How to Fix a Sewer Gas Smell

This project explains how to fix a sewer gas smell coming from under the bathroom sink by replacing the one-way air admittance plumbing valve.

How to Fix a Sewer Gas Smell

A distinct sewer gas odor was coming from the bathroom sink area. At first I wondered if there were a leaky pipe or such. Sniffing around, it was clearly strongest under the vanity. I discovered the Studor® Mini Vent, a plumbing air admittance valve had gone bad, allowing sewer gases to back flow into the house.

Studor Mini Vent under Kitchen Sink
Studor® Mini Vent under the Bathroom Sink

Studor Mini Vent

The Studor Vent is a one-way air admittance valve that allows air to flow into the drain pipe but not out into the room. Running water going down the drain pipe creates a vacuum that could siphon the U-trap dry. The purpose of the U-trap (also called a P-trap or J-trap) is to create a water seal to prevent sewer gasses from coming back up the sink drain. The Studor Vent allows air into the pipe to break the vacuum preserving the water seal in the U-trap.

The Studor vent is commonly installed under the sink at least 4″ inches above the weir of the trap in situations where connecting to a vent pipe to the roof is not practical. A quick inventory of my house found a Studor Mini Vent is installed at nearly every bathroom sink and the kitchen sink.

Studor Mini Vent Air Admittance Valve

Studor Mini Vent Air Admittance Valve

Plumbing Air Vent Malfunction

This vent is about 8 years old and malfunctioned because flies and mosquitoes (yuck!) from the sewer line had clogged up the valve, preventing it from fully closing to seal out the sewer gases.

Studor Vent Clogged with Insects

Studor Vent Clogged with Insects

Install the New Sewer Vent

A new Studor Mini Vent was purchased at the local home improvement for about $23.00. The old vent was removed with a pair of Channel Lock Pliers because the threads were tight due to the hardened plumbers putty on the pipe threads. Aside: Do not use Plumber’s Putty on the Studor Vent threads! The plumber made a mistake when the original vent was installed. Be sure grasp the main pipe firmly to minimize torquing the PVC glue joints when unscrewing the old vent.

The threads of the new Studor Vent are wrapped with a two or three turns of PFTE thread seal tape or Teflon Thread Seal Tape (if using the Du Pont name brand tape as I am), then new vent screwed on hand tight:

Studor Mini-Vent Installation

Studor Mini-Vent Installation

Replacing the Studor Mini-Vent only required 5 minutes or so to do the job.

Fix a Sewer Gas Smell: Studor Mini-Vent Replacement

Fix a Sewer Gas Smell: Studor Mini-Vent Replacement

Studor Mini Vent installation instructions are available on the manufacturer’s web site here (see page 15).

Additional Studor Mini Vent installation photos are available here about 1/2 down the page.

Faucet Connectors

While I was working on the sink, I also replaced the original cheap gray plastic hot & cold water faucet hook-up lines with heavy-duty vinyl faucet connectors reinforced with a braided stainless steel wire. I worried those cheap gray lines might burst and flood the house some day. An easy job, just turn off the faucet water supply valves inside the sink cabinet and the rest is working with two adjustable wrenches.

Hope this saves you some money.

Bob Jackson

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22 Responses to How to Fix a Sewer Gas Smell

  1. Suzette C November 22, 2009 at 5:33 pm #

    This is probably what’s going on with my house. My kitchen stinks. Especially under the cabinets.

  2. Pat O'Neal August 27, 2012 at 9:33 am #

    The Studor Air Admittance Valve comes with a limited lifetime warranty, even though after 8 years of service and the flies and mosquitoes where the problem. A simply call to our 800 447 4721 number would direct the customer to the closest supplier for replacement. We do not pay for labor to reinstall the air admittance valve. We only replace the product. Please note that “Teflon” type plumbers tape should be used not plumbers putty. The putty could contaminate the diaphragm and is stated on the valve.

    Regards,

    Pat O’Neal
    Director Technical Services
    Studor

  3. Rolin N October 2, 2012 at 2:25 am #

    I recently experienced a sewer odor problem under my bathroom vanity. I traced the odor to my Studor Mini-Vent which was installed 12 years ago when the house was built. I removed it (plugged the sewer vent with a plastic bag with masking tape) and noticed that inside was dirty with a residue and laden with dead bugs. I shook all the bugs out and cleaned the unit with an all-purpose household cleaner. To completely remove the residue behind the plastic grill, I poured a small amount of household bleach in the unit and let it soak overnight. The next day, I rinse out the unit and blow dried inside with a hair dryer to ensure that it was completely dry before reinstalling. I has been a week and the sewer smell has not returned. So, unless the valve/diaphragm is damaged, cleaning maybe an alternative to replacement.

    • Bob Jackson October 2, 2012 at 4:04 am #

      Nice restoration work. The Studor Valve has a lifetime warranty as noted in Pat O’Neal’s comment should you prefer a replacement.

