An improperly installed drain hose is by far the most common defect that I find with dishwasher installations, but it's also one of the easiest things to get right. In the photo below, which was featured as a Structure Tech Photo of the Day, there are several plumbing defects, two of which are related to the dishwasher drain. Can you spot them?
Problem number one is that the drain is connected to the sewer side of the trap. The water that always sits in the bottom of the P-trap is what prevents sewer gases from coming in to the house. The dishwasher drain must be connected before the P-trap, not after it, which is what was done here. With this improper installation, sewer gases have the potential to come back in to the dishwasher. The diagram below right shows a proper installation.
In the diagram above, right, you'll notice that the dishwasher drain makes a high loop underneath the kitchen sink. This is the minimum requirement on every dishwasher drain installation - it's required by every manufacturer of dishwasher, and it's also required by the Minnesota State Plumbing Code (section 4715.1250). In fact, our plumbing code says "as high as possible under the countertop." An alternative to installing a high loop is to install an air gap at the kitchen sink - it's a device that actually mounts above the sink. I can't imagine why anyone would do this if they didn't have to - how ugly. In some parts of the country, these air gaps are required no matter what. Too bad for them.
Although new dishwashers come from the manufacturer with the drain looped up at the side of the dishwasher, every installation manual still requires this high loop underneath the sink.
I've heard different reasons for why an additional loop is required under the sink, so I decided to contact the manufacturers directly. I sent out an email to eight dishwasher manufacturers, asking them this question:
"A high loop is required on the dishwasher drain in the installation instructions for all of your dishwashers. What is the purpose of this? Doesn’t the high loop that is incorporated in to the side of the dishwasher achieve the same thing? Any insight or commentary in to this matter would be greatly appreciated."
Here are the responses I received:
To summarize, the reasons for the high loop are to prevent potential backflow of water in to the dishwasher and to prevent improper drainage of water. Even though dishwashers come from the factory with the drain looped up high against the side of the dishwasher, this is not an acceptable substitute for the high loop underneath the kitchen sink. In Minneapolis, the lack of a properly installed high loop under the kitchen sink requires repair for their Truth-In-Sale of Housing evaluations.