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.

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 Bathroom Sink

Studor Mini Vent under Bathroom Sink

How to Fix a Sewer Gas Smell

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 Mini Vent Clogged with Insects

Studor Mini 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

Copyright © 2016   Reproduction strictly prohibited.

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57 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.


    Pat O’Neal
    Director Technical Services

  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,

  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.

  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.

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


    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.


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


        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.


        • 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.

    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.


  12. Steve January 3, 2015 at 9:31 am #

    I’m stumped. I have studor vents in my attic and one of them is actually allowing backflow of liquid & gas. I have replaced the vent and still have the same problem. There is a shower & toilet that are sharing this vent. This is the highest point in my house. It has actually caused there to be enough fluid that my veiling above the toilet is showing wet / leak signs. There was a shower remod about 2 years ago and there has not been a problem until this last couple weeks. The shower has 2 shower heads & I know there is a large volume of water flowing when both are on. There is no slow draining of the shower, & toilet works fine. But we have this problem now with the vent leaking. How can there be fluid up that high? Does anyone have any idea what is going on? Help please….

    • Bob Jackson January 3, 2015 at 11:55 am #

      Hi Steve,
      Is your home connected to a municipal sewer system or do you have a septic tank?

      > How can there be fluid up that high?
      Recognized that a Studor Air Admittance Valve (AAV) is a one-way valve that only allows outside air to enter into the Drain Waste Vent (DWV) system (i.e. plumbing vent pipes) to protect the trap seal for the sink, shower and toilet by relieving the vacuum caused by flowing water. The Studor AAV can’t solve positive pressure problems.

      The Studor Design Criteria and Installation Instructions states on page 4:

      “Certain public sewer systems may exert a positive pressure on the connected building sewer. The positive pressure may be from a forced main, the proximity to the sewage treatment plan, overtaxed public sewer mains, high pressure cleaning equipment, or a mountainous terrain. The pressure can be dissipated by having a vent extended to the outdoors.”

      Something is producing a high positive pressure in your DWV system, overwhelming the Studor AAV one-way valve and pushing waste water through to the attic. The positive pressure may originate from the municipal sewer system, septic tank or a design flaw in your home’s DWV.

      At least one outdoor vent is required by the plumbing code per article 903.1 Required vent extension:

      “The vent system serving each building drain shall have at least one vent pipe that extends to the outdoors.”

      Do you have an outdoor vent pipe? If not, replacing the problem Studor vent in the attic with vent pipe through the roof should be easy enough.

      Contact a licensed plumber to review your plumbing vent design and see if a roof vent will solve the problem.


  13. Steve January 3, 2015 at 12:19 pm #

    Thanks for the quick reply Bob. Yes, my house is connected to a sewer system. My home is about 15 years old. There is a sewage station / lift about 1/2 mile away. I have about 3600 sq foot house with basement & all the faucets & toilets & drains are all using AAV mini’s. There is a Single roof penetration pipe as well. There is def positive pressure. There is no standing water in the pipe up there. Just lots of condensation. I looked this am. I was wondering if a Maxi Vent may be more suitable up there as the shower produces a lot of water flow with 2 heads. It’s on the same main drain line a the toilet in that same bathroom. I have a plumber coming out Monday but am trying to understand what could be going on. There was not a problem a few weeks ago. Any other thoughts? Adding a PAPV to help with the positive pressure?
    Thanks – Steve

  14. Steve January 3, 2015 at 12:40 pm #

    Thanks Bob. So I am on a sewer system, I do have 1 roof vent pipe, I’m about 3/4 mile from a lift station.
    House is about 10 years old, all the other mini AAVs are doing fine no issues. Just this one is a problem. 3 Studor minis in the attic. Would changing it to a Maxi help or having a PAPV added? I have a plumber coming out but not until Monday night and trying to understand as much as possible. When talking to him he said he’d never heard of this situation which has me nervous. Any other thoughts? Thx

  15. Steve January 3, 2015 at 12:41 pm #

    Correction – my house was built in 99.

    • Bob Jackson January 3, 2015 at 2:30 pm #

      > Would changing it to a Maxi help or having a PAPV added?
      The Studor P.A.P.A. (Positive Air Pressure Attenuator) is a bellows type device similar in operation to a water hammer arrestor that’s intended for transient/surge positive pressures in high-rise buildings. It’s expensive, too… about $500. You could probably have a new roof vent installed for less money that will also solve for continuous venting issues.

