Because a dishwasher drains into your kitchen's plumbing system, there is a slight possibility of dirty drain water siphoning back into the dishwasher and contaminating clean dishes or being drawn back into the fresh water supply. To prevent this, building codes require some method of creating an air space in the dishwasher drain hose.
What Is Dishwasher Venting?
Dishwasher venting prevents dirty water from re-entering the dishwasher. There are several common ways to do this: creating a high loop in the drain hose, connecting the hose to an air gap fitting, or installing a check valve on the drain line.
In any case, it is important to check local building and plumbing codes before installing a new dishwasher. The local code rules dictate the legal requirements in your area. Contact your city's building department or visit online for more information. In some areas, a high loop is the minimum requirement, but in many other jurisdictions, an air gap must be installed with all new dishwashers.
High Loop Dishwasher Connection
In the high loop method, the drain line of the dishwasher is looped up as high as possible under the countertop before it runs back down to connect to the drain system. In some areas, this high loop method is an allowable option, but make sure to check. Because the top of the loop is located above the flood level of the dishwasher, it makes it unlikely that drain water can be siphoned back into the dishwasher.
The high loop method is not as reliable as an air gap, but it can help to prevent contaminated water from being drawn back into the dishwasher. Where allowed, the high loop method frees up a sink mounting hole that can be used for a soap dispenser, water filtration, or instant hot water dispenser.
Air Gap Dishwasher Connection
An air gap connection was once the preferred method of connecting a dishwasher drain, though its practicality has faded. An air gap is a small metal or plastic fitting that is installed on the countertop or sink and usually has a decorative cap. The device has two hose fittings on its underside, below the sink or countertop. The incoming dishwasher drain hose connects to one fitting, while a secondary hose runs from the other fitting to the kitchen sink's drain pipe or garbage disposer.
The air gap works by allowing fresh air into the drain hose if there is a backup causing negative pressure. This eliminates any suction force that can draw water back into the dishwasher chamber. Air gaps are designed to fit into an existing mounting hole on the top of the sink (or they can mount to any type of countertop). They are available in a variety of finishes to match your faucet and sink.
While using an air gap connection is still doable, it is no longer the preferred method and is not as practical as using a high loop or check valve.
Connecting the Dishwasher to the Drain
The dishwasher drain hose will always run from the dishwasher up to the air gap or high loop first, then it runs back down to connect to the drain system. How you connect to the drain system depends on whether or not you have a garbage disposal installed under your sink.
When there is a garbage disposal installed, the dishwasher drain line should run down from the air gap or high loop and connect to a side nipple on the disposal, where it connects with a hose clamp. Do not bypass the garbage disposal if it is present. By draining the dishwasher through the disposal, larger pieces of food residue will be captured in the disposal, where they can be ground up when the garbage disposal is next used.
When there is no garbage disposal present, the dishwasher hose should run down from the air gap or high loop and connect to the sink drain by means of a drain tailpiece with a branch fitting. The dishwasher hose is fitted onto the ribbed branch fitting and secured with a hose clamp. The dishwasher hose must connect to the sink drain before the drain's P-trap. If the hose connects after the P-trap, sewer gasses can flow up into the dishwasher hose and into the dishwasher.