Improperly Installed Dishwasher Drains: A Very Common Defect

One of the most common defects that I routinely find when inspecting homes is improperly installed dishwasher drains. All dishwasher manufacturers require either a high loop or an air gap. Each helps prevent dirty water discharge back into the dishwasher where it could contaminate silverware, dishes, glasses, etc. that you thought were now clean and sanitary.

High loops are easier to install and are the preferred method that we see in the eastern US. In some areas of the country, such as out west, air gaps seem to be more common. For this article, we’ll discuss high loops only.

In this example, the dishwasher’s drain hose (the corrugated flexible tubing that looks like an esophagus) connects to a garbage disposal. The issue is that the high point of this drain hose is its disposal connection. If water backs up in the sink or disposal, it will run right back down into the dishwasher by gravity and contaminate whatever is inside.

This graphic illustrates how the dishwasher drain hose should be run. Using a pipe clamp, the top of the high loop should be secured to the underside of the kitchen counter or to a point on the cabinet wall (as high as possible) under the counter. Even if there is no disposal installed, the high loop is still needed and the dishwasher drain’s outlet gets connected to the sink’s drain pipe (upstream of the sink’s drain trap), such as in the photo below.

High loops are required by every dishwasher manufacturer yet most dishwasher installers, it appears, don’t actually read the installation instructions. It is also required by the International Residential Code (IRC) in section P 2717.2:

P 2717.2 Sink and dishwasher –
“The combined discharge from a dishwasher and a one- or two-compartment sink, with or without a food-waste disposer, shall be served by a trap of not less than 1½ inches (38 mm) in outside diameter. The dishwasher discharge pipe or tubing shall rise to the underside of the counter and be fastened or otherwise held in that position before connecting to the head of the food-waste disposer or to a wye fitting in the sink tailpiece.”

Most dishwashers nowadays come with a loop in the drain line mounted to the side of the dishwasher (it’s not visible once the dishwasher is installed into the kitchen cabinetry) although a high loop adjacent to the sink under the countertop is still also required. Repairing this defect is pretty straightforward and can be performed by a licensed plumber.

Another issue related to dishwasher drain lines has to do with them being connected downstream of the sink drain’s trap. All plumbing fixtures need to have a trap installed in order to keep sewer gases out of the home. (The only exception is for toilets as they have a built-in trap). A plumbing trap holds water in it which creates a blocking point to prevent sewer gases coming from drain plumbing downstream.

If the dishwasher drain is connected to the sink drain downstream of the sink drain trap (see photo to the right), it is possible for sewer gases to enter the home through the dishwasher. The sink trap can’t provide proper sewer gas protection for the dishwasher in this case.

Matthew Steger is a Certified Level 1 Infrared Thermographer, an ASHI Certified Inspector (ACI), and an electrical engineer. He can be reached at matthew@thehomeinspectorsnotebook.com. No article, or portion thereof, may be reproduced or copied without prior written consent of Matthew Steger.