      Upon further consideration, I don’t believe you have a positive air pressure problem. The Studor Mini-Vent is rated for 40 inches of water column pressure and that much positive pressure would cause your toilet, shower and sink drains to bubble.

      I’m betting on the Studor vent is freezing in the winter cold and causing the rubber flapper to stick open. You mentioned it’s serving the shower at the highest point in your house, the problem only began a couple of weeks ago (when it started to get really cold) and replacing the vent didn’t solve the problem. Hot water from the shower is steaming up the vent pipe and freezing on the valve flapper. Warm moist air exits the open vent, condenses and weeps down the pipe onto the drywall ceiling. Try wrapping the vent pipe with water pipe heat cable for a quick fix. You can buy plumbing heat cable at the home improvement store.

      If the valve is indeed freezing open, you’ll need to convert it to an outdoor roof vent for a permanent fix. BTW – all of my Studor vents are located indoors so I’ve never encountered a freezing vent issue.

      Please post back with what you find.


      • Steve January 24, 2015 at 9:16 am #

        I appreciate all your feedback & advice. The plumber couldn’t tell if the vent was freezing or why is was failing but I had the vent ran through the roof so it was open air. I had 11/2″ that I changed to 2″ and went through roof with that. Problem solved, but I still have 2 other Studor vents in the attic venting other bathrooms but they seem to be fine.
        Thanks for the follow up note as well. Your site was very helpful!

        Steve – Indy

  16. paul February 16, 2015 at 1:45 pm #

    Every time the outdoor temperature reaches the single digits, we get an awful sewage odor from under our kitchen sink. We have a Studor vent. When the temperature returns to the 20’s the smell disappears. What do you suggest for curing the problem?

    • Bob Jackson February 16, 2015 at 8:11 pm #

      Moving to Florida would cure the problem. :-)

      Seriously, I wonder if a roof vent is freezing and blocked due to condensation inside the pipe. Perhaps leaves and other tree debris has fallen into the pipe making it susceptible to freezing and clogging. Sewer gases then build up in the Drain Waste Vent (DWV) plumbing system and leak out under the kitchen sink because the Studor vent is worn out and/or clogged with insects as mine was. Have you tried replacing the Studor vent? Also take a look at the roof vents with a strong flashlight. Squirrels can chew up vent pipes, too.

  17. paul February 17, 2015 at 11:00 am #

    Thanks for the quick reply, Bob! Florida sounds nice about now, but in the meantime I’ll try your suggestions. Thanks again.

  18. scott February 25, 2015 at 9:43 pm #

    our master bath sinks on 1st floor on one year old home stink . Gas smell coming from overflow of undercounted sink. Two plumbers have looked and one cleaned out p trap and then later replaced p trap. Smell used to be mostly when we turn water on and dissipates after running water for a bit, now it smells when we walk in room. Very cold temperatures now. We asked both plumbers about putting in studor vents and both said no because vent is on roof. No other bath room smells and neither does kitchen. We can’t take this smell much longer. Do you think a studorvent would work? could vent on roof be problem?

    • Bob Jackson February 26, 2015 at 7:22 pm #

      If the plumber replaced the P-trap that’s good because it eliminates questions about an improper trap. Other people have had problems with sewer gas odors in really cold weather. I wonder if the roof vent is covered with snow or ice? Perhaps leaves or a wasp nest is partially blocking the vent pipe and that’s making it susceptible to icing over more easily? See this article on InspectAPedia.

      Something is causing the P-trap to be siphoned breaking the water seal allowing sewer gases to waft out of the sink drain. A Studor vent is easy to install with minor modifications to the drain plumbing and will prevent the P-trap from siphoning. See the P-trap and Studor vent installation steps about 1/2 way down the page in this project.

      Let me know if the Studor vent solves the problem and if you find anything blocking the roof vent pipe.


  19. Jim Shields March 5, 2015 at 2:20 pm #

    Sewer gas coming from bathroom sink drain after water is started. Smell stops within 3 seconds. Have a self venting device installed behind p-trap and have replaced it twice. Suggestion was made to add a second p-trap to eliminate problem. There is no vent pipe attached to bathroom sinks….This is in a Mobile Home. Any advice would be appreciated…

    • Bob Jackson March 5, 2015 at 7:49 pm #

      Double P-traps are forbidden by the plumbing code:

      P3201.6 Number of fixtures per trap. … Fixtures shall not be double trapped.

      and double P-traps will create more problems.

