Dishwasher drainhose to sewer pipe, bad idea?

Discussion in 'Plumbing Forum, Professional & DIY Advice' started by jlubra1, Feb 6, 2007.

  1. jlubra1

    jlubra1 New Member

    Our dishwasher smells, not uncommon. Our dishwasher drainhose connects straight to the garbage disposal. I don't think there is a "loop". But that is to prevent water backup I believe.

    We are thinking of connecting the drainhose directly to the sewer line in the basement. We would use PCV piping with a trap, and have an adapter at one end to accept the dishwasher drain hose.

    I do think the smell is simply caused by not running the washer enough, and letting dirty dishes and glasses sit too long in there. Or, simply not adding baking soda to the disposal.

    Is this project a sound one? I'm wary it may cause worse problems, like a possible sewer back up into the washer. And I just don't like the idea of opening a hole in the sewer line. But this isn't my call, I'm the helper.

    Do you agree? I appreciate your feedback.

    Last edited by a moderator: Sep 28, 2007
  2. jimbo

    jimbo Plumber

    San Diego
    You cannot and must not make a direct connection to the sewer. It needs to be an indirect connection, such as you get going into the disposer, or through an air gap into a tee.

    If your machine smells, it is probably not pumping out completely.
  3. jlubra1

    jlubra1 New Member

    You say "can not" and "must not". Is this a Code violation?

    Also, in this case, can you explain an indirect connection? Thank you.
  4. norcal1

    norcal1 Plumber/Owner

    Seems like a waste of are definately not going to help improve the smell.

    Why go to all that work, when you have a perfectly fine connection at the garbage disposal?

    The p-trap under your kitchen sink keeps the sewer gases from coming into your dishwaher drain hose. Again, it doesn't make any sense to do what you are planning on doing.
  5. jlubra1

    jlubra1 New Member

    I will make sure to run the washer and disposal every night this week, see if that helps curb the odor.
    Last edited: Feb 7, 2007
  6. leejosepho

    leejosepho DIY scratch-pad engineer

    No, but it would *produce* a lot of sound when the new hose immediately begins to double as a sewer vent and begins to gurgle, followed by belching up plenty of sewer odor when its trap is sucked dry.

    Now if you already had a vented line running down to the sewer line, you *could* put a tee in that line and add a trap to receive your drainhose output ... but you already have precisely that all tucked in very nicely right there under your sink!
  7. dubldare

    dubldare Plumber/Gasfitter


    As long as there were no direct, physical connection between the standpipe of the trap and the hose from the dishmachine. This would mean that if the trap were fouled, the waste could freely spill.
  8. leejosepho

    leejosepho DIY scratch-pad engineer

    I think I understand ... like with a washing machine hose simply hanging loose down inside a stand pipe with a trap at the bottom? I do not understand why it must be that way (other than for venting), but I do understand that is how it is done. Overall, however, I was not trying to be a plumber here. Rather, I was trying to reiterate to jlubra1 that the existing drain connection is fine and is not the source of the odor problem.
  9. dubldare

    dubldare Plumber/Gasfitter

    We don't care about venting when it comes to the inlet side of the trap. The atmosphere and gravity take care of that for us.

    Venting exists solely to protect the water seal of the trap (and to ventilate the sanitary sewer system, but we won't get into that now), which prevents the sewer from communicating with our world.

    The reason for no direct connection (with a trap below the dishwasher) is sanitary in nature. If that drain should become plugged, we do not want the waste to contaminate the dw. Therefore, the water should spill before it can encroach the dishmachine.

    This is why air gap fittings or high-loop air breaks are used normally. Most codes prohibit wastes from one story from discharging into another, so the idea we're talking of is thrown out the window.

    When the dw discharges on the same story, prior to the sinks trap with a high loop, the sink will be backed up before water can encroach the dw. With an air gap fitting, it the same scenario, you will have a ton of water on the countertop before the dw can be compromised.
  10. Cass

    Cass Plumber

    It is illegal to connect directly to a trap.

    Like jimbo said, it must have an air gap.

    I would run a little bleach through the dishwasher a few times this week separate from the dishes.

    Is the garbage disposal old?

    If there is no high loop with the DW hose make one.
  11. jlubra1

    jlubra1 New Member

    We don't have this loop in the drain hose at present. The hose simply connects from the dw straight to the hole in the disposal.

    What is the function of the loop? To prevent drain-water backing up to the DW due to the pump action? Does it help minimize odor coming from the disposal?

    I would like to make this loop, as well as the other simple measures mentioned, before attempting something complex and risky.
  12. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    New England
    As already mentioned, the loop prevents a stoppage from allowing the waste to back up into the DW.
  13. The need for air explains to me why I hear so much gurgling in my kitchen sink drain. I have no disposer and no air gap. The dishwasher is connected to the sink drain just before the P trap.

    If I renovate the kitchen and want to eliminate totally the noises the dishwasher makes when draining, what are my options?
    a.) buy a more modern dishwasher? (I have no idea if they drain silently.)
    b.) install a standpipe (like for a washer) -- which still makes noise, doesn't it?
    c.) do something a little more complicated, with P traps, air gaps and disposers...

    I don't know what to do.

  14. I had a family member today call me to do the rerouting of drain lines for a new kitchen.

    As I started to explain the scenario of "what you had" and "what you will have" became a distortion match between differing opinions.

    I mention the vent NOW has to be ran to the roof, whereby there was none before but that is not my concern.

    Also, I mentioned the mandatory Air Gap which was a feeler question since I know granite tops are going in necessitating the 5th hole.

    Once again, they stated they didn't have one before and have no intentions of doing it now.

    Family or not I ditched the work. My distant cousin is going to do the work and I already know that there is going to be a S-trap under the sink with a direct connection to the dishwasher. This line is going to be joined with 1.5" jumping off a laundry tub because they do not want to dig the floor up and catch the 4" clay underground and turn out 2" like it should be. (DFU load on 1.5" is 2)

    It's going to be an awesome looking kitchen but it's not going to drain...and the first time too much vegetation goes down the drain and it backs up, it's heading right towards the dishwasher.

    It's amazing how people try diligently to avoid codes, and think there's nothing wrong with it.

    I'm glad I don't have financial limitations that could force me to "look the other way" and take on jobs like these and install the plumbing incorrectly.
  15. kordts

    kordts In the Trades

    exurban Chicago
    How about running some ice cubes through the disposer?
  16. hj

    hj Moderator & Master Plumber Staff Member

    Cave Creek, Arizona

    The dishwasher is pumping water into your sink drain. It will ALWAYS make a noise unless it is connected directly to the drain, and that would create potential health/sanitary problems.
  17. adrianmariano

    adrianmariano New Member

    My dishwasher is connected to the disposal. It drains pretty quietly. I think I hear the sound of the pump in the dishwasher but no gurgling. Sometimes if I forget to run the disposal the sink will start silently filling up as a reminder from the dishwasher waste water. So I would say that there is no significant noise even though the drain is not directly connected.

    Is it a sufficient air gap for my dishwasher if the drain hose makes a loop up to the top of the cabinet and then descends to connect to the disposal? (It would seem like the disposal would provide the air gap.)
  18. kd

    kd New Member

    I think DW drain works this way, but please correct if I am mistaken. (1) Airgap- DW pumps waste water up to the airgap, DW waste flows into disposal or a T above the P trap. After the DW shuts off, a small amount of DW waste in the tube flows by gravity back into the DW-- it is visible in the bottom of the DW. (2) Loop up to the bottom of the countertop- It functions the same way, except if you have a clog in your sink drain, or the disposal is stuffed with garbage the disposal will fill with standing water and it will siphon into the DW after the DW shuts off. This is bad news so if you have a loop you should make sure that your disposal and drain are clear.
  19. jlubra1

    jlubra1 New Member

    Final thoughts and questions for anyone in this thread

    1. First off, our drainhose does have the "high loop" before connecting to the disposal. Now, some amount of water siphons back into bottom of dw after running, but the appliance rep told me this is normal, helps keep the dw seal from drying out.

    2. And I still maintain the smell is simply from letting dishes sit too long in the dishwasher. I've been sticking (literally) my nose in the sink drain, I never smell anything odd from the disposal.

    3. Also, this direct dishwasher-only-to-sewer connection I mentioned from the onset ignores something important, I think. Because it would be an independent run, there would be no utilization to an air vent or air pipe, whatever you call it - you know, those pipes sticking out of our roofs.

    4. I don't quite know their purpose, and what happens if you don't have it, all I know is you need it.

    5. Someone mentioned that the dw pump would suck the trap water out, followed by sewer gas! If I'm following you, this would be due to the action of the pump, after it pumps out the rinse water, there is then a slight siphoning back up that occurs?

    6. Finally, someone mentioned a building code violation, how is that?

    So, I'm no plumber or handy man, I'm learning from you guys. I do know to be cautious. It's the homeowner who gets these ideas, (ex engineer) then he's off to the races.......

    Please do not hesitate to comment on my points and questions. Thanks!
  20. Replacing air and water, as water flows downhill

    Almost all these points show you need to understand venting. Think of air as being not compressible. Think of a straw and how you can block the flow of water by blocking the flow of air - with your finger on the top of the straw.

    Same thing with fast and large volumes of flowing water. Wouldn't happen if all drains were a trickle like a finger rinse or toothbrush rinse. Sucking the P trap dry is what people say when they mean that your water draining will suck dry ANOTHER P trap in the system, to get the quantity of air it needs to keep flowing. It can also suck dry its own P trap too. A moving "plug" of water displaces air as it moves through the pipe. It needs air on both sides for it to keep moving, otherwise it slows down to a trickle, and it won't do that in any well-behaved fashion at all.

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