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Is Negative Pressure Causing a Problem in Your Home?


Ever have the smell of your fireplace inside your home?  It’s not uncommon.  Here’s a question I got from a reader named Rob last month:

We always have a smoke smell from our fireplace, more on some days than others.  I had the chimney cleaned really well and the damper closed and it was fine.  Then we just built our first fire since cleaning this weekend and the smokey smell is back.  It has a metal damper in the firebox. I assume somehow negative pressure is forcing air back down the chimney especially when we run bathroom fans or the dryer I suspect.

What’s going on here?

Do you have negative pressure?

Rob suspects negative pressure is behind the fireplace smells in his home.  First, when we talk about negative or positive pressure, we’re talking about the pressure difference between two places, usually indoors and outdoors.  A negative pressure in the house means the indoor pressure is lower than the outdoor pressure, which can cause air to move from outdoors to indoors.

But that infiltrating air needs a pathway to do so.  For air to move across the building enclosure, you need two things:  a pressure difference (positive or negative) and a pathway.

It’s quite easy to find out if a house is indeed running a negative pressure.  All you need is a manometer, and the nice ones cost over $1,000.  You could get one with lower precision and spend less than $100.  You also could use a magnehelic gauge for less than $100.

Or you could do it the easy way.  With the house operating at what you suspect is a negative pressure, go to a door and open it just a bit.  If you stand inside and put your face near the crack in the door, you’ll feel the air blowing on you if there’s a negative pressure.  You can feel it with your fingers, too, and it works even better if you lick them (part of what Joe Lstiburek calls the “look, lick, and squirt test”).  If you still can’t feel any air movement through that opening after you lick your fingers, you don’t have enough negative pressure to worry about, at least not right there.

If you have positive pressure, the air will be moving toward the outdoors.

The 3 causes of pressure differences in buildings

Three factors are responsible for pressure differences in buildings:

Wind, of course, can create pressure differences between inside and outside.  Hurricanes and tornadoes can induce pressure differences big enough to blow the roof off of a house.  That’s why building codes require hurricane straps in high-wind areas.

Stack effect is a density-of-air phenomenon.  Warm air is less dense and thus rises when surrounded by cold air, like an air bubble in water.  That can create some pretty big pressure differences, as you can see in the video below.  The two factors that affect the magnitude of stack effect are height and temperature difference.  That’s why it’s so strong in that video taken in a 44 story building on a cold day in Denver.

And then there’s the wild card of mechanical systems.  Exhaust fans in your bathrooms and kitchen, of course, will induce some negative pressure.  The more airtight the house, the bigger that negative pressure will be.  Let’s put some numbers to that idea. 

My house currently has a blower door test result of 5,200 cubic feet per minute at 50 Pascals of pressure difference (5,200 cfm50).  That’s pretty high, but it’s a 3,800 square foot house built in 1961, and it was close to 7,000 when I bought it last May.  But let’s get back to the main point.  With an air leakage rate of 5,200 cfm50, my house won’t show much of a negative pressure if I turn on a 50 cfm bath fan.  It takes 5,200 cfm to induce a 50 Pa pressure difference, so 50 cfm will be lost in the noise.

At the other extreme, some Passive House projects have air leakage rates of 100 cfm50 or even lower.  Two 50 cfm bath fans would induce a 50 Pa pressure difference across such a house.  That’s a lot of negative pressure for a house, which is why Passive House designers and builders worry so much about the mechanical systems they use.

Other mechanical systems that can create pressure differences are clothes dryers, heating and air conditioning ducts, powered attic ventilators, portable air conditioners, and supply-only ventilation systems.  Atmospheric combustion appliances draw in air from the space around the appliance and send it up the flue, but that’s generally small compared to the pressure differences created by the others I listed.

Finding the source of negative pressure

If you suspect negative pressure is causing problems, the first thing to do is figure out which of the three causes of pressure difference is most likely to blame.  Does the fireplace smell (or whatever symptom you notice) occur only when it’s windy outside?  You can’t stop the wind, but you can see about eliminating the pathways for air to move by doing some air sealing and closing the fireplace damper.