      Can you e-mail photos of your sink drain plumbing with the P-trap and self-venting device to bob[at] so I can see how it’s installed? Replace the [at] with the @ symbol.


  20. Joycee Nielsen October 29, 2015 at 3:54 am #

    I have a mobile home and there’s a bad smell (not rotten eggs but like ammonia and rotting meat) in the cabinet under the bathroom sink. I thought it was a dry trap so I put water in every drain (2 sinks, bath tub and shower) in that bathroom but it didn’t go away. We have the self venting pipes so I replaced every air admittance valve in the house, but now the smell comes from the air duct and under the cabinet. The smell gets worse when the heat or central air is on, but just from that bathroom! It’s so strong that I now have a sore throat, stuffy nose and constant headache. It has been a month since I first smelled the odor. Is it sewer gas? Is it from a pipe near the air duct? Or could it be a dead rodent in the air duct? And can I die from sewer gas if I’m exposed to it for a month or two? Please help!

    • Bob Jackson October 29, 2015 at 8:22 pm #

      Sewer gas in my experience has a definite fecal odor, definitely not like ammonia. However, decaying plants and animals can give off an ammonia odor. The New York Dept of Health – The Facts About Ammonia states “It is also produced when plants, animals and animal wastes decay.”

      > Or could it be a dead rodent in the air duct?
      I believe you’ve identified the problem. Based on your description smells “like ammonia and rotting meat” my bet is you have a dead rodent. With the colder temperatures mice and other critters are looking for warm places and one has probably gotten in the crawlspace or wall when it kicked the bucket.

      > The smell gets worse when the heat or central air is on, but just from that bathroom!
      The central air system is picking up the odor and moving it around making it worse.

      I’d call a plumber because he can verify if it’s a plumbing problem, check the crawlspace and they’re accustomed to dealing with messy situations. Have a rake and plastic trash bag on hand to dispose of the dead rodent.

      > And can I die from sewer gas if I’m exposed to it for a month or two?
      See the Sewer Gas Health Effects on Wikipedia.

      Let me know when you find the cause of the odor.

      Good luck!

  21. Amar Patel November 8, 2015 at 3:26 pm #

    I have a sewer smell in the basement, only when someone takes a shower on the top floor. It smells for a while and then the smell dissipates only to return when someone takes another shower.

    My guess is something blocking the roof vent partially or fully.

    Any quick tests to check this? Or do I need to bite the bullet and hire a Plumber? to check out the roof vent issue?


    • Bob Jackson November 8, 2015 at 5:02 pm #

      Sewer gas needs a pathway to enter the home, often through a dry drain trap or malfunctioning air admittance valve. It could be the roof vent is blocked which may cause a sink trap to get sucked dry or at least pull out enough water such that the water trap seal is broken. If you have roof access it’s easy enough to shine a flashlight down the roof vent.

      Do you have plumbing fixtures in the basement – utility sink, bathroom or floor drain? Run a 1/2 gallon of water down the drain to ensure the trap is full, then run the shower. Listen for air and water gurgling noises at the drain. Sniff around to try and isolate the odor to a particular drain.

  22. S Miller November 16, 2015 at 10:46 am #

    We noticed that we had a sewer gas smell coming from our bathroom sink drain after our friends visited last week. They use cloth diapers and rinsed (at least one messy one to my horror) in the sink.

    So we found the culprit–a hair clog at the end of the drain stopper which trapped a calling card from their bundle of joy and a clump on the flapper that holds the stopper in place. My husband removed the mess from the drain and stopper and we used drain cleaner. The P trap is full of water and the drain is clean, looking down with a flashlight. I have the window open in there–how long before the smell clears out?

    • Bob Jackson November 16, 2015 at 8:39 pm #

      Nice friends!

      I’d bleach everything – vanity top, sink, faucet, handles and hand towel rack! My guess is the lingering diaper odor is poo residue stuck to the drain pipes. There might be dribbles on the floor and bathroom rug, too.

  23. concerned homeowner February 5, 2016 at 10:43 am #

    I am having some trouble with a supposed sewer gas smell. When our heater runs, it comes through my air vents, but only after water has been run. It is only in one bedroom, which is on the end of my house. It is horrible, especially at night after everyone has taken a bath/shower. I had our air vents cleaned. They said that should remedy the problem, but it didn’t. We called a plumber. They said they fixed the problem (a plumbing vent) but that the odor is still there because it takes a few days to dissipate. Is that correct? Being that I have kids, I am very concerned about sewer gas in my vents.