  4. Barbara Bowles November 18, 2012 at 1:16 pm #

    Dear Bob, thank you for clearing this problem up for me. I just built a house and moved into it the toilets were the last to be installed. So once I move in an started using the plumbing the smell started coming from the cabinets and drains in the kitchen and bathroom. I complained to the builder and he said he didn’t smell anything. I quit trying to get rid of the odors and went off for the weekend and invited him back and he said he still did not smell it. When his wife walked in she said OMG what is that awful smell. He replaced the bathroom but not the kitchen. Now the odor makes me gag when I try to cook so I am eating out I keep the bathroom, laundry room, and bedroom closed so the smell isn’t so bad. Now after reading your comments and website I think I can replace the one in the kitchen, without dealing with the builder.

    • Bob Jackson November 18, 2012 at 5:31 pm #

      Hi Barbara,
      Have you called you local Building Dept. and asked if the inspector would check your plumbing to verify it’s up to code? Explain that your new house has plumbing ventilation problems.

      Take care,
      Bob

  5. Ray Mickey January 4, 2013 at 5:55 pm #

    Terrible odor under our bathroom vanity (has a studor mini vent). Home only 2 yrs. old. Builder had plumber come – when he removed studor vent it was full of mold as well as all the pipes?? He installed new pipes and new vent but odor is coming back. HELP! How do you know if a studor vent is not working? I know mold is a health risk – what about sewer gas?

    • BobJackson January 5, 2013 at 9:48 am #

      Hi Ray,
      Based on your description – 2 year old home, excessive mold in the vent pipes, clogged Studor vent, plumber replaced the pipes, recurring sewer gas problem – I think your home plumbing vent system is flawed. The premature failure of the Studor Mini-Vent (which has a lifetime guarantee) is just a symptom of a more fundamental plumbing problem.

      Does the sewer gas odor go away for days or weeks after the Studor vent is replaced? Is the Studor vent installed correctly per the manufacturer’s Design Criteria and Installation Instructions? I would hire a licensed plumber (not the one sent by the home builder) to review your plumbing vent system to determine if it’s in compliance with the Building Code, the Studor vent is correctly located and installed, and there’s no underlying problem such as dry (not glued) PVC pipe joint that’s leaking air.

      Please write back when you find the problem.
      Bob

  6. Garret February 24, 2013 at 2:41 pm #

    For anyone experiencing sewer gas smell but cant find where it’s comming from, read on…
    I bought a 5 year old home post and beam home and had random aweful sewer gas smells that i couldn’t nail down. I eventually tracked the problem to malfunctioning Studor vents. I replaced two that were periodically malfunctioning, but the odor from one part of the house seemed to be coming from a cabinet that had no vent in it. After a year of frustration i finally started ripping walls out. Low and behold the fools who built the house had sheetrocked a Studor vent behind the wall of a closet!
    I know sometimes there is no choice, but Honestly, if you can avoid using these vents, do so at almost any cost. They seem to be WAY more trouble then they are worth.

    • BobJackson February 24, 2013 at 4:00 pm #

      > Low and behold the fools who built the house had sheetrocked a Studor vent behind the wall of a closet!
      That is bad! I’m wondering if the vent pipe was supposed to have been routed to the vent pipe on the roof? Or it would’ve been easy to install two PVC elbows to route the vent pipe outside the drywall for access to the Studor vent.

      > I know sometimes there is no choice, but Honestly, if you can avoid using these vents,
      > do so at almost any cost. They seem to be WAY more trouble then they are worth.
      Studor vents have a lifetime guarantee and are probably the best AAV product on the market. The blame is on the goober who improperly concealed the vent behind the drywall.

      The problem with not using the Studor vents “at almost any cost” is no one wants to pay for routing all vents to the roof. The home builder and subcontractors are often trying to cut corners (maximize profit) and home buyer’s demand low prices. Most people won’t appreciate or pay for design and mechanical upgrades that they don’t understand or see… until it becomes a problem.

      The Studor vent is a good solution in approved locations and easy to replace if not hidden behind the drywall.

      Thanks for sharing your story.
      Bob

  7. Paul March 21, 2014 at 10:13 am #

    Hi.

    I have a 18 month old remodeled Kitchen. The Main sink and Dishwasher are attached to the septic drain system by a professional Plumber and he used an Air Admittance Valve.

    We have a musty odor when drain plugs are not blocking oder from drain. could the cause be an AAV flaw

    • Bob Jackson March 21, 2014 at 11:24 am #

      A malfunctioning Air Admittance Valve (AAV) could be the problem and causing the drain trap to siphon dry, which breaks the trap water seal and allowing sewer gases to waft out of the sink drain. Replacing the AAV will cost about $25 for a new part and I’d do that first because it’s the easiest repair.

      Do you have a garbage disposal? Is the odor only coming from one drain if your sink has two basins/drains? It might be a problem with the garbage disposal not working properly.