Does it happen only when it’s cold?  If so, the stack effect may be to blame because it increases with temperature difference and most places have bigger temperature differences in winter.  But you also run the heat when it’s cold, so it could be unbalanced duct leakage.

To determine if mechanical systems are inducing a significant negative pressure, turn on the one(s) you suspect and do the cracked-door test again.  Then turn them off and see if there’s a difference.  That also works for pressures induced by unbalanced duct leakage.  Do the door test with the heating system on and then again with it off.

If you have the fireplace smell in your home like Rob does, finding the source of the negative pressure and reducing it should solve the problem.  You don’t want to stop using bath fans, range hoods, and clothes dryers, though, so keeping the damper closed when the fireplace isn’t being used should help.

Remember:  You need both a pressure difference and a pathway to have air movement, so eliminate one or the other or both and you solve the problem.


Related Articles

The 3 Rules of Air Leakage

Heat Rises…and Falls — Stack Effect, Air Movement, & Heat Flow

The Surprising Building Science History Behind the Revolving Door

The Sucking and the Blowing — A Lesson in Duct Leakage


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This Post Has 52 Comments

  1. This is so interesting and
    This is so interesting and timely. I have a furnace exhaust going between my house and my neighbour (picture here: ) and just got a Plume air quality monitor. This morning at 4 AM the NO2 spiked way up. Bathroom exhaust outlet is just a few feet away from furnace outlet, so if there is any negative pressure in my house that’s a nice pathway right back in for the furnace exhaust. Something is going on.

    1. Lloyd, your exhaust fan wall

      Lloyd, your exhaust fan wall cap may well be the entry port for the furnace exhaust.  If your home is operating under a negative pressure, there’s probably a pathway through that exhaust fan duct when the fan isn’t running.  If you’ve ever looked at the damper in a typical exhaust fan, you can see that they aren’t meant to stop all backdrafting.

    2. Lloyd, you should consider
      Lloyd, you should consider converting to an electric boiler soon. Apart from flue gas potentially entering your home, it looks like some flue gas may be recirculated through the boiler via the concentric termination causing efficiency loss and corrosion in the combustion chamber over time. Also, the flue is contacting a brick wall, maybe the neighbours house. Sooner or later those bricks will start to spall and I’m thinking you’ll be on the hook for it.

  2. I have a client with this
    I have a client with this exact same issue we’ve been trying to fix for over a year without luck yet. Large 1.5 story house on conditioned crawl/bsmt. Great room/kitchen is wide open with tall vaulted ceilings, but the designer put the wood burning fireplace in the corner of the room and the flue only goes up maybe 5-7 ft before the pipe penetrates the foamed roof line and the majority of the flue pipe is exposed above the roof. Client always has smoke smell when they open the glass doors on the fireplace…no issue when they’re closed. There were lots of air leaks from the 2nd story I had them foam up to reduce stack effect and they fixed some issues in the crawlspace too, but didn’t solve it. I don’t think the combustion air intake into the fireplace is anywhere close to big enough either, and the flue pipe being exposed mostly above the roof line is contributing to the flue draft issue. Any other thoughts??

    1. Eric, I think they may be

      Eric, I think they may be fighting that cold flue as long as the fireplace stays in that configuration.  With a lot of the flue outside in the cold, it’s hard to get air moving up into it and out the top.  Sealing the leaks at the top of the house definitely should have helped, though.  I doubt the combustion air intake has anything to do with the problem.  They’re mostly ineffective anyway, from what I understand.

  3. Dr. Bailes,
    Dr. Bailes,
    Could you please address the stack effect in Coastal areas that experience ‘reverse’ stack effect due to hot outside and cold inside. There are some studys from Thailand but most simply address warm building and cold climate outside. My findings have been that there is definite air movement downward in low rise condos ( 5-7 stories) that is exasperated by penetrations from plumbing (especially sewer chase), electrical and any bone headed renovator punching holes over the years. Having some serious issues on Siesta Key, FL. Any suggestions about how to prove it exists, remediate and finally will a blower door accurately diagnose this movement of air during baseline operation prior to testing?

