the trap under the sink, or in the drain line beyond the trap. Since this type of blockage would be obvious just by running water into the sink, it is likely that the blockage would have already been discovered. To correct this, first check for a blockage in the garbage disposal and clear it if you find a blockage. If there is no blockage in the disposal, remove the P-trap and clear any blockages in the trap or in the drain line beyond the trap. If the blockage is too far down the line, this step may require a plumber.
  4. If the dishwasher is connected to a garbage disposal and it has always leaked water from the air gap while the dishwasher drains, then it is possible that the knockout plug on the disposal inlet was never knocked out. It happens! Disconnect the power to the disposal, and use a screwdriver and hammer to punch out the plug. A pair of long-nose pliers will be helpful to fish out the plastic plug from the disposal.

To minimize the possibility of a future clog, ensure that there are no kinks in the hoses. Also, make sure that none of the hoses is too long. There should be very little slack in either hose after they are installed because slack (and the corresponding low spot in the hoses) can allow debris to collect and cause a clog.

See our post about Dishwasher High Loops

© 2021 Mike Morgan

This article was written by Mike Morgan, the owner of Morgan Inspection Services. Morgan Inspection Services has been providing home, septic and well inspection services throughout the central Texas area since 2002. He can be reached at 325-998-4663 or at mike@morganinspectionservices.com. No article, or portion thereof, may be reproduced or copied without prior written consent of Mike Morgan.


Like this post? Share it!