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The Most Common Dishwasher Installation Defect

By In Dishwasher Drain On July 20, 2010


An improperly installed drain hose is by far the most common defect that I find with dishwasher installations, but it’s also one of the easiest things to get right.  In the photo below, which was featured as a Structure Tech Photo of the Day, there are several plumbing defects, two of which are related to the dishwasher drain.

Can you spot them?

Dishwasher Drain

Wrong side of trap

Problem number one is that the drain is connected to the sewer side of the trap.  The water that always sits in the bottom of the P-trap is what prevents sewer gases from coming in to the house.  The dishwasher drain must be connected before the P-trap, not after it, which is what was done here.  With this improper installation, sewer gases have the potential to come back in to the dishwasher.  The diagram below right shows a proper installation.

Dishwasher Drain Marked Up Dishwasher Drain Diagram marked up

No high loop

In the diagram above, right, you’ll notice that the dishwasher drain makes a high loop underneath the kitchen sink.  This is the minimum requirement on every dishwasher drain installation – it’s required by every manufacturer of dishwasher, and it’s also required by the Minnesota State Plumbing Code (section 4715.1250).  In fact, our plumbing code says “as high as possible under the countertop.”  An alternative to installing a high loop is to install an air gap at the kitchen sink – it’s a device that actually mounts above  the sink.  I can’t imagine why anyone would do this if they didn’t have to – how ugly.  In some parts of the country, these air gaps are required no matter what.  Too bad for them.

Dishwasher Drain Loop Photo Dishwasher Drain Loop Diagram

Although new dishwashers come from the manufacturer with the drain looped up at the side of the dishwasher, every installation manual still requires this high loop underneath the sink.

Dishwasher Integral Drain Loop

I’ve heard different reasons for why an additional loop is required under the sink, so I decided to contact the manufacturers directly.

I sent out an email to eight dishwasher manufacturers, asking them this question:

“A high loop is required on the dishwasher drain in the installation instructions for all of your dishwashers. What is the purpose of this? Doesn’t the high loop that is incorporated in to the side of the dishwasher achieve the same thing? Any insight or commentary in to this matter would be greatly appreciated.”

Here are the responses I received:

  • Kenmore: The high loop or air gap must be used to prevent potential backflowcontamination 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.
  • Maytag: No response after 29 days.  Boo.
  • Whirlpool: No response after 29 days.  Boo.
  • Amana: No response after 29 days.  Boo.

To summarize, the reasons for the high loop are to prevent potential backflow of water in to the dishwasher and to prevent improper drainage of water.  Even though dishwashers come from the factory with the drain looped up high against the side of the dishwasher, this is not an acceptable substitute for the high loop underneath the kitchen sink.  In Minneapolis, the lack of a properly installed high loop under the kitchen sink requires repair for their Truth-In-Sale of Housing evaluations.

Reuben Saltzman, Structure Tech Home Inspections – Email – Minnesota Home Inspections

        



About the Author

Reuben

Reuben is a second generation home inspector with a passion for his work. He grew up remodeling homes and learning about carpentry since he was old enough to hold a hammer. Reuben has worked for Structure Tech since it was purchased by Neil in 1997, and is now co-owner of the company. Reuben’s favorite customers are the ones who have a lot of questions; he grew up thinking he was going to be a school teacher because he enjoyed teaching others so much. In a sense, that’s a lot of what home inspections are about, so Reuben truly does what he loves. Reuben has an A.A. degree in liberal arts and has attended most of the Building Inspection Technology classes at North Hennepin Community College. Reuben and his wife are the proud parents of two young childen, Cy Alexander and Lucy Nicole, and have a German Shepherd named Stanley. With two young children Reuben doesn’t have much free time, but he still tries to play disc golf as often as possible during the summer. Reuben lives in Maple Grove, MN. Professional Qualifications / Memberships: *ASHI Certified Inspector *President, ASHI Heartland Chapter *Member, Minnesota Society of Housing Inspectors (MSHI) *Licensed Minneapolis Truth-in-Sale of Housing Evaluator *Licensed Saint Paul Truth-in-Sale of Housing Evaluator *Licensed Maplewood Truth-in-Sale of Housing Evaluator *Licensed Hopkins Truth-in-Housing Evaluator *Licensed Robbinsdale Point of Sale Evaluator *Affiliate Member, Southern Twin Cities Association of Realtors

