How to Install a Dishwasher Drain

If your home improvement plans include a new dishwasher, you'll need an alcove as close as possible to the kitchen sink so you can draw hot water from the kitchen faucet supply and connect the dishwasher drain to the sink drain. The alcove is usually right next to the sink cabinet, but it can be separated by intervening cabinets up to a maximum of 10 to 12 feet depending on the dishwasher manufacturer's specifications.

The dishwasher drain hose connects to the sink drain before the P-trap. If the sink has a garbage disposal, you should find an inlet port on it for the dishwasher drain hose, but if there's no garbage disposal, you have to connect the drain hose to the drain tailpiece, which involves installing a tee. The drain hose must make a high loop or connect to a fitting called an air gap before you make the connection, and the method you should use depends on the code requirements in your area.

When you connect a dishwasher drain to the sink drain, the potential exists for the dishwasher pump to siphon wastewater from the sink and contaminate the dishwasher. To prevent this, the Uniform Plumbing Code (UPC) requires a dishwasher air gap — a small, dome-shaped fitting that mounts to the deck of the kitchen sink or to the countertop. You connect the dishwasher drain hose to the inlet port of the air gap and then you add a short drain hose to the outlet port of the air gap and connect the other end to the sink drain. The air gap includes an air vent that equalizes pressure in the drain line to prevent backflow.

The International Plumbing Code (IPC) recommends air gaps but doesn't require them because it also allows the use of a high loop, which means the drain hose makes a loop, or arc, underneath the sink that rises to the sink rim or the underside of the cabinet. The rationale for the high loop is that the sink can't overflow past its rim, so there should always be enough air in the loop to prevent siphoning. Whether or not you have to install an unsightly air gap or you can employ a high loop depends to a great degree on whether your local plumbing code is based on the UPC or the IPC.

Most western coastal states as well as the northern ones west of Michigan and a few others are UPC states. It's important to check with a plumber or inspector, though, because some state codes aren't specific and leave the decision to local authorities. It's also important to check the installation instructions for the dishwasher. For example, GE dishwasher instructions, referenced by the InterNACHI Inspection Community, specify that the connection on the sink drain tee must be at least 18 inches above the floor to employ the high-loop method.

Most manufacturers supply a 6-foot, 6-inch length of corrugated drain hose with a new dishwasher. It has rubber fittings on both ends that fit 5/8-, 3/4- and 1-inch stubs, and you secure the ends with metal pipe clamps, which are also supplied. If the distance from the dishwasher to the drain exceeds 6 feet, you'll need to buy a longer hose. For any installation involving an air gap, you'll need a short hose in addition to the long drain hose. You can buy a dishwasher drain kit that includes everything you need from the dishwasher supplier.

If you're connecting the drain directly to the drain tailpiece, you'll need a tee, and rather than gluing one into your existing tailpiece, it's easier to replace the old tailpiece with a new one that has a built-in tee; this is called a branch tailpiece. A plastic one costs less than $5, while a metal one costs about $15. The best way to connect to a double-sink drain is to replace the tee connecting the sink waste arms to the P-trap with one that has an inlet port for the dishwasher, as pictured at Whitten Plumbing Solutions.