      If the AAV is working, another possibility is rather than a P-trap you might have an S-trap drain configuration which are especially prone to water siphoning. S-traps are no longer allowed by the plumbing code. See Primer on S Traps by Reuben’s Home Inspection Blog. Given the kitchen was remodeled only 18 months ago by professional plumber I doubt you have an S-trap, but this is easily verified.

      Suppose replacing the AAV doesn’t fix the problem and you don’t have the problematic S-trap drain. Most drain traps are assembled with threaded connections. When you smell the odor from the sink drain, carefully take apart the trap being careful not to spill the water in the trap. Hold the trap level and check the water level; the water should seal the U bend. If the water level in the trap is too low and allowing air (sewer gases) to pass through then something else is causing water siphoning.

      By way of comparison, take a look at my dishwasher, garbage disposal, AAV and kitchen sink drain plumbing in these projects:
      * How to Replace a Dishwasher – Part 7
      * How to Replace a Garbage Disposal

      If all else fails, call a different license plumber to check the other guys work.

      Let me know when you find the root cause.

      Thanks,
      Bob

      • Paul March 23, 2014 at 9:07 pm #

        Bob,

        Thank you for the assistance. The trap was full but the odor was coming from the trap pipe. My wife had poured some flower water down the drain that was from moldy and old from a flower vase. We won’t do that again. I let the trap set a chlorine solution and the odor seems to have dissipated.

        Paul

        • Bob Jackson March 23, 2014 at 9:35 pm #

          Your welcome! Yeah, the moldy water must’ve stuck to the drain pipes. That’s why I pour mop bucket water in the toilet instead of the sink.

  8. jim love April 26, 2014 at 9:22 pm #

    I have replaced the vent under the kitchen sink but cannot seem to get rid of the smell. I cannot seem to locate the promblem?

    • Bob Jackson April 27, 2014 at 10:11 am #

      Hi Jim,
      Have you tried bleaching the sink? It could be something in the sink drain, pipes or trap. Pour a 50/50 mix of liquid laundry bleach and warm water in the sink drain and let it sit for 15 minutes. A 1/2 gallon should be plenty to flush out the trap.

      Also see my advice to Paul dated March 21, 2014 about checking the garbage disposal and possible trap siphoning.

  9. jim love July 27, 2014 at 9:09 am #

    I have a bad smell under my kitchen sink. I have changed the studor vent. The smell comes and goes?

    • Bob Jackson July 27, 2014 at 12:49 pm #

      The odor may be caused by trap siphoning or something rotting in the garbage disposal or drain pipes. See this discussion for more info.

  10. Esteban Martinez September 2, 2014 at 11:43 am #

    Been in home 30 yrs , no mini Studor & have never had smell . I have septic tank which has been pumped out . Noticed smell a few days ago . Ran bleach thru Dishwasher & Washer plus poured some down roof vent . Could I have blockage in septic tank ?

  11. Robert October 21, 2014 at 11:46 am #

    Should an AAV be able to handle a high flow (400 cfm) exhaust fan?

    We have a new house (2 months) with 400 cfm exhaust fans in the bathrooms that work exceptionally well. One of the bathrooms has a AAV (ProFlo) located in the vanity which is the source of sewer gas when the exhaust fan is on with the window and door closed. If the window is opened about an inch then the smell does not occur. The plumber replaced the original AAV and the problem is not nearly as bad but still noticeable.

    Is this a defective AAV?

    Does size matter from a reverse flow sealing persective? When searching I noticed that they have different DFU values like 6 or 20 DFU for 1 1/2″ pipe.

    Do some brands perform better then others for this condition?

    Any recommendations to help solve this problem are appreciated. I prefer not to have to open the window everytime the fan runs, especially in very hot or cold weather.

    Thanks
    Are there high performance Studor vents that can handle the vacuum caused by a high flow fan?

    • Bob Jackson October 22, 2014 at 10:51 pm #

      Hi Robert,
      A 400 cubic feet per minute (cfm) bathroom exhaust fan strikes me as too large unless your bathroom is huge, say 20ft x 20ft or greater which is more than a 2 car garage.

      Since opening the door or window prevents the sewer odor by relieving the back pressure, I think the too large bath fan is pulling sewer gases from Air Admittance Valve (AAV) under the bathroom sink. But AAV’s are one-way valves designed to prevent that from happening under normal conditions. For example, the ProFlo PFAAV20 is “100% functionally tested at 1/4″ H20 and 30″ H20 ensuring trouble free performance”. 30 inches of water gauge is a lot of pressure!

      Perhaps it’s not the AAV but an air leak at a plumbing joint? Are the AAV threads sealed with Teflon plumber’s tape? Is there bad PVC solvent weld joint? Sprinkle some talcum powder over the AAV valve while the fan is running with the window and door closed to look for air currents indicating the AAV backflow preventer is failing. Spray soapy water on the pipe joints and look for bubbles. But do not light a match or candle to make smoke because sewer gases can contain methane and explode.

      You can contact ProFlo and ask if bathroom vent fan may be overpowering the AAV.

      Let me know what you find.

      Thanks,
      Bob

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