117 Comments

  • ashwani bansal 10 MONTHS AGO

    I bought a dish washer recently but it is not working.....The machine keeps stopping every now and then ....what can be the readon

  • Rafal 10 MONTHS AGO

    I have Kenmore Elite Stainless steel dishwasher. We paid top of the price since we thought it will be great machine. Unfortunately since beginning we have some mold that is collecting everywhere inside dishwasher. We tried everything. Special liquids, vinegar, we cleaned it troughly in and out. I checked our hoses. We got bad smell and mold. Is there anyone who knows how to help? IN old house we had old Kenmore machine and it was working fine. Never smell. This Kenmore Elite we operate last 3 years with lots of problems. Please help.

    • Reuben Saltzman 10 MONTHS AGO

      @Rafal - I think you're looking for an appliance repair web site / forum.

  • Mike S 10 MONTHS AGO

    My dw drain is connected to the air gap directly. It bothers me that the drain water comes into the sink. After I replaced my air gap the dw drained great but i hate it coming into the sink. If I change this which won't be easy where do I put the air gap?

  • Jack Irick 10 MONTHS AGO

    The is not sufficient (>14") vertical space between the garbage disposal inlet from the dishwasher and the bottom of counter top. I plan to rig a check value at the inlet to the disposal. Any comments and any suggestion for a value brand or connectors.

  • Mike 11 MONTHS AGO

    If you don't think you need an air gap for back flow prevention and cross contamination, go drain that sink trap into a clear glass and drink it. Go ahead, I'll wait. You are connecting your dishwasher to your sewer, even though it's above the trap, the check valve in the dishwasher will not prevent cross contamination, unless it is specifically designed to do so, and if it is, it will require annual testing and certification. An air-gap device will eliminate the problem. Ugly? So what? Better than grunge in your dishwasher.

  • josh 11 MONTHS AGO

    I just installed a new dw and as soon as I finished I found black gunk in a shower I'm the basement and my washing machine. Any chance that it's related to the new install or just a separate issue?

  • Kathryn Soeder 11 MONTHS AGO

    I suddenly have a slow draining kitchen sink. The dishwasher backs up into it and sometimes when I run the disposal it backs up into the other side, It goes down but not like it used to. I did notice that my dishwasher drain line was just draped across under the sink and installed into the drain for the disposal free sink. (rental house) I took every pipe fitting I could apart and have snaked everything including the vent pipe outside on the roof. I am not finding much in the pipes. When I take the plug out of the bottom of the trap, water drains just fine from both sides of the sink. When I put it back and run water in the sink, it comes out into the dishwasher drain line and some comes out of the plug opening. It's acting more like an air pressure problem than a clog, but I'm out of ideas of what to do next and I can't afford a plumber. I doubt landlord would pay it. Thoughts please?

    • Reuben Saltzman 11 MONTHS AGO

      @Kathryn - what you're describing sounds like a clogged drain, which isn't a problem with your dishwasher. Do you have old steel drain lines? How long of a snake did you run down the drain?

  • Damon 11 MONTHS AGO

    Reuben, I removed my dishwasher to install tile and when replacing it I did not coil the drain hose beneath the DW as the original installer did. Will a vertical loop in the slack drain line put excess pressure on the pump or increase the risk of backflow, or will the this be okay?

  • Jimboep 12 MONTHS AGO

    I am planning to go through the floor, hook up to the main drain line running to the septic tank, and build my p-trap and standpipe behind the dishwasher. I am not planning to go through the roof with a vent for it. The standpipe for my clothes washer is the same way, only air is able to get in between the discharge hose and the standpipe. For the dishwasher, it doesn't have the same hook end on the discharge hose, so I thought I would hook it up to the standpipe just like I would to the drain under a sink. Do you think I need to put some time of cap on the top of the standpipe (about 22 inches above the floor)? Or do you think I can just leave it open? This is why I was kinda wondering if it would be better to get an air gap and mount it somehow to the top of the standpipe. I want to do this right the first time. I'm certainly interested in an informed opinion. Thanks for your help.

    • Reuben Saltzman 12 MONTHS AGO

      @Jimboep - I've seen dishwashers connected directly to standpipes without air gaps many times, but never lived in those houses to figure out if it caused a problem.