    • Bob Jackson February 7, 2016 at 2:45 pm #

      > When our heater runs, it comes through my air vents, but only
      > after water has been run. It is only in one bedroom, which is
      > on the end of my house.
      I’m thinking it’s not sewer gas leak but mold or perhaps a dead rodent in the ductwork serving that one bedroom. If your Drain Waste Vent (DWV) piping were leaking the sewer gas wouldn’t be sucked into the bedroom heating duct because the duct is pressurized and a break or hole would blow air outward instead of sucking it in.

      Inspect the heating & cooling ductwork that serves the bedroom. Trace the duct branch line from the bedroom all the back to the main duct. Look for open joints, a rodent hole, wet spots and mold. Mice are drawn to warm spots and like to nest in the duct insulation; it’s more common if your home has a crawlspace instead of a basement. Your nose will guide you. It’s straightforward to cut open a section of flex duct to isolate the odor and install a sheet metal duct splice afterward.

      It would help to know the details of your bedroom air duct installation – location (attic, under the floor, basement or crawlspace), round flex duct or rigid, etc.


  24. Mary March 19, 2016 at 3:33 pm #

    Hi Bob,

    I was wondering if you might have some advice or guidance regarding the rotten egg/sulfur/sewer gas odor I’ve been experiencing?

    Here’s the problem:
    When I run water at the kitchen sink, bathroom sink, bathtub, washing machine, or flush the toilet, I often experience a horrible, foul, gas-like odor being emitted with the water. The odor can be foul enough to stink up these rooms for several minutes, or it may be barely noticeable, and sometimes it’s not present at all – there’s no pattern to it – although, lately, the odor seems to be worse in the evenings. The odor happens mostly on cold settings and as the water is heating up.

    A little background about my building and apartment:
    The building is brand new (finished construction in 2015). I’m the first person to live in the apartment – it’s on the top (4th) floor. The odor began on the first day I moved in (June 2015) and has never gone away. My neighbors are not experiencing this problem – it’s isolated to my apartment. We’re on city water and there are no natural gas sources. Maintenance said there’s no rooftop vent for my apartment – only a Studor valve, which is under my kitchen island.

    Things that maintenance and I have tried (none have solved the problem):
    – When I moved in, the hot water heater wasn’t draining (it hadn’t been put together properly), so maintenance fixed it, thinking that would solve the odor problem, but it didn’t.
    – The hot water heater has been flushed multiple times, with water running in my apartment for 10-15 minutes each time.
    – The anode rod was replaced in the hot water heater (nothing was wrong with it).
    – The temperature was increased on my hot water heater and left up for over a week, and flushed a couple of times at the high temperature, but the odor remained.
    – The p-valve under kitchen sink was replaced (nothing was wrong with it).
    – The Studor valve under the kitchen sink was replaced (nothing was wrong with it).
    – Bleach was poured down the drains (the odor still occurred – even minutes after the bleach was used).
    – The traps have water.
    – The toilet is sealed to the floor properly.
    – At the bathroom sink, kitchen sink and bathtub, I’ve plugged the drains, run the water, collected the water in a glass and the water in the glass stinks (the odor isn’t bouncing up from the drains – it’s emitted with the water from the faucet, or when I flush the toilet, the tank emits the odor).
    – A plumber visited my apartment (and of course the odor wasn’t present), so the plumber dismissed everything I’ve been experiencing – frustrating.

    Two things block the smell completely: the water filter showerhead I installed and my filtered water pitcher (for drinking).

    A recent observation: I noticed a small bulge in the wall behind my shower. No signs of water damage, but when I run hot water in the bathroom sink, I hear pipes thumping, exactly where that bulge is located. Not sure if there’s a connection.

    What could be creating this odor? A cracked pipe? A cap that’s fallen off the pipe? Problems with the venting system? What can I do to resolve this? My property manager and maintenance manager seem to have given up and I need some guidance. Any advice you may have would be appreciated. Thank you for your time.

    • Bob Jackson March 19, 2016 at 4:53 pm #

      To summarize:
      * The rotten egg/sulfur odor has been isolated to the fresh water supply.
      * The odor is present in both the hot and cold water sides.

