The Dishwasher Air Gap Story

This article appeared in our September, 2009 newsletter.  This brings us to the present.  Articles will be posted from now on as they are published in the newsletters.

            This is going to be news to a number of people but that little chrome tower to the side of the kitchen faucet is not supposed to leak water into the sink.  Well, that's not totally right.  If everything is O.K. it is not supposed to do that.  If there is a problem it is supposed to do that.  Here is the story.

            That little chrome tower mentioned above on your kitchen sink is the dishwasher "air gap."  If you have a dishwasher that drains into your sink drain system, an air gap is required to be there by code.  It is a safety device.  Typically the dishwasher pumps out the dirty wash and rinse water either into the garbage disposal or directly into the side of the sink drain tail piece (the pipe under the sink).  This is what happens in sinks where there is no garbage disposal.  If there were no air gap, some of the water in the garbage disposal or the drain could be sucked back into the dishwasher when the pump shuts off.  This could contaminate your clean dishes.  Also, if there was a bunch of stuff in the garbage disposal or the drain and it was enough to prevent the flow of waste water from the dishwasher, the pressure could damage the dishwasher pump and seals.  It would be a real mess.  So some bright person created these air gaps to prevent that from happening.

            Please refer to Ken's primitive illustrations below to help visualize this.  They are pretty crude and not exact but they should help do the job.

            The first illustration shows the air gap tower as you see it sitting on your sink.

            The second illustration shows the inside of the air gap.  Again, there are some variations on the layout but the principle is the same.  The air gap is essentially two tubes.  One larger diameter tube with a smaller diameter tube inside of it. The smaller one is longer and extends down below the sink.  The dishwasher discharge hose is connected to the part of this tube that is under the sink.  The larger diameter tube also extends below the sink.  The bottom of this tube is connected via another hose to either the garbage disposal side connection or the sink drain side connection.

            When the dishwasher pumps out waste water it flows up through the small tube and spills over into the larger tube.  The water then flows by gravity down the drain line to the garbage disposal or sink drain.  It could be said that the water in the air gap has "jumped the gap" between the two tubes.  Once the dishwasher pump shuts off, it cannot suck water back into the dishwasher.  There is a "gap" there.

            Now suppose there is some "stuff" clogging the drain connection at the garbage disposal or the sink drain.  The water cannot drain out that way.  It backs up into the big tube and spills out the overflow hole in the side of the air gap cover.  This water then flows into the sink.  It does not get back into the small diameter pipe and get back to the dishwasher.

            So, whenever the dishwasher empties waste water and the water flows from the air gap overflow hole and into the sink, this is a signal that there is a stoppage in the system between the air gap and the garbage disposal or sink drain.  The hose needs to be disconnected and the stoppage cleared.  usually the stoppage is right in the part where the hose connects to the garbage disposal or sink drain.

            Now it is a fact that some people think the water flowing out of the air gap overflow hole into the sink is how the thing should work.  Believe it or not, I have discussed this with people who have been in their house for years and it was draining that way since the day they moved in.  They thought that was the way it should be.  Some even argued with me about it.  Technically they are correct.  This is the way it should be if there is something wrong.  They just did not get the "something is wrong" part.  Now you know the whole story.