  • Jimboep 12 MONTHS AGO

    Hi Reuben and Ava, I think my situation is similar to Ava's. I am about to install a dishwasher that is not next to a sink. I am thinking I will put in a separate stand pipe with p-trap. I'm not sure how to connect the drain line to the stand pipe, since it is different from the one for a washing machine. I would like to connect it on the side of the standpipe near the top, then cap off the top of the standpipe. But I'm afraid that if I do this, since there won't be any way for air to get into the standpipe, this could create a siphon and draw water back into the DW. One of the Manufacturer responses mentioned putting and air gap on the stand pipe. Anyone know how to do this? Ava, it sounded like your problem may have been that there was no way for air to get into the left stand pipe. Does this make sense? What did you finally end up doing? Great posts, I would appreciate any insight you can give me. Thanks

    • Reuben Saltzman 12 MONTHS AGO

      @Jimboep - if the drain / standpipe is properly vented there shouldn't be any siphon. http://www.structuretech1.com/2013/01/plumbing-vents-why-houses-need-them/

  • Jonah 1 YEAR AGO

    Thanks Reuben! Dishwasher is now working great!!

  • Jonah 1 YEAR AGO

    I'm installing a new dishwasher, and was wondering about best location for the drain line. The old installation had the drain line entering the top of the cabinet to go to the sink drain. But with the new dishwasher this pinches the drain line behind the machine. I am thinking of drilling a new hole at the bottom of the cabinet, where the dishwasher won't pinch it, then I can do the high loop inside the sink cabinet. Does that work? Or should I just keep the original drain line hole? Thanks!

    • Reuben Saltzman 1 YEAR AGO

      @Jonah - yes, your idea would work fine. That's a common way of doing it.

  • Joel 1 YEAR AGO

    On occasion, when my garbage disposal has not been cleared, my dishwasher will leak water onto the floor and not drain properly. Clearing the disposal solves the problem and it then drains fine. My question is why does this happen? I would think that if the pump just couldn't get rid of the water it would just sit in the bottom and the float valve would prevent the machine from moving on in the cycle or bringing in additional water? What am I not understanding here? Is this a design glitch and if not...is there something I can do to prevent this from happening b/c I sure my family will continue to forget to clear the disposal before running the dishwasher....thanks!

    • Reuben Saltzman 1 YEAR AGO

      @Joel - sorry, but you'd have to ask an appliance expert. After I close the door and hit start on my dishwasher, I don't know what happens inside there.

  • Veanna 1 YEAR AGO

    We had a garbage disposal installed 2 years ago by a professional tech it fell off sometime between Saturday and Sunday. We went to clean a pan when we got home Sunday letting the water run to get hot. Had water come flowing onto the floor and into the basement through the holes for the plumbing. We got that cleaned up but we feel that the company that installed the disposal should be responsible for the expense of reinstalling the disposal. I've never had a disposal just fall off.

  • Naji 1 YEAR AGO

    What a great article. Saved me from tearing up my sink. Thank you

  • Ali Khan 1 YEAR AGO

    Just moved in to my new house and my BOSCH dishwasher smells. I read your blog and checked the installation of my dihwasher. It seems like my sink drain and dishwasher water disposal pipe apparently do not have a P-Trap and unlike most of the pictures online they are being drained towards the floor and not towards the wall. Wondering how should I address this issue.

    • Reuben Saltzman 1 YEAR AGO

      Ali K - I recommend you hire a plumber to take a look at your setup and correct it if necessary.

  • G 1 YEAR AGO

    Thank you, thank you, thank you soooo much!! I keep hearing it is user error for our dishwasher being backed up. Not after reading and looking, I can see it is the fault of the person whom installed it! We also had issue with the plastic pieces being found after it backed up the first time (of about a hundred times, lol).

  • Granny Corkie 1 YEAR AGO

    In above post, "Dishwasher had a terrible "STINK", not "sink"" Oops!