      Is your building on city or well water? If well water then my guess is the water supply plumbing to your apartment bypasses the building’s water softener/filter system. I once owned a home in Southwest Florida served by a well and the water stank like sulfur until I had a Kinetico whole house water treatment system installed.

      Ask the building owner to call the plumber again and trace the pipes back to the source. Is your apartment plumbed differently than the other units? Saddle valves can be installed on the supply pipes where they enter your apartment (e.g. upstream of the hot water heater) to sample the water. This can isolate if the odor originates within your apartment plumbing or if it’s coming from the building plumbing. When you smell the odor at the sink, open the saddle valve to catch a sample in a bucket to check if the odor is present there too. Since the odor comes & goes monitor it for a week and keep a written record. With this knowledge you’ll hopefully narrow the possible sources of the odor.

      > but when I run hot water in the bathroom sink, I hear pipes thumping
      That’s water hammer in the copper water pipes. A common issue when the faucet is shutoff causing the momentum of the water to rattle the pipes. Completely unrelated to the odor.

      Let me know what happens.

      • Mary March 19, 2016 at 11:21 pm #

        Thank you so much for your quick reply, Bob! I’m on city water and my apartment *should* be plumbed like the other apartments. I’ve been keeping a written record since February of when the odor occurs. It’s completely random, although, most mornings, it stinks in the bathtub (as the water is heating up), and about 75% of the time the toilet tank stinks after I flush it. Those are the only consistent things. The washing machine, kitchen sink and bathroom sink are very random (half the time I use them, they smell and the other half, they’re normal). That’s an interesting idea about the saddle valves. I’ll mention it to the maintenance manager and if they agree to install it, I’ll follow your advice and continue to keep a written record of what’s happening. I sincerely appreciate your help!

        • Bob Jackson March 20, 2016 at 2:53 pm #

          After reading up on this I think the problem is your hot water heater. Although you’ve turned up the temperature to kill bacteria and replaced the anode rod, hydrogen sulfide gas producing bacteria may still be growing in the heater. The magnesium anode rod is often the cause.

          See these articles:
          * Your Household Water Quality: Odors in Your Water by the UGA Agricultural and Environmental Services Laboratories:

          To address this [rotten egg odor] problem, the following
          actions are suggested:
          – Make sure that hot water smells, but cold water does not.
          Often, the smell originates from a magnesium rod in the
          hot water tank.
          – If the heater has a magnesium rod, a licensed plumber
          can replace it with an acceptable alternative
          such as an aluminum rod.

          * Does your water smell like rotten eggs? by the National Environmental Services Center:

          Water heater modification: If H2S odor is
          associated primarily with the hot water system,
          modifying the hot water heater may reduce the
          odor. Replacing the water heater’s magnesium
          corrosion control rod with one made of aluminum
          or other metal may improve the situation.

          * Why does my water smell like rotten eggs or sewage? by the City of Garden Grove, CA. Note the hot water heater issue at the bottom of the page.

          The recommended fix is to replace the standard magnesium anode rod an aluminum or aluminum/zinc alloy rod and disinfect the hot water tank with several pints of drug-store grade hydrogen peroxide.

          The How To Eliminate Water Heater Odors by the Clean Water Store is very detailed. They sell a Water Heater Odor Killer in-line filter.

          Given your hot water heater installed incorrectly (which has since been corrected) that may be why the bacteria/odor got started. It’s difficult to completely sanitize the heater and water pipes. Replacing the magnesium anode rod with the same isn’t a solution. The reason you sometimes notice the rotten egg odor on the cold water side of the tap is our noses are very sensitive to hydrogen sulfide gas and it’s probably mixing with the cold water on the inlet side of the water heater. Recall hot water heaters don’t have one-way inlet valves so some gas diffusion may occur especially if the water hasn’t been run overnight.

          A saddle valve on the inlet and outlet sides of the water heater may be telling. Install the saddle valve as far upstream as possible, but my recommendation at this point is to (best) replace the entire water heater or at least install a non-magnesium rod and flush with hydrogen peroxide to any kill bacteria in the pipes.

          • Mary March 20, 2016 at 11:19 pm #

            Oh my gosh, Bob – this is SO helpful! I can’t thank you enough. I thought my hot water heater was a ‘dud’ from the beginning. I asked the plumber who visited my apartment if the hot water heater was a ‘lemon’ and he insisted that none of this was being caused by the hot water heater. That didn’t make sense to the maintenance manager and me – we still thought the hot water heater was involved somehow. But it definitely makes sense that if the hot water heater wasn’t draining properly to start with, bacteria could’ve built up during that time. Thank you for that insight!