    Thanks Karl Peer, Energy Rater, FSEC QA

    1. Karl, reverse stack effect is

      Karl, reverse stack effect is definitely a thing.  I wrote about it briefly in my stack effect article linked to in the Related Articles section above.  You can prove it exists using the cracked-door method I described in this article or using the technique Mac Sheldon described in his comment below but using an attic hatch in the ceiling instead of the fireplace.  Air sealing helps a lot with remediation because you can’t get more hot air leaking in from the attic than is able to leak out at the bottom of the house. 

      The baseline measurement during a blower door test may help a little, but it will depend on the house.  It’ll work better in a 2 or 3 story house than in a single story house on a slab.  If the baseline is very positive on a calm day with all the mechanical systems off, it may be because of reverse stack effect.

      1. Dr. Bailes,
        According to a report written by an investigator working for my insurance agency, my home experienced depressurization combustion reentry. An air quality test disclosed high levels of VOC’s in my basement when the boiler was running.
        Now its nearly four years later, and I continue to have problems…once the heat goes off my basement is so moist that the fluorescent lights won’t go on unless i run a fan and two dehumidifiers. The moisture somehow travels through my house and my clothes and bedding are completely damp. At times, my ears block up…like I just got off a flight.
        I believe there is negative pressure in my home…could this be caused by a gas leak in the street migrating to my sump pump….this is most of the moisture is?

  4. This is one of the many
    This is one of the many reasons that I don’t like fireplaces–of any kind. I have only had them because the previous owner had them. Oh well, to each his own.

    1. Roy, I’m with you on that one

      Roy, I’m with you on that one.  Fires should be built outside in the firepit!

  5. OK first of all, who is still
    OK first of all, who is still enduring wood burning fireplaces? 🙂

    Convert to a proper gas retrofit, and get on with life! Eliminate the smell of smoke but be sure to include a CO detector to the project.

    And second of all (then I’ll go away :)), Allison, where did you get that photo of the home atop your always informative article?

    Don’t believe I’ve ever seen a home with such a lovely patina of weathered OSB, on the outside! And on the chimney, too 8-).

    1. Steve, yeah, I’m with you,

      Steve, yeah, I’m with you, especially if it’s not a sealed combustion unit.  As I said to RoyC, fires should be built in the fire pit in the backyard.

      I hadn’t noticed that similarity before, but yes, it does lokk kinda like weathered OSB, doesn’t it?  It’s actually brick, though, and I took the photo in Pittsburgh back in 2011 when I visited a friend there.  I took a few photos of this house because I’ve never seen brick laid that way anywhere else.

  6. I have seen (smelled?) what I
    I have seen (smelled?) what I consider to be a bigger issue with negative pressure in houses: Sewer odors. I was involved with a very tight research house that often had a bad sewer odor and I finally figured out that it was due to two factors. First, there was a very efficient and quiet bathroom exhaust fan that would often get left on because it was so quiet that you couldn’t tell it was running. I could easily measure the negative pressure when it was running, a couple tenths of an inch of water. So the other obvious factor was a dry drain trap somewhere. I kept pouring water in drain traps, but it didn’t help. I finally figured out that it was coming from a floor drain in a mechanical closet that did not have a trap. I contacted the plumber who did this installation and he admitted that he forgot to put in a trap. How do you forget to put in a trap? It turns out that he was smarter than me. For floor drains, he uses a dry trap which is just a rubber flapper valve that can be inserted in the drain after construction is complete. It worked great and you don’t have to worry about it going dry. No more sewer smell. This was cheaper than installing a fireplace to cover up the smell 😉

    1. Roy, you anticipated a future

      Roy, you anticipated a future article there.  I’ve got another one planned about negative pressure and sewer gases with a nice little video from a LinkedIn friend. 

      I didn’t know about the flapper trap either, but that’s a pretty cool idea.  And what a relief that you didn’t have to install a fireplace!  (That was hilarious, by the way!)

      1. Allison, in your next article
        Allison, in your next article on negative pressure or any other future articles, are you going to discuss how to keep back-draft dampers on kitchen, bathroom, and dryer exhaust fans from rattling when the wind blows? I live in windy Oklahoma and this is