Q: For the past few years I have noticed a water discharge from my dishwasher air gap. I just replaced the hoses from the dishwasher and installed a new air gap, but I’m still getting a discharge when I run the dishwasher. Any ideas?

A: The leak is caused by water backing up from the drain into the air gap. Air gaps prevent water from siphoning backward, a possibility in the unlikely event the water level in the sink is high enough that it backflows into the dishwasher.

An air gap is a simple device that allows air to break the flow of wastewater into the dishwasher. When water under pressure backs up into the air gap, it’s discharged through the holes in the air gap.

Air gaps are required for plumbing code compliance in most but not all jurisdictions. Kevin has no air gap for the dishwasher in his kitchen in Idaho. Rather, the local code allowed that the dishwasher discharge hose be run directly to the garbage disposal, but mounted as high under the sink cabinet as possible to prevent backflow into his dishwasher.

Water coming out the air gap should not be a regular occurrence, though occasionally it might happen when, for example, someone forgets to run the garbage disposal. Regular water discharge from an air gap points to a restriction in the water flow. Since you’ve replaced the hoses and the air gap, the problem is either with your installation or somewhere downstream.

The first thing to check out is that the 7/8-inch (inside diameter) hose coming from the air gap to the disposal (or tailpiece if you don’t have a garbage disposal) is as short as possible, not kinked or bellied. In other words it should run downhill its entire length. Because the hose is new, we presume it’s clear of any debris or obstruction. It won’t hurt to check it out, though. If it’s a ribbed flex hose, consider replacing it with a smooth rubber hose, as the ribs might eventually cause an obstruction.

Next, make sure the plastic plug in the garbage disposal is completely removed. Sometimes the plug isn’t totally removed when the disposal is installed. To check out the opening, remove the discharge hose from the garbage disposal inlet and probe the hole with a screwdriver to make sure the plastic plug is completely removed. If you dislodge something, unplug the disposal, reach into the disposal with your hand, and fish out the plug.

If the disposal intake pipe is clear, the next thing to check out is the discharge side of the disposal. An "L"-shaped fitting attached to the side of the disposal discharges into a "P-trap." Disconnect the trap and clean it out. Next look at the waste line entering the wall to make sure it’s clear.

Hopefully, after you go through these simple steps the obstruction will be clear and the dishwasher will run properly. If not, the next thing to do is to run a plumber’s snake down the drain to clear any obstructions in the drain line.

A simple thing we recommend doing is to wash off the majority of food debris before placing the dishes in the dishwasher. We know that many manufacturers say you don’t have to, but rinse your plates before they go in the dishwasher. After all, a dishwasher is a sanitizer, not a food disposer.

For a visual of how air gaps work, check out this YouTube video: http://www.youtube.com/watch?v=jLWJ9wkYfLo.

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