            Last month, maintenance accidentally turned off the breaker for the water heater, so I was only getting cold water in my apartment for 24 hours. I could still smell the odor, but it was nowhere near as bad. When they turned the water heater breaker back on, the smell was intense again. If the hot water heater was off, though, is it possible that I would still smell the odor? The odor was faint, but it was still there. Is it sort of like what you were describing in your message about why I smell it in the cold water now?

            The hydrogen peroxide is a great idea! I read about that online and we haven’t tried that, yet. I’m glad you mentioned it. I’ve also been wondering about the new anode rod that maintenance put in the hot water heater. I read online that some are not ideal and can actually make the smell worse (as you were mentioning). The email I have from the maintenance manager states that he replaced the “sacrificial anode rod” and “replaced both upper and lower heating elements, both upper and lower thermostats.” I don’t really know what all that means.

            I’m going to run all of this by the maintenance manager tomorrow – the non-magnesium rod, flushing the water heater with hydrogen peroxide, the in-line filter and installing the saddle valves. Thank you SO much! You’ve been the only person who’s helped me. I’ll keep you posted with what happens.

  25. Mary March 21, 2016 at 6:47 pm #

    Hi Bob,
    I met with my apartment property manager and maintenance manager this afternoon. It turns out that the maintenance manager installed a non-magnesium anode rod in the hot water heater when he switched out the rods a few months ago. He installed an aluminum copper rod, which should’ve eliminated the odor, but I read that an aluminum-zinc rod is the best to use because it’s something with zinc that removes the odor (but maybe copper is better than zinc in this circumstance?). He also said that at the same time (months ago), he poured hydrogen peroxide into the water heater. I didn’t know that happened.

    After our discussion this afternoon, he came up to my apartment and luckily, the odor was very faint coming out of the kitchen sink – so he was able to smell it. The water coming out was cold water when it produced the smell, so he feels strongly that the problem is coming from the cold water supply (through the pipes from our city’s water). The problem is that no other apartments around me have this issue. I’ve done ‘sniff’ tests of my neighbor’s water and it’s completely normal.

    The odor intensifies in the evenings (maybe because that’s when I’m using it the most). The odor is definitely in the cold water, but I also smell it in the warm water (as the water is heating up – but maybe that’s because there’s still cold water mixed in with it).

    The maintenance manager is going to put more hydrogen peroxide in my water heater and let it sit there all day, then flush it out in the evening. I honestly don’t think it’s going to help, but I’m open to trying anything. The last time he did this (when I didn’t know he had put hydrogen peroxide in the water – I thought he had just flushed it), the smell went away for about 24 hours, then came back.

    I’ve been instructed to call our city’s water department since the odor is coming from the cold water supply. If no other tenants are having this problem, though, and we’re all on the same water supply, I’m not sure what the city can do about this, but I’ll give it a try.

    Management is looking into a whole house water filtration system for my apartment, but they’ve already expressed that if it’s too much money, it won’t be a possibility. So I don’t have high hopes for that being a solution.

    The maintenance manager doesn’t believe that the odor is sewer gas and doesn’t think it’s a vent problem. What could it be, though??? What kind of gas could get into the cold water supply and ONLY come to my apartment and no one else’s apartment. I’m totally baffled by that.

    I’m also confused about where the sewage pipe is in relation to everything else since I have a studor valve that does all the venting. I’m used to venting that goes through the roof and since we don’t have that, I’m puzzled by how all the plumbing works. I understand the basics of the studor valve, but is it possible that there could be a crack in the sewage pipe (somewhere) and somehow getting into the cold water supply? None of this makes sense. If someone in an adjacent apartment had a water leak or busted pipe, it seems like we would’ve noticed it by now (at some point during the past 9 months).

    Anyway, the theory now is that the cold water is the culprit. I’m not sure what can be done for this, but something is not right if my neighbors all have normal water and I don’t – something has to be messed up inside the walls, floors or pipes. I should’ve been a plumber – then I’d know what to do. :)

    Thank you again for your help.