  • Granny Corkie 1 YEAR AGO

    Reuben, I posted a comment on August 11 re: my stinky, un-usable Bosch DW & the hose loop issue. Several weeks since, the installer from the place that sold me the Bosch came by, but said I didn't need a high loop since I had an air gap coming out the top of my sink. (And, my DW pump is working fine). He did admit that the DW had a terrible sink, however. I called Bosch about the smell. They said that if I don't have a high loop, they can't help me with the issue. Period. The hose currently runs from the dishwasher, along the cabinetry floor for 3'-4', (it runs through another narrow cabinet before it gets inside of the area under my sink). It then runs up to the air gap and the side of my garbage disposal. The installer says it's all hooked up correctly. Also - When the DW drained, I could easily see the water gushing into the disposal, so the hose is not blocked. (I've used both disposal & drain cleaner "just in case", but never noticed a smell coming up into my sink - only in the dishwasher & my dishes there). I can, however, see some water sitting in the part of the hose that is running along the floor of the narrow cabinet next to the dishwasher. It's been nearly 2 months since I've been able to use my DW!!! Would your opinion be to still have a high loop put in, (requiring a longer hose), even though I have the air gap? The company from whom I bought the DW is anxious to help if they can. I'd sure like to be able to use the doggone thing again! (It's just under two years old.)

    • Reuben Saltzman 1 YEAR AGO

      @Granny - an air gap is the best type of installation, and is required here in Minnesota on commercial installations. A high loop is allowed as an alternative in residential installations. If you have an air gap at your dishwasher, you definitely don't need a high loop.

  • Suz 1 YEAR AGO

    Thanks for the quick response. I reread the Bosch DW Manuel and it instructs me to open the door all the way until I hear the water drain out (I heard it gurgle). Then I lifted the clear drain hose from the DW under my sink so the old water sitting in the hose could enter the garbage disposal yesterday and no smells yet! Living in the south I'm too familiar with the smell of mold and mildew. The smell was like an old dirty wet dish rag or sponge. I had sewer smells during construction before the plumber put in the pipes to the sewer system, and it wasn't that smell! I wouldn't be able to stay in the house. My nose or sense of smell is very sensitive. That said I'm still curious if there are alternate plumbing methods to fixture double sinks (1w/disposal) without a p trap? And, why would anyone design a DW so that dirty water sits in the drain hose until the next wash cycle of dirty water pushes the old cycle out? Thanks again!

    • Reuben Saltzman 1 YEAR AGO

      @Suz - good question. It should be quite simple to move the hose around a little so water never sits stagnant.

  • Suz Swanson 1 YEAR AGO

    I have new construction home with kit island and new Bosh DW which has the plastic hose loop coming through the top of the inside cabinet under the double sink - which sounds to be the correct loop. After that plastic drain line comes from the top it then falls to the floor of the cabinet then travels back around several more inches until it goes back up to enter the garbage disposal drain which is about 10" from the cabinet floor. As you can imagine the hose is full of water. It seems too long and the bosh installation only says not to cut the hose. Which is what I thought should be done so water doesn't sit in it. Which could be the cause of the moldy mildew smell? My problem has to do with a mildew smell coming from my kitchen sinks. I've tried everything to clean them and the garbage disposal to no avail. I don't notice any smell from the dishwasher. there are one to two people using dishes and we always scrape and rinse dishes and don't really use the garbage disposal. A home inspector told me (before i noticed the smell) there is no p trap and the plumbing would be difficult to get to for repairs with the lines going under the floor of the cabinet island. (Which is hopefully where a p trap may be?). I could send pics. I'm not sure if its the hose holding water and how to remedy if it can't be shortened, or if its the plumbing connections without a visible p trap? Any questions or suggestions? Thanks for being our professional out there to help us solve these crazy issues!

    • Reuben Saltzman 1 YEAR AGO

      @Suz - if there is no trap underneath your sink, that's definitely the source of your odor problem. It's not mildew you're smelling, but sewer gas. That's a health hazard that you should have a plumber correct.

  • Granny Corkie 1 YEAR AGO

    Thanks for the great site - you MAY have solved my problem. [For Kevin who wrote some time ago that Bosch did not require a loop, my written manual specifically states to use one.] However... My installer looped the drain hose behind the dishwasher, then the hose runs along the floor of my inside cabinet and straight up to my garbage disposal! The DW smells so bad that I had to stop using it several weeks ago. I had to soak all of my dishes in vinegar because the DW smell permeated them. On one of the last runs, the water in the dips at the top of my "clean" drinking glasses was a bit "milky" looking. Bosch just said "run it with vinegar". I ran 3 loads with no dishes & vinegar, removed & cleaned the spray arms & filter, and washed down the stainless steel inside - and it still stinks. I've also used a cleaner on my disposal & my other sink drain, although there was no odor in either one. After all that, two weeks not being run at all, it still smells inside. An installer from the company from which I bought the DW 1 1/2 yrs ago is coming tomorrow - and I'm showing him YOUR article! By the way, Google "How to fix an odor from a dishwasher", and you'll get hundreds of hits - mostly by people with Bosch DWs. What gives?

