My job as a home inspector can be quite interesting with unusual things popping up that one doesn’t normally see. One thing that isn’t included in that unusual list is the dishwasher drain’s high loop or air gap device. I’d say a good 25% of all the dishwashers I look at lack proper configuration of the drain line. First let’s take a look at what a high loop and air gap device is and why you want one.
Dishwashers are normally located in the kitchen near the sink. The easiest way to plumb the dishwasher drain is often into the sink drain piping. A plastic hose about ¾” in diameter runs from the dishwasher to the drain line under the sink. The hose is attached to the garbage disposal or Tee’d into the piping upstream of the trap. It is important the connection is before the trap. Traps are designed to prevent sewer gases from entering the home. If the drain line is installed downstream of the trap, sewer gases can rise through the line straight into the dishwasher. Besides the objectionable smell, sewer gas also contains poisonous hydrogen sulfide and methane. This is the reason traps are required on all drain lines in the home.
A high loop is simply taking the drain hose from the dishwasher and mounting it as high as possible under the counter top before it loops down and connects to the sinks drain line or garbage disposal. What this does is form a trap inside the hose. The reason it has to be as high as possible is to get it above the level of water in the sink. If the high loop is below the water level in the sink, dirty sink water can flow back down the drain hose into the dish washer. You don’t want dirty sink water spraying around the dishwasher during its rinse or dry cycle do you?
An air gap device takes things a little further. In addition to running the drain line above the water level in the sink to prevent dirty water from flowing back into the dishwasher, they also provide an air gap. An air gap is simply a break in the line that lets in air to prevent siphoning. Under certain conditions if the city water line were to loose pressure, the city line can become contaminated by your dirty water siphoning back into it. An air gap device prevents this. Air gap devices are usually mounted on the sink to the side of the faucet. Two hoses attach to the bottom of it below the counter top. One hose in from the dishwasher, and another out to the sink drain line or garbage disposal. There normally is a slot cut into the air gap device that allows water to flow out in case the downstream hose becomes plugged. If you see water running out of the air gap when the dishwasher is draining, check your line to the under sink drain and the under sink drain lines for an obstruction.
Requirements vary by municipality. Be sure to check with your local building inspector if a high loop meets code or you need to install an air gap device.
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