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How To Prevent Garbage Disposal Backing Up Into Dishwasher

Garbage disposals come with a dishwasher drain connection, the purpose of which is to drain water from the dishwasher into the garbage disposal. But sometimes, this flow can reverse due to various reasons, and the dirty water from the disposal ends up in the dishwasher. How to prevent this backflow?

To prevent garbage disposal from backing up into the dishwasher, ensure that all the drain lines are free of clogs and the disposal’s knockout plug is removed. Also, use a high drain loop or an air gap to prevent water in the sink and the garbage disposal from draining into the dishwasher.

Let us see in detail why your dishwasher is filled with dirty water from the garbage disposal and how to prevent this from occurring in the future.

Garbage disposal backed up into dishwasher

Why does garbage disposal drain into the dishwasher?

As you know all garbage disposals come with a dishwasher drain connection. This is a standard 1/2 inch connector to which the dishwasher drain hose is connected to. So when the dishwasher drains the dirty water along with all the food debris, they end up in the garbage disposal. This setup helps to grind the food wastes before sending them down the drain pipes.

Unfortunately, sometimes the flow from the dishwasher to the garbage disposal can reverse and the dirty water from the disposal can end up in the dishwasher.

The main reason garbage disposal backs up into the dishwasher is the lack of an air gap or a high drain loop. For a new installation, this can happen if you forgot to remove the knockout plug. Water can also backup if the disposal or the drain pipes are clogged.

Install an air gap or add a high loop

When you connect a dishwasher drain line to a garbage disposal, the connection slopes towards the dishwasher. This means that the water from the garbage disposal will easily flow into the dishwasher. But that is now how it should work. You can prevent this backflow by using a small device called ‘air gap’.

A dishwasher air gap is a small cylindrical device sitting on your sink. It usually sits near the faucet. An air gap prevents water in the sink from draining in the dishwasher. It has two hose fittings under it. When you have a garbage disposal and a dishwasher, the dishwasher outlet hose connects to one fitting of the air gap, and another hose from the garbage disposal connects to the air gap’s other fitting.

The air gap prevents the backflow of water from the garbage disposal into the dishwasher since it sits higher than the dishwasher connection. Since water doesn’t drain against gravity, the air gap’s higher position prevents water in the garbage disposal from draining into the dishwasher. The air gap also lets fresh air into the drain hoses, eliminating any suction forces that can back up water into the dishwasher.

Shown above is a video detailing the installation of an air gap. You can do it by yourself if you have the necessary tools. The most difficult part of the installation is drilling the countertop; the rest is easy.

If you don’t have an air gap, adding a high drain loop also works. This is very simple to do. All you have to do is clip the drain hose connecting the garbage disposal and the dishwasher to the underside of your countertop. Make sure that the loop sits higher than the dishwasher inlet of the garbage disposal. Otherwise, this won’t work.

High loop installation

Clear clogs in the garbage disposal and drain pipes

Clogs in the garbage disposal or drain pipes will prevent the dishwasher from draining. This will also cause the water from the sink and the garbage disposal to flow into the dishwasher since there is no other path for it to flow. Unclogging the drain pipes and the garbage disposal is the only solution here.

Try unclogging the drain pipes using a sink plunger. The larger the plunger, the better. Place the plunger’s cup over the drain hole and run some water just enough to seal its sides. Make sure that all sides of the plunger head are sealed tight. Push the plunger down and upward. This moves the water up and down through the drain pipes, loosening any clogs.

Plunging a sink

You will know when the clogs are removed because it will be much easier to pump the plunger then. Remove the plunger and run the garbage disposal to see if the water is draining into the dishwasher.

If the clogs are too big, a plunger won’t do the job. You will have to disconnect the drain pipes and clean them one by one. Clogs can happen anywhere, so make sure that all the pipes are inspected and cleaned. The pipes can be cleaned using a wire brush and a plumbing snake. Once they are cleaned, you should also clean the garbage disposal to prevent future blocks.

Here are the detailed guides to unclogging and cleaning garbage disposal and drain pipes;

Remove the knockout plug from the garbage disposal

The dishwasher connection of the garbage disposal is blocked with a plug. You have to remove this plug before connecting the garbage disposal to the dishwasher. But many people during DIY installation connect the dishwasher hose without doing this. As a result, the dishwasher will flood when it is turned on. If this happens, immediately turn off the dishwasher and remove the dishwasher knockout plug in the garbage disposal.

It is difficult to remove a dishwasher plug without dismounting the garbage disposal but not impossible. Here is how to do that;

  1. Turn off the garbage disposal and unplug it from its power source.
  2. Disconnect the dishwasher connection and the hose from the dishwasher
  3. Insert a screwdriver through the dishwasher inlet of the garbage disposal and use it to hammer the knockout plug gently. The plug will fall into the garbage disposal.
  4. Shine a flashlight into the disposal through the sinkhole and locate the plug. Remove it with the help of tweezers or tongs.
  5. Reconnect the dishwasher hose plug in the garbage disposal.

Please make sure that you get the knockout plug out of the garbage disposal. They are not safe to be ground, and running the garbage disposal without removing the knockout plug can damage the disposal’s walls and its grinding components.

If you don’t have enough space under the countertop to perform this, you will have to dismount the garbage disposal, remove the knockout plug and mount the disposal again.

My name is Thomas Anderson, and I have worked briefly as a plumber. Even though I am currently working on a different career path I still help out my friends and family with their plumbing issues. This is a hobby blog where I share my knowledge with the audience through random articles. Through this blog, I hope to address some common questions about garbage disposals.