An all-too-common problem that I find with renovated kitchens with new dishwashers and/or new disposals is when the dishwasher drains through the overflow/vent cap on the sink rim like that shown in Figure 1:

  Figure 1

The dishwasher should not do that! I've had lots of Clients tell me, "It's always done that! We thought it was supposed to!" Uh, no. If you don't have a cap like that shown in the picture (possibly a different color), you might have what is called a "high loop" and I usually find them installed wrong as well.

So no cap or a dishwasher draining through the cap means that you should call your plumber, preferably one who didn't do the original work.

By the way, make sure that the slots in your dishwasher drain cap faces the sink so that if your main drian does get clogged, you won't have water like this going all over the countertop and the floors. Children love to rotate these caps, so there's no telling which way yours is facing right now.

Readers who would like additional knowledge about this problem can continue reading.

Although a dishwasher draining like that shown usually means that the dishwasher drain is clogged, it's the source of the clog that often is at issue. Sometimes it's the grease, corn, beans, fat, etc., that has clogged the drain. More often, though, especially in renovations with new disposals, it's the disposal, not the dishwasher.

The diswasher drain hose is routed from the dishwasher up to this drain cap and then down to the disposal. Part of the reason for this system is to prevent the backflow of dirty water from the sink into the dishwasher where clean dishes might still be sitting.

There's a small plastic knockout piece in the disposal that needs to be knocked out before the dishwasher hose is connected. Many DIY home owners don't know about the knockout, and there are some newbie plumbers who either didn't know about it or simply forgot to remove it.

If your dishwasher does not have a vent cap on the sink rim or countertop by the sink, it probably has a "high loop" system to prevent backflow. Two common types are shown in Figure 2 and Figure 3:

Figure 2

Figure 3

The top of the loop should be as high as possible in the sink cabinet in order to prevent water from a full sink backflowing into the dishwasher. Some plumbers would consider these two high loops to be acceptable, believing that the top of the loop only need to be as high or slightly higher than the bottom of the sink; however, that doesn't take into account filling a sink full of water and then letting it drain.

"When would I have a sink full of water?" Granted, not very often since most of us don't wash dishes by hand anymore, but I was over at a friend's house a couple of weeks ago when mommy was washing three-week-old baby in the kitchen sink. When I saw that, I did quickly glance to make sure they had a dishwasher vent cap.

Better safe than sorry.

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