Connecting a Dishwasher Drainage Pipe to a Double-Sink Plumbing System

Dishwashers are a saving grace with regards to time, effort, and preference, but their installations can seem tricky at first glance. Double kitchen sink plumbing with dishwasher certainly sounds like a headache, but it’s not! If you’re looking to install your dishwasher yourself then read on as we discuss the steps involved so as to make it as easy and straightforward as possible!

Why Is a Drainage Setup Necessary?

Once your dishwasher is done doing its primary job, it needs to send that mucky water out and in to your homes drainage system to get rid of it safely. If this is not done, or done incorrectly, that water will simply dirty up your dishes again, create a mess everywhere, possibly flood your kitchen, and the list of potential disasters goes on. Trust me, this is something you want to do and get right the first time!

Things You Will Need

Installation Instructions (No Garbage Disposal)

Before we break down the steps on how to connect a dishwasher drain, you need to know which one of the two installation categories you will fall into. In some areas and countries plumbing legalities stipulate that you have to install an airgap on your countertop or sink if you are running a dishwasher. The drain will link up to this airgap and then to your sink drains. Double sink dishwasher setups with an airgap may not look as appealing but they are required by law in some places. I suggest you look into that before doing anything else. If it is not plumbing law for this to be done you can simply use what’s called a high loop which we will discuss further down.

If You Require an Air Gap

First, turn off the water supply completely. Then, cut the required hole in your countertop or sink basin. Be sure it is big enough to nicely fit the airgap stem.

Now, feed the airgap stem through, ensuring you have left one rubber washer on the top end. Once it’s through, place the other washer over the stem, followed by the compression nut. Tighten the nut whilst holding the top end of the airgap that is above the hole. Once everything is fairly snug, grip the top end of the airgap with a pair of pliers and use another pair to tighten the nut underneath. The required washers and nuts will come with the airgap kit.

Now run your dishwashers drain pipe up to the smaller outlet on the airgap stem. Attach it using the compression nut or hose clamp provided with the kit. Be sure it is tight. I suggest you also check the clamp on the hose on the dishwasher-end of the drain pipe. Ensure that it is liked correctly and fastened well, or the water will never even make it as far as the airgap!

Use a measure tape to check the distance from the larger outlet on your airgap to the drain line for your kitchen sink. Cut the spare piping provided with the kit to the correct size. Now attach the one end of the pipe to the second outlet on the airgap stem the same way you attached the dishwasher pipe to the first outlet.

If there is no dishwasher wye connected to the bridge pipe between your sink baskets you will have to add one. To do this, cut a piece out of the bridge piping the exact length of the wye excluding its broader attachment rims. Take this piece out and fix the wye on in its place, with the little dishwasher inlet pip facing upwards and towards your wall. Fasten each end of the wye to the bridge using the compression nuts. Remember to put the rubber compression washers in first as this ensures it’s all water-tight.

If your double sink plumbing system uses a y-type connection between your sinks, the process is the same except done vertically instead of horizontally. But it is extremely important that you install the wye above the trap in your system! If installed below it there is a risk of cross-contamination.

With the wye in place, attach the loose end of the drain pipe leading from the airgap to the inlet on the wye. Use a compression nut with a washer to clamp it on nice and tight. If there was already a wye installed simply follow the last sub-step!

Now that your drain setup is complete, turn the water supply on again and run your dishwasher to do a leak check. If there are any leaks, trace them and tighten those joints further.

If You Don’t Plan to Use an Airgap

If your drain installation system features no dishwasher wye connected to the bridge pipe between your sink baskets you will have to add one. To do this, cut a piece out of the bridge piping the exact length of the wye excluding its broader attachment rims. Take this piece out and fix the wye on in its place, with the little dishwasher inlet pip facing upwards and towards your wall. Fasten each end of the wye to the bridge using the compression nuts. Remember to put the rubber compression washers in first as this ensures it’s all water-tight.

If your plumbing system uses a y-type connection between your sinks, the process is the same except done vertically instead of horizontally. But it is extremely important that you install the wye above the trap in your system! If installed below it there is a risk of cross-contamination

Check the connection on your dishwasher to the drain pipe to ensure it’s well fastened. Now run the pipe up and loop it around. Use a clamp or bracket to fasten the loops top end to the underneath of your counter or against the back wall in the corner between wall and counter. It is imperative this loop is as high as possible to prevent water from the sink or dishwasher running back down and into the dishwasher.

Take the loose end of the drain pipe coming out of the loop and attach it to the small inlet on the wye. Use the compression nut and washer to fasten it. Your pipe should now be running out of your dishwasher, around in a high loop, and down into the wye between your sink baskets.

With everything set up, turn your supply on again and run your dishwasher to leak-check your installation. If any leaks are found tighten the responsible joints’ compression nuts more.

If You Have a Garbage Disposal

If you have a garbage disposal the process is far simpler as there is already an outlet designed into the garbage disposer that is specifically for dishwashers. You simply need to create a high loop with your dishwasher drain pipe as mentioned above, that you attach to the back wall or counter underside. Then attach the pipe end to the disposal inlet and clamp it with a compression nut and washer, or a hose clamp. These will be provided in the dishwasher installation kit. Done!

Most Common Installation Defects

If the wye is connected to your drainage system after the trap in your piping, it can lead to numerous problems such as sewer gas leaking in to your dishwasher, water running back into the dishwasher, cross-contamination, and more. It could even lead to a backup that floods your kitchen. It is imperative that the wye is installed in the bridging or directly after the bridging of your sinks and before the S or P trap that these lead to.

If you are not using an airgap to prevent back-run, it’s of the utmost importance that you loop your dishwasher drainpipe as high as possible to break pressure and prevent water from being able to flow back in to your dishwasher. If this is not done you could get mould, moisture build up, dirty dishes rinsed in the same water they were washed with, and much more that could go as far as threatening your health. This is why some areas have laws demanding the use of an airgap. Always ensure your loop is high, unkinked, and unhindered!

And that is how to install dishwasher drain to double sink. It need not be difficult at all! If you follow this guide you would be doing it right the first time and can rest easy knowing you pulled off a nifty DIY project successfully. I wish you luck in your installation, and enjoy the convenience of your new dishwasher!