It is one of the most common “defects” that I see when inspecting a home – however, it is also one of the easiest ones to improve or fix!
A quick background: Dishwashers pull water in from the home’s water supply line and use it to spray and wash dirty dishes and utensils. Once done, the dirty water is pumped out of the dishwasher to the home’s waste system through a drain line which is usually under the kitchen sink.
It is required by most jurisdictions and manufacturers that the dishwashers drain line be installed with a high loop. So why the fuss? The high-loop prevents the dirty water that is being drained from being inadvertently pulled back into the dishwasher or even siphoned back into the water system. I’d say about one third of the homes we’ve inspected do NOT have a high-loop installed properly or at all! Keeping your waste water away from your dishes and utensils and your water supply is key here!
The high loop (or air gap) prevents potential back-flow contamination of the dishwasher. With a properly installed high loop drain line you will become aware of the disposal backing up as it will back up into the sink in lieu of into the dishwasher.
Here’s a photo of a near perfect install of a high-loop (near because it actually could have been mounted a few inches higher. The things inspectors get excited over, right?