  26. Carol March 30, 2016 at 12:00 pm #

    Hello Bob,

    We live in the high desert of Central Oregon. We live in a 18 year old manufactured home. We have had a pretty rough winter with lots of wet snow and ice. Our master bath has double sinks. One side of vanity recently had that awful sewer smell, the other side was fine. We changed the trap vent Sunday and all seemed well until this morning. The SMELL is back. My husband swapped the right side trap vent with the left side to see if it made a difference. The smell is gone again.

    There is no snow or ice on our roof right now but we have had very cold nights and mornings. I have noticed over the past two plus months that when our main bath toilet is flushed it echoes. I called the septic guy and he said it may be a roof vent plugged with ice. At that time there was no smell in the house. Our son who is a plumber (he lives three hours away) said our bathroom sinks are not vented through the roof. He said to try a better trap vent. The ones available in our very small town are less than $5.00. Would the roof vent pipes cause the problem we are having with smell even though all the sinks in our house have trap vents? This smell problem is driving us crazy, we are not so handy at DIY stuff and I have a nose like a bloodhound. Do we need a plumber, roofer or septic guy? What is your best educated guess? Thanking you in advance for any help.

  27. Carol March 30, 2016 at 12:10 pm #

    Hello again Bob,

    This is an addendum to the above. You mentioned the P trap issue several times. It just dawned on us that we seldom use the second sink. Maybe that is the problem? Not enough water in the P trap. Maybe the water in the P trap is evaporating to a low level? I hope that is it.

    • Bob Jackson March 31, 2016 at 7:03 pm #

      Hi Carol,
      The odor may be caused by any of the issues you mentioned.

      Although the sinks are not directly vented through the roof vent, the sinks eventually connect to the main sewer pipe to the septic tank. Septic tanks create a lot of gas as microbes digest the waste and this is normally escapes through the roof vent. If the roof vent is clogged it will cause a pressure buildup (back pressure) which can force sewer gases past the sink P-trap. The under-sink Air Admittance Valve (AAV) is a one-way valve that only lets air in but blocks back pressure air (sewer gas) from escaping into the living areas. So if the roof vent is blocked the septic tank gases may percolate past the water seal in the sink P-trap. Replacing the AAV won’t fix this particular problem.

      > I have noticed over the past two plus months that when
      > our main bath toilet is flushed it echoes.
      Echos as in a really loud gurgling sound as the toilet trap water seal momentarily opens at the end of the flush cycle? That indicates the toilet is drawing in an excessive amount of air to replace the volume of water going down the sewer pipe to equalize the pressure. Is there a bathroom sink nearby? Check if the bathroom sink has an AAV and if so replace it. The AAV may be stuck and not allowing displacement air to be drawn into the sewer pipe during the flush cycle.

      > He said to try a better trap vent. The ones available in our very
      > small town are less than $5.00. has the Studor Mini-Vent:

      Write back when the problem is fixed!


  28. Darren Lougheed April 13, 2016 at 4:29 pm #

    Hello Bob,
    I read your article and the Studor Mini Vent could be my problem. The sewer gas smell coming from the laundry room is unbearable when the weather is cold outside. It only happens when the temperature is pretty cold. It does not happen in warmer winter weather or in the summer. I just discovered the mini vent in the laundry room while working on another project. I am going to change the mini vent as it about 10 years old. Any thoughts as to why the smell only occurs during cold weather. Thank you for any help.

    • Bob Jackson April 14, 2016 at 5:44 pm #

      Hi Darren,
      If the sewer gas odor only happens during cold weather it may be the rubber flapper inside the vent stiffens due to the cold and doesn’t close completely. Replace the vent and see if that fixes it.

      Another possibility is the roof vent pipe is freezing and clogging. Assuming the Mini Vent is working, the sewer gas could be bubbling back through the laundry sink or clothes washer P-trap. See a related discussion dated March 31, 2016 for details.

  29. John July 12, 2016 at 12:23 pm #

    I just recently installed two sinks in my remodeled bathroom. i only get sewer smell from one of the sinks. each sink has its own vent pipe so im not really sure why the one sink produces the smell. each sink was plumbed the same way with the same p trap set up. totally baffled. any ideas? I have no ventured onto the roof to see if the vent pipe is blocked yet and thats my only thought.

    • Bob Jackson July 14, 2016 at 7:04 pm #

      Were there one or two sinks before the remodel?

      Are the two bathroom sinks some distance apart, say on different sides of the bathroom? Is the problem sink much closer to the toilet?

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