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(Fix) Kitchen Sink Only Drains When Garbage Disposal Is Running

Garbage disposals are simple machines that grind food wastes into a pulp and dispose of them through drain pipes. However, they can sometimes run into problems.

If your kitchen sink only drains when the garbage disposal is ON, the culprits here are the clogs in the drain pipes. This problem is solved only by unclogging the drain pipes using a plumber’s snake. By making them clog-free, the sink will drain without the downforce from the disposal.

This can also be due to problems with the plumbing if this happens after making changes to it.

Why does the sink drain when the garbage disposal is on?

A garbage disposal runs with the help of an impeller. This impeller, while running, creates centrifugal force. Even if the drain is clogged, the downforce created by this centrifugal force is enough to force the standing water past the clogs and drain the sink. This is why sometimes a sink with standing water in it drains when your garbage disposal is turned on.

Reasons why sink won’t drain unless garbage disposal is on

The reasons why the sink won’t drain unless your garbage disposal is on are;

  1. Faulty plumbing
  2. clogs in the pipelines
  3. Lack of proper venting

1. Faulty plumbing

A lot of people install a garbage disposal themselves. It is a simple job in most cases. But those DIYers with not much experience with plumbing can make some simple mistakes that prevent the water from draining naturally from the sink.

Here are the two common plumbing mistakes people make;

a. The drain pipe into the wall is installed higher than the pipe from the disposal

Water drains with the help of gravity. So when you install a garbage disposal, its outlet should always sit higher than the drain pipe going into the wall. But when people install garbage disposals, especially on deep sinks, the disposal drain pipe’s position can be lower than that of the pipe that goes into the wall. This will prevent the free flow of water and cause it to stand in the sink unless it gets a push, which is what the garbage disposal does when it runs.

Height of garbage disposal drain and pipe going into the wall

This arrangement can also lead to constant clogs in the drain pipe. Even if you clear the clog, it will come back sooner or later. The best solution here is to get rid of the garbage disposal and connect the pipes directly. And put your food waste in the garbage bin instead.

Why do I say the best solution is to remove the garbage disposal? Can’t you lower the drainpipe? This may sound easy, but you can run into many problems with this new arrangement. The drain pipes, aka branch drain lines that run through the wall, have a slight downward slope to help the water drain. If you lower the drain pipe from the kitchen sink, this can invert the slope of these horizontally running pipes, which will lead to massive drainage issues. So the more practical solution would be getting rid of the garbage disposal.

b. The depth of the p-trap is too much

A p-trap is a U-shaped pipe that sits between the garbage disposal’s pipe and the drain pipe that goes into the wall. It is mainly used to trap food wastes and other debris from the sink, thus preventing clogs in the drain pipes.

Sometimes, when the depth of the p-trap is too much, it can prevent water from draining. When the trap is deeper, the water needs to exert more pressure to move out of it.

That being said, there is no definitive guide to selecting the p-trap height. If there are no clogs in the pipes and there are no other problems with the plumbing, you can try changing the height of the p-trap and see if the water drains correctly.

2. Clogs in either the drain pipes or the garbage disposal

Another reason why the sink only drains when the disposal is running is clogging in either the drain pipes or the garbage disposal or both. These clogs may not be strong, which is why they let the water pass when the garbage disposal is turned on. To drain the water, you have to get rid of these clogs.

30 minutes 30 minutes

Here is how to find and clear these clogs;

  1. Open the p-trap

    When the sink has standing water in it, remove the p-trap and see if the water is draining. Make sure you take precautions to prevent the kitchen from flooding. If the sink has too much water, you can first run the garbage disposal until there is only a quarter of water left. If the water is draining easily, then the clog is in the pipeline going into the wall. If the water is not draining, then the clog is in the disposal.

  2. Unclog the disposal

    Your garbage disposal clogs because there are food waste residues closing the tiny holes in its shredder rings, thus preventing the water from going through them. The solution to this problem is cleaning the disposal. The best way to do this is to grind loads of ice cubes. Since the garbage disposal is running, it is not jammed, and so we can safely grind ice cubes.
    To unclog the garbage disposal, fill the garbage disposal with trays of ice cubes until it is full. Then run hot water and start the garbage disposal. You will see a lot of debris backing up into the sink and then draining. I recommend keeping the p-trap open and putting a bucket underneath it. This is because there can be other clogs in the pipeline, which can make this process hard, and we can avoid that by keeping the p-trap open.
    Here is a list of other ways to clean any debris left in a disposal.

  3. Clean the p-trap and the drain pipes

    Use a plumber’s snake to clean the p-trap as well as the drain pipes. Try to snake the branch drain pipes as far as you can.

  4. Refit everything and check

    Refit the pipes, run water into the sink and see if it is draining without running the disposal. If the problem continues to persist, there may be clogs further down the drain pipes running through the wall. At this point, it is better to seek the help of a plumber.

3. Lack of proper venting

A plumbing system needs to be vented properly to avoid the smooth flow of water. If the venting system is not working correctly, it can cause slow sink drains. If your kitchen sink drain comes with a Studor Vent, check it for any blocks. Sometimes the debris from the sink can accumulate in these vents and block the airflow.

Studor vents are easy to open and clean. They are also easy to replace. If you don’t have one of these on the drainpipe and suspect that the drainage problem you are having is due to a lack of proper venting, you can install one of these to solve it. However, I would recommend you get the help of a professional plumber before making any changes.

I hope these little tips help you to clear whatever is preventing your sink from draining. But when in doubt, always ask a plumber; trying to DIY this can cause more problems than it is worth.

My name is Thomas Anderson, author of DisposalQA. I have 15 years experience working as a plumber in CA, and this is where I answer common questions about garbage disposals.