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How Do Dishwashers Drain?

A collage of dishwasher drainage.

Dishwashers drain through a drain plug that connects to a building’s sewer or septic system.

If you hate kitchen chores, odds are good that your automatic dishwasher is your favorite appliance.  When these gadgets were first introduced to the market several decades ago, they promised to make this dreaded job go away.

Instead, early dishwashers required users to spend a lot of time on rinsing and rewashing, making a lot of potential customers question their real value.

Today, dishwashers are a lot more sophisticated.  Most modern machines require very little pre-washing, and many high-end models only require the user to place their dirty dishes in the machine.  At the end of the wash cycle, however, a lot of people wonder where exactly the dirty water, suds, and food particles go.

Dishwashers work by spraying hot water into the washing tub or bay.  By spraying at both a high temperature and pressure, the water is able to remove food particles.  

This water must be removed before the wash cycle is complete. Typically, this is done when the dishwasher opens the valve that leads to the drain pipe.  

Where Does the Drain Pipe Lead?

Close up photo of a professional repairing a dishwasher drain pipe.

The drain pipe that leads away from the dishwasher is often connected to the same drain pipe that is used for the wastewater from the kitchen sink.  

If a garbage disposal is installed on the kitchen sink, this means that the main drain pipe is just a few feet away from the dishwasher. This pipe contains the shared wastewater from both the kitchen sink and disposal as well as the dishwasher.

While this is a much more efficient way to build drain pipes than installing a completely separate drain system, it does mean that it is possible to get a backup of water in the kitchen sink from the dishwasher.  Typically, this occurs when there is a blockage in the shared drain pipe.

Essentially, the shared drain pipe can become clogged when food or other materials are forced down the dishwasher drain pipe.  Water cannot get past the blockage, so it travels the only way it can, back into the sink.  

If this occurs, wait until the dishwasher stops running and let the water drain out as much as possible.  Then, run the garbage disposal to eliminate any blockage that is “ahead” of the disposal apparatus.  This usually clears the problem.

In some cases, however, the dishwasher drain is connected behind the garbage disposal, or there is no garbage disposal connected to the sink.  In this case, the best method for removing a blockage is to use a commercial drain cleaner, followed by running hot water through the pipe for a few minutes. 

Why Is My Dishwasher Not Draining Properly?

When a dishwasher isn’t draining properly, it can be due to several different reasons.  Be sure to check all of these areas.

Drain Pipe

Close p photo of a clogged dishwasher drain pipe.

The most common reason for a dishwasher not draining properly is a block or clog in the drain line.  This is often the result of not pre-washing the dishes enough or washing items down the kitchen sink that are too large.

Filter and Valve

At the base of each dishwasher is a filter that prevents very large food particles from washing down the drain pipe.  When this filter gets clogged, however, there is a good chance that even water won’t be able to pass through.  This will cause the water in the dishwasher to drain much slower than usual.

Similarly, large food particles can also get stuck in the valve, forcing it to stay shut or not open all the way.

Seals

Close up photo of a professional repairing a damaged dishwasher seal.

If the seal around the washtub is damaged, odds are good that the dishwasher will start to leak.  In many cases, people assume that this means the machine isn’t draining properly, since the water will usually start to leak after the tub is filled.  

How Can I Fix a Dishwasher That Isn’t Draining?

If your dishwasher isn’t draining fast enough (or at all), there are some things you can try before calling the repair person.

1.  Make sure the drain pipe is clear.  If your dishwasher is connected to your kitchen drain pipe, this can be a fairly easy fix.  If you have one, turn on your garbage disposal while running hot water.  Allow the disposal to run until the water in the sink can flow freely through the drain.  If there is no garbage disposal, use a commercial drain cleaner.

2. Clean the filters.  Many dishwashers actually have two filters, one in the base of the tub near the drain valve and another one near the top of the machine. 

Keeping these filters clean is an essential part of making sure your dishwasher runs at peak efficiency and actually gets your dishes clean.  Learn how to remove and clean all of the filters in your dishwasher, and keep up with them on a regular basis.

Close up photo of a professional cleaning a dishwasher filter.

3. Check your seals.  If water is spilling onto your floor, you probably have a bad seal.  While this isn’t a problem with the drain, it’s a good idea to fix it anyway.

If none of these fixes work, it’s time to call a repair person.  A professional will be able to replace a valve or determine if there is a problem with the wash cycle that is affecting the performance and drainage of the dishwasher.

Related: How to clean a dishwasher properly