Laundry Room Sewer Smell Solved – Plumbing Trap

By Todd Fratzel on Plumbing

Sewer Smell – Washing Machine Plumbing Trap Problem

Recently a good friend of mine asked me what might be causing his laundry room to have a sewer smell. Obviously this was a serious problem that needed to be corrected immediately. Not only are sewer gasses an unpleasant experience but they can be a safety concern. If you smell sewer gasses in your home you should try and diagnose the problem sooner than later.

familyGuy Peter stinky 72 240x300 Laundry Room Sewer Smell Solved   Plumbing TrapI started quizzing my friend about any recent changes in their home that might have contributed to the sewer smell. He asked me if washing machine mold might be causing the smell or some type of dead rodent.

He wasn’t sure that anything had changed other than they had recently purchased a new washing machine. I knew there was likely something wrong with the plumbing trap (p-trap) which was accounting for the sewer gas smell.

The culprit ended up being the washing machine hose that was incorrectly installed by the appliance delivery guys. Read on to see what happened!

Plumbing P-Traps

Plumbing traps (p-traps) create a trap in the drain pipes which retain a small amount of drain water which in turn creates a water seal to prevent sewer gasses from entering the living space of buildings. Traps come in several different configurations but they all work the same way by trapping water in the pipe to seal out the sewer gasses. Traps are present for all sinks, showers, toilets and drains (such as washing machine drains).

Typical Washing Machine Drain Connections

Washing Machine Drain Box 300x225 Laundry Room Sewer Smell Solved   Plumbing TrapToday most washing machines come supplied with a flexible, corrugated plastic drain hose. The hose is easily inserted into a standard washing machine drain box (see photo). The washing machine drain box is hooked up to the houses waste piping that contains a P-Trap inside the wall (or exposed in older homes).

With this set-up waste water comes out of the plastic washing machine hose and empties into the drain box pipe. The waste water fills the trap below and creates a seal that prevents the sewer smell (gasses) from entering the home. See the next diagram for a typical washing machine drain set-up.

Correct Washing Machine Hose Drain Detail Laundry Room Sewer Smell Solved   Plumbing TrapIn-Correct Washing Machine Drain Hose Set-Up

The following diagram shows what happened at my friends house. The delivery guys from the appliance company delivered the new washing machine and hooked it up. The ended up pushing the flexible plastic washing machine drain hose into the drain box and past the P-Trap.

Incorrect Washing Machine Drain Hose Detail Laundry Room Sewer Smell Solved   Plumbing TrapAs you can see in this diagram the flexible washing machine drain hose prevented waste water from filling the trap. This allowed the sewer gasses to pass around the house and trap and into the house. To fix this problem all he had to do was pull the washing machine drain hose out of the drain box until it was only inside by about 8 inches or so.

Obviously this was a rare situation and one that I’m still scratching my head about. For the life of me I can’t figure out what the delivery guys were trying to do. However, this just shows you how important the plumbing trap is and how it works.

About the author

Todd Fratzel

I'm full time builder for a large construction company in New Hampshire. I run their design-build division that specializes in custom homes, commercial design-build projects and sub-divisions. I'm also a licensed civil and structural engineer with extensive experience in civil and structural design and home construction. My hope is that I can share my experience in the home construction, home improvement and home renovation profession with other builders and home owners. I'm also the author of Tool Box Buzz and Today's Green Construction. Please feel free to contact me if you have any questions, suggestions or you'd like to inquire about advertising on this site.

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30 Comments

  1. Shohana says:

    Did you have to pull it back to stop getting the smell? We have same issue for a while. The smell comes and goes. The cloths does npt smell but we get it often…specially early in the morning and when we come back home from out side.

    • Todd says:

      Shohana – The hose should only be inside the drain tube a few inches as shown in the sketch. If the end of the hose is touching the trap water it’s too far.

      • jJim says:

        Todd,
        I need some advice.
        I too have the sewer(septic tank in my case)odor.
        My house is a view-out with the washer on the main floor.
        I have a sump pump in my basement with a floor drain about a foot away from it for the a/c and water heater.

        When we do laundry and the machine cycles it’s water out you can
        see water rise in the floor drain and it spills over into the sump pump.

        I had the septic tank cleaned out last month thinking that would take care of the odor, but it hasn’t.

