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Why does dishwasher only drain when unhooked from sink?

Just bought a new place. Wife did load of dishes in dishwasher and then noticed about one inch of water had not drained. I did some research and assumed there was a Clog someplace so I took the thing apart and found nothing. I called a handy man who told me to unhook the dishwasher tubing from under the sink and try to drain dishwasher. Drains without hesitation. However when I plug back on its the same problem. Sink does not back up when running. Tubing under sink has the rise and fall before connection.

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Hi @sleepincow ,

How strong is the flow of water in the drain hose?

It may be that the drain pump doesn't supply enough pressure to ensure that the flow can get over the "loop" to the sink pipe connection. If it seems like a weak flow it could be a faulty impeller or motor in the drain pump.

What is the make and model number of the dishwasher?

Some dishwashers use the same pump for washing and draining.

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I just noticed now that the dishwasher connects to the plumbinG on the sewer side of the trap. Would this have any impact on the pump

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Hi @sleepincow ,

The dishwasher drain hose must be connected before the P trap, not on the sewer side of the trap. The water "held" in the P trap prevents gases coming back into the dishwasher from the sewer.

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The high loop is to prevent potential backflow of water into the dishwasher and to prevent improper drainage of water.

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Would this not allow the dishwasher to drain as well?

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Hi @sleepincow ,

The drain pump has been designed so that it supplies enough flow pressure to cope with the high loop.

The loop is in the plumbing code of most places.

This is what some manufacturers have to say re the high loop:

Kenmore: “The high loop or air gap must be used to prevent potential backflow contamination of the dishwasher. Local plumbing codes generally dictate the requirements in your area. Section 807.4 of the Uniform Plumbing Code states: “No domestic dishwashing machine shall be directly connected to a drainage system or food waste disposer without the use of an approved dishwasher airgap fitting on the discharge side of the dishwashing machine. Listed airgaps shall be installed with the flood level (FL) marking at or above the flood level of the sink or drainboard, whichever is higher, or separately trapped with the airbreak located on the stand pipe.”

GE: “If an air gap is not required, the drain hose must have the high loop from the floor to prevent backflow of water into the dishwasher or water siphoning out during operation.”

Bosch: The high loop in the drain hose of your dishwasher is to keep water from settling in the hose if it were hanging down any lower or horizontally. This keeps the drain hose dried out and keeps any odors from backing up into the dishwasher.

Viking: In testing our dishwashers, we have found that the additional high loop in the back of the dishwasher is required for proper draining of the water. We have seen when this piece is not applied that over time the consumer will have issues with the water back up and causing issues with proper drainage and water pooling in a particular area.

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I had same problem. Dishwasher would drain when not connected. Check your plumbing that the dishwasher connects to under sink mine was clogged up with fat and grime, I took it apart cleaned all tubing under sink and it worked great.

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