Q: I am installing a dishwasher and a garbage disposal in my home. The dishwasher instructions include two ways of installing the drain hose.

The simpler way is to attach it at the sink waste, before the trap. The other way seems more complicated and costly to attach it to an air gap that then attaches to the garbage disposal. I would prefer the first option. What are the pros and cons of each way?

A: The purpose of an air gap is to prevent wastewater from siphoning from the disposal into the dishwasher. The same thing can happen if you improperly install the drain hose directly to the drain, even if you attach it to an inlet above the trap.

An air gap is exactly what it sounds like. It is a small fixture that attaches to the top of the sink (or sometimes the countertop) that breaks the flow of wastewater as it moves from the dishwasher to the house’s drainage system.

Two small lengths of pipe extend from the bottom of the fixture. One acts as an inlet, and the other acts as an outlet. The drain hose from the dishwasher is attached to the inlet.

Another hose attaches to the outlet and connects to the garbage disposal, which drains the wastewater from the dishwasher into the house’s drain lines.

Of course, if a garbage disposal is not installed, the dishwasher water drains from the air gap into the drain, above the trap.

If local codes allow, you may forgo installing an air gap and install the drainage hose with a large loop between the point where it discharges from the dishwasher and enters the sink drain. Think of a bell-shaped curve. Despite illusions created by magicians and “Ripley’s Believe It or Not,” water does not run uphill.

Make sure to attach the drain hose as far up in the cabinet as you can, using hose clamps to secure it, when you run it between the dishwasher and the disposal.

Bottom line here: This is really a plumbing code question. Check with your local plumbing inspector to see if air gaps are required.

An air gap provides greater protection against wastewater siphoning from the disposal into the dishwasher. But if you put too much dish soap in the dishwasher, it will bubble out of the air gap.

If you are installing a new sink with your new dishwasher and disposal, we recommend you use an air gap. If you are salvaging the existing sink, be sure it has a hole for the air gap. If it doesn’t, you’re better off choosing the bell-shaped-curve option.

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