You dishwasher and kitchen sink share the same drain line. If the drain line is blocked, running the dishwasher will result in water backing up and filling your sink.
Wastewater from a sink enters the drain line from an opening at the top of garbage disposal (if you have one) and exit at the bottom into the drain trap. Plumbing a dishwasher is usually very tricky and if not properly done you will always have water pooling at the base of the dishwasher or even a smelly dishwasher.
When a sink is filling with water when running the dishwasher, it is a sign of a clogged drain line, blocked sink air gap or an obstructed dishwasher drain hose. If you have a new garbage disposal, it is possible that the drain plug was not removed.
Since the sink and dishwasher and the sink share the same drain line, you should not run the dishwasher when the sink is clogged. This will only result in more water backing up through the sink. Unclog the sink first.
When water from the sink is going into the dishwasher, it means the drain line is clogged restricting the flow of water, forcing it to back up to the dishwasher. Unclog the sink drain and also consider installing a dishwasher air gap which will prevent water from flowing back to the dishwasher when the drain line is clogged.
Caution: Remember to always turn off or plug out the garbage disposal and dishwasher before starting troubleshooting them.
Let us now look in more details why your sink fills up when running the dishwasher.
1. Clogged Drain Line
As I have already mentioned, the sink and dishwasher share the same drain line. This means that if there is a problem with the drain line, it will affect both fixtures.
If the drain line is clogged, there is no way for water to pass through it and is hence forced to back up through the sink drain. It does not even have to be a full clog. A partial clog can result in the sink filling up when running the dishwasher and then the water will drain out slowly.
Usually, whenever you have a clogged kitchen sink drain, the clog is inside the P-trap. A P-trap is the U-shaped bend under the sink. All fixtures in your house have one.
Its main function is to trap potential drain clogs, preventing them from clogging the drain line farther away, where unclogging would be way harder.
Due to its shape, the P-trap is also full of water at all times. This water acts as a barrier, preventing sewer odors from coming into the house via the sink.
There are several ways to unclog a kitchen sink drain. You can consider snaking it or breaking down the clog using baking soda and vinegar.
My favorite method however involves disconnecting and cleaning the P-trap. It is an easy and fast method to have your line draining well again.
If you have a double kitchen sink, it is also the best method since it allows you to clean the tee which connects the second sink to the main drain line.
Here is how to unclog a Kitchen sink drain:
- Start by removing the items you have stored under the kitchen sink.
- Place a small container under the P-trap. This will prevent any water inside the trap from spilling on the floor.
- Disconnect the P-trap. Since the P-trap has a short and long arm, I always recommend disconnecting the shorter arm to allow the water in the trap to flow out via gravity. Remember that the connections are in most cases hand-tight and you may not even need a wrench to loosen them.
- Pull down the P-trap and inspect inside. Can you see gunk? Check also the pipe connected to the wall as well as the tail piece (pipe connected to sink drain opening). Remove as much gunk as you can.
- Connect back the P-trap.
- Run the dishwasher and check if wastewater has stopped backing up through the sink drain.
And basically that is how to fix a sink filling up when running a dishwasher as a result of a clogged drain line.
2. Blocked Air Gap
If water is coming out through your sink’s air gap when running a dishwasher, the air gap opening is blocked. This is usually caused by food scraps from the dishwasher as well as the use of too much or the wrong type of detergent.
In the old way of plumbing dishwashers, the drain hose used to be looped underneath the sink countertop to prevent backflow of water into the dishwasher in case of a clogged drain line.
In modern plumbing, we use a simple and effective device called a dish washer air gap. This is the knob you see next to the kitchen sink faucet.
An air gap prevents the back flow and therefore contamination of the dishwasher by wastewater in case of a clogged drain line. It works on the simple principle that water cannot siphon back through an unpressurized gap of air.
If there is a clog somewhere on the air gap, it means that the wastewater cannot flow out into the drain line and can also not flow back to the dishwasher. In that case, water will start to shoot from the top of the air gap and enter the sink.
Clearing a clog from the air gap is quite easy and fast as well. You can do it from the top or disconnect the drain house from underneath the sink and snake it from here.
For more information on the same check out the video below.
3. Plugged Garbage Disposal
Have you installed a new garbage disposal lately? If you have, there is chance that you forgot to remove the drain plug and as result the dishwasher cannot drain, forcing it to fill the kitchen sink with wastewater.
New garbage disposals come with drain plugs installed on the opening where you are supposed to connect the dishwasher drain hose. They are also installed since some people need a garbage disposal but do not have a dishwasher.
If the opening is left open, the garbage disposal will be leaking every time it is being used. You should however remove the plug when hooking up the dishwasher drain.
Just grab a pair of pliers and disconnected the dishwasher drain hose from the garbage disposal. Check if the drain plug is there and pull it out. Connect back the drain hose.
And that is how to fix a dishwasher backed up into the sink. I hope this guide was helpful. If you are unable to fix it on your own, fell free to consult a professional plumber.