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Installing a Dishwasher drain

My dishwasher is not directly next to the sink, there is a corner cabinet and another cabinet that separate the two. Therefore I'm planning on draining the dishwasher through the floor into the basement directly into the drain pipe there.

My instructions say I need at least 18" loop or an air gap in order for the dishwasher to operate properly. Is this only if I'm draining "upwards" to the sink? Or do I still need that when draining straight downhill to the drain in the basement.

What is the purpose of the air gap or loop? Is it some sort of pressure thing with the pump? Or is it simply to make sure dirty water doesn't come back into the dishwasher? If it's to eliminate dirty water, could I simply put a one way check valve in the drain line to keep it from back flushing?

I asked these questions on another plumbing forum and all I got was smartalec remarks and was told that I needed to hire a professional plumber. I want to do the job right, but I also want to understand what it is that I'm actually trying to accomplish.

Thank you!

Blue RidgeParkway
Re: Installing a Dishwasher drain

one thing that is important is the direction of the waste stream. is your proposed dishwasher drain location downstream or upstream on the waste stream of your kitchen sink? sharing the same horizontal waste line?

any chance you'll have a disposer on your sink now or in the future?

how about venting what's your plan?

your dishwasher uses a pump to drain not just gravity or centrifugal force assisting gravity. check valve like for a sump pump is a no no often the dishwasher can shoot solids some have a built in grinder, and detergent scum kind of like why you dont put a check valve on a washing machine either - dishcharges of lint and detergent scum and dirt. anyone who has had their air gap get clogged and had the dishcharge spray all over the counter top knows this is always a possiblity.

the high loop on the drain line with or without the air gap depends on your local codes. not only to prevent reverse syphoning but also to break vacuum and prevent wet venting into the dishwasher. the bottom of the dishwasher compartment being lower than the sink basin.

your model dishwasher will have instructions and specifications for installation. it may or may not have a high loop designed in where the drain hose is attached to the side of the frame.

you'd need more information about your waste drain waste venting system and the specific model and type of dishwasher and installation type. drawer models/combo warmer trays and platform installs of cabinet style dishwashers are becoming more common.

if you look into how a island loop vent works also called a chicago loop and understand a bit more about dwv plumbing it might help you understand. a practical lesson using a straw dipped in a glass full of water then putting your finger over the top of the straw to seal it as you withdraw the straw filled with water might also help you understand. keep in mind that the pump for discharge on the dishwasher also does the recirculation and fresh water distribution in the dishwasher with its different chambers. there is connection within the dishwasher to both potable plumbing and waste. also the flood rim of the dishwasher when the door seal is working is up near the top of the cabinet.

if you have a corner you might bring the drain hose after its looped high on the side of the cabinet down under the bottom of the corner cabinet in the toe kick area then back into the sink cabinet and up to the air gap on the sink deck - running diagonally short run just behind the toe kick at the corner? the exact distance and your dishwasher specifications would determine how much of a drain hose extension your dw pump can handle. the drain hose extensions are usually connected with stainless steel bands that are torqued down but must be accessible - so you can configure your toe kicks with screws or use magnet brackets to hold in place for access - just an idea depending on your location and type of electrical connection you are planning on using - got to keep that in mind should there be a leak from dw pump, drain line, or cabinet or if your dwv gets clogged and backs up to the vent or a horizontal rise.

p.s. you also have to keep a water seal/trap going and that is supposed to be accessible and above the floor that the fixture or in this case dishwasher is on.

Re: Installing a Dishwasher drain

Most dishwashers have no way to retain the water in the tub of the dishwasher without the loop. They eject the water via the pump. Most codes rule that the dishwasher waste be indirectly conveyed to the drainage system. This means through a trap inlet of another fixture. even if it is not right next to he kitchen sink you can replace the provided hose with a longer hose witch is usually 7/8 i.d. min. preferably 1 inch.

If you go through the floor then you strap a loop of tubing to the wall and the run the remainder through the floor. The DW has a check vale on it.

The air gap is a mechanical device that prevents an cross connection hazzard. It seaparates the DW hose from the drain and is usually mounted to the counter top. if you go to the basement it would be affixed between the hose and the trap.

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