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. There are two common ways to do this.
One method for venting is to loop the dishwasher drain hose up under the sink cabinet. This connection method is known as a high loop. Another method is to route the drain pipe through an air gap fitting that mounts to the top of the countertop or sink.
Dishwasher venting can be somewhat confusing for the do-it-yourselfer. The best strategy is to follow whatever local code guidelines require for dishwasher venting. Some local codes will specifically require the use of an air gap when installing a dishwasher, while in other areas the high loop method is an approved alternative.
It is important to check local building and plumbing codes before installing a dishwasher. In many cases, you can look on the website for your county’s building and safety department to find the code requirements. If you can’t find the information online or if you have any doubts, call them and ask. It is also a good idea to read the manufacturer’s recommendations for installation.
The Air Gap Method
In many areas, the air gap method is the only one you're allowed to use when installing a dishwasher. An air gap is a small metal or plastic fitting that is installed on the countertop or sink, with two hose fittings on the underside. The dishwasher drain hose connects to one fitting on the bottom of the air gap, while a secondary hose runs from a second air gap fitting to the house's drain system. The air gap works by allowing fresh air into the drain hose when any negative pressure occurs, eliminating any suction force that can draw water back into the dishwasher chamber.
The air gap fitting usually fits into an existing mounting hole on the top of the sink, and they are available in a variety of finishes to match your faucet and sink.
The High Loop Method
In the high loop method, the drain line of the dishwasher is looped up as high as possible under the sink 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 nowhere near as effective 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.
Connecting to the Drain
The dishwasher drain hose will always run from the dishwasher up to the air gap or high loop first, then loop back down to connect to the drain system. How you connect the other end of the hose 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 loop down from the air gap or high loop and connect to a side nipple on the disposal. This ribbed nipple is sized to accept the dishwasher hose and is secured to it 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 remain 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 loop 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.