  • John Carver 1 YEAR AGO

    Interesting that of the manufacturers that responded, only Viking actually addressed the question - why two loops? And the reply from Bosch doesn't even make sense: a dried-out hose keeping odors from backing up? As there's agreement that the loops prevent backflow and siphoning, I can see 2 sources for that - water remaining in the drain hose and water flushed down the sink. A loop by the sink would keep water from the sink running down into the hose; a loop by the DW would keep the water in the hose flowing back to the DW. What do you think? I came upon this site trying to find a way to deal with a dishwasher that's a long way from the drain (15'). No option to drain to a different location. Still looking for a creative solution.

  • Rafal Kujszczyk 1 YEAR AGO

    I purchased Kenmore Elite Dishwasher. It was top of the line model. Last 2 years mould is building up all over the place inside dishwasher. On the racks, walls etc. We tried to clean it with vinegar, bleach, expensive cleaners, baking soda etc. Nothing is working. Also there is some odor. What can cause this problem? Any solution? Please help.

    • Reuben Saltzman 1 YEAR AGO

      Hi Rafal, I recommend asking this question to the manufacturer or at a forum for appliance repairs.

  • MP 1 YEAR AGO

    Reuben, first off great information on this topic Thanks!! Would it make a difference if I connect my drain hose into the drain pipe that runs across from 1 sink to the other? I'd still use the high loop and it's before the p-trap as well, just not sure if there are any hazards to that ? Thanks again, MP

    • Reuben Saltzman 1 YEAR AGO

      MP - I think the dishwasher drain is supposed to be connected to a vertical pipe, not horizontal. I've seen horizontal connections done before and I've never made a big deal about it though.

  • Pam 1 YEAR AGO

    Is this something that I could do myself, or should I hire a plumber? It was something noted on my home inspection.

    • Reuben Saltzman 1 YEAR AGO

      @Pam - this is probably something you could do yourself.

  • Nigel 1 YEAR AGO

    I just installed a Whirlpool DW and for the most part I think I followed all the directions. Before shoving the DW back into place and securing it in place with screws, I wanted to run it for a couple of minutes and then stop it and hit drain. Because part of the drain pipe is that white semi-transparent plastic, I could see that after the drain ended there was still some water in the drain pipe. It's a new DW and the water used for this "test" was obviously clean. So I have 2 questions.. First, is there suppose to be water in the drain pipe after the DW is finished running a full cycle? And secondly, wouldn't that water that gets trapped or left over in the drain pipe normally be dirty water? Thx for help.

  • chuck 1 YEAR AGO

    I have a 4 year old Viking dishwasher with the pan under components keeps filling I have after removing kick plates and running it through a cycle identified a leak around dishwasher drain hose where it leaves the back of dishwasher I am assuming I will need to pull dishwasher out from cabinet to either replace or repair this hose? If not how can I fix with the least amount of hassle Chuck

    • Reuben Saltzman 1 YEAR AGO

      Hi Chuck, I recommend you contact someone who repairs appliances or try an appliance repair forum.

  • Josh 1 YEAR AGO

    Hi, I have installed a new sink and I am using an air gap on top of the sink. Ever since the new sink ha been installed my dishwasher is spewing water out of the air gap. I tried using the high loop first so that I didn't have to have the air gap on the counter but water was coming out under the sink so that's why I went back with the air gap but I'm having the same issue. Any ideas I what it could be? I was thinking that the drain pipe into the disposal may be too long and it goes below the disposal and then tries to come back up into the disposal. Could this be the issue? Thanks

  • Mike seat 1 YEAR AGO

    Reuben, We have had this smell coming from our dishwasher since we moved into our house 6 years ago. It was a new home construction. My wife has finally had enough and wants me to find a solution. The hose appears to be installed correctly and we do have an air gap. Do you have any ideas as to what might be the issue? Our dishes most often smell horribly. Thank you!

    • Reuben Saltzman 1 YEAR AGO

      Mike - do you have a Bosch? I've heard they're super quiet because they don't have a food grinder, so food needs to be manually removed. If that's not it, I recommend you contact the manufacturer.