        What do you think?
        P-trap?
        clogged vent pipe?
        sewer line clogged (under basement floor)?-the clean out just outside the house is not clogged.

        Please help-the wife is getting upset.

        Thanks,
        Jim

        • Todd says:

          Seems as though something isn’t vented properly or the hydraulic grade (height) of the floor drain is very close to the level of sewage in the tank. You really should get a licensed plumber (other than who built the place) to come look at it.

  2. Kirstin says:

    I have a similar problem, but my “drain smell” only seems to happen when I do a load of laundry after a few days. Then it goes away until I let it sit for a while again. As long as I use my washer a few times a day, no smell, otherwise the next time i have a horrid smell in my laundry area. Could this be something similar>

    • Todd says:

      Absolutely. Do you have a drain hose on the back that goes into a washing drain in the wall? Did you check to see how far the hose goes into the wall drain?

  3. Mary says:

    I have the same problem except I can smell the sewer gas bad when I have the washer running and the a/c unit on. Coud this be something different?

    • Todd says:

      It’s likely that they are related. I would certainly check the drain hose first. See if that is the problem. It might be that your A/C system is creating negative air pressure that’s sucking the sewer gas out of the trap.

  4. Brandy says:

    We just bought a new house and we have an oder pretty bad from our washer drain. My washer hose is only in about an inch because it had a guard to keep it from going in too far. I have used rid-x, bleach and drano type cleaers with no luck. Our house was custom built by the original owner. How do we find out if there is even a p-trap hose installed? Is it even possible for a house to be built within the past 8 years to not have a p-trap?

    • Todd says:

      Brandy – You can figure it out with an inspection camera. Most plumbers today have a very small camera that’s attached to a long snake like flexible rod. They should be able to put it in the drain and very quickly determine if a trap exists. Sure sounds like you don’t have one. You should get this looked at ASAP as it could be a serious health concern.

  5. idc says:

    I have a hose leaving my washing machine that fits into an L shaped piece of Pvc pipe. The Pvc pipe then leads to a drain on the floor and sits loosely on top of the drain. We are having the same smell you describe here. So bad so I can’t sleep at the moment. Please help! I think I need a more airtight piping…I know very little plumbing but its the holidays and I can’t get my husband to do anything about it even if it weren’t

    • Todd says:

      First of all if the pipes are loose and sewer gas is getting into your home it’s a serious health hazard. If you or your husband can’t fix it you really need to call a plumber. Washing machines MUST be drained into a trap and water tight plumbing.

  6. Dave Bowers says:

    Hi Todd,

    I have a drainage problem, but no oder. My HE washing machine pumps up about 7 feet to the main house line going to septic. From the main line I have a p-trap and a vent, and they both seem to work fine. The problem I am having, is (grey) water is back-flushing into the machine when the spin cycle stops. This is the water that is still in the hose and doesn’t make it to the discharge drain. It makes sense that this would happen, however, the manufacturer says it can be as high as 10 feet (max). My question is why doesn’t it work? The manufacturer will not hold to their printed claim, and I have now tried everything to fix it. I have extended the flex hose to leave a large trap below the washer line; I have added in a check valve, and nothing has worked. Now I have removed everything and decided to drain into the shower. The problem I am having now is water is being suctioned back to the washing machine because there is no vent in the line to the shower. Please offer a solution or something to try. My next trail is to instal a PVC Y with the machine’s flex pipe going into it, the other end venting up, and then connecting to the 1″ flex pipe to the shower, which in theory should have a p-trap installed.

    Thoughts???? Wife is about to pull her hair out and scalp me I think!!
    Dave

    • Todd says:

      Dave – While most manufactures say they can pump a certain amount of vertical head the reality is that never works well AT ALL. In fact, in some jurisdictions the code requires that you dump the water into a sump and pump it up to the sewer. Honestly, if you want to get it done right, create peace in your home, it’ time to get a licensed plumber out to look at it. Wish I had better advice but these type of situations really do require a professional. Good luck!

  7. Jim says:

    A drain box was installed in our new laundry room.
    I just discovered there is not a trap in the drain
    line. Instead of cutting the pipe and installing a
    trap,is there a way to seal the washing machine hose
    where it enters the drain box?
    Thanks,
    Jim

    • Todd says:

      NO! The gases could still enter the washing machine. Did a licensed plumber install that? It needs to be corrected immediately as it’s a big health risk. Traps are one of the VERY most important features in all drain lines.

  8. John says:

    Thanks for the article. What you described is exactly the problem I had (down to the installers of our new washer being the guilty party).

    I pulled those back….problem solved!

    • Todd says:

      John – Glad you found the article helpful. I sure hope you’ll consider signing up for our Newsletter or sharing the article on social media. We sure could use the help and exposure.

  9. Anonymous Coward says:

    Thank you so much! I could never pinpoint the location or cause of the mysterious smell that started the day our new washing machines arrived. I thought it might have been related to our water softener, which drains right into the same drain pipe. You just solved my household mystery.

  10. Karen says:

    Good evening Todd,

    I’m installing a new tub and tub surround. I’m changing the hot n cold water pipes which are Copper to Pex tubing (shark bite fittings etc) my question is concerning grounding. I believe my pipes are grounded, so when taking a shower if lightening strikes it is redirected. When I change it to Pex then how can I ground it? Can I add a tracer wire, or is there something else I can do. Appreciate any advice. Safety is a number one concern to me.

    Thanks,

    Karen A-G

    • Todd says:

      I’m not plumber, but I can tell you that I’ve never seen a ground wire installed in the new homes we build that use PEX piping. The only time we see one is a tub with a motor (hot tub, soaking tub, etc). You can certainly check with your local building code official or a local plumber to confirm this.

  11. Jean Hawks says:

    I have a problem with my washer drain. It seems to be a drain problem but I can’t really identify it. The drain pipe is allowing water to spill out at the beginning of the drain cycle through the holes in the rubber gasket at the top of the drain pipe (standpipe). We opened the clean-out to allow water to drain to the outside. Nothing comes out the clean-out. It doesn’t act like a blockage exactly but it certainly isn’t draining properly. My floor is going to be ruined if this continues. What else can I try?

  12. Andrea says:

    We have three bathrooms with showers/tubs. When I use one of the showers in the upstairs bathroom, I notice the odor which was mentioned coming from the laundry room. I do not notice it as strong when we do any laundry. Any ideas on why the odor is so strong when this one bathroom shower is being used?

    • Todd Fratzel says:

      Most likely there is poor venting at the washer drain or the trap is being siphoned out. Sounds like a job for a plumber to evaluate. Good luck.

      • Andrea says:

        Thanks Todd… after reading more of the other questions/answers, we also noticed that we had the drain hose installed to far down into the pipe. We pulled it back and re-inserted it about 6″, and I didn’t notice the smell in the laundry room this morning after my shower. We will keep an eye on it. Thank You!

  13. Denise says:

    Hi Todd! I have the same sewer smell issue. Im not doing any home improvement but i did recently purchase a new he washer and dryer a few months ago. I started to notice a sewer smell when i would use my washer but not only that when it was not in use, the smell would be stronger. Now we could smell the sewer smell from our shower drains and most recently the kitchen sink! I have a septic system. We purchased our home 3 years ago and we were first time homebuyers. We don’t know anything about this. I understand you are not a plumber but any advice you can give, would be greatly appreciated! Thank you!

    • Todd Fratzel says:

      Did you at least check to see that the washer drain hose isn’t too far into the drain? It’s also possible that when the washer is draining, if the venting for that branch isn’t proper, that the drain water is siphoning the other traps dry, letting gas release from the other drains. You may need to get a plumber involved.Typically, this type of issue has nothing to do with the fact that you’re on a septic.

  14. Susan says:

    We moved our laundry room from first floor to basement. Was fine for a few years. Then we started noticing an egg/sulfur smell when doing laundry – but only where old laundry room used to be (now pantry). Nowhere else in house, not even new laundry room.
    We now had new leech field installed with filter system(old system had no filter)
    Now we are getting a septic smell in pantry. Again no where else but old laundry room.
    It is only slight while using water for showers, dishwasher, sink, etc. but very stinky when doing laundry. Someone suggested old laundry room pipe was not capped or perhaps something with old drain.
    Can you advise. Could this small be from old pipe left in wall in old laundry room that was not sealed off right
    Thanks
    Susan

    • Todd Fratzel says:

      Susan – That’s exactly what the problem is. If that laundry drain wasn’t capped, you’ll get sewer gases out of it because there’s no water going down it to fill the trap. This should be a very simple fix. It’s important that it be taken care of soon as sewer gases can be dangerous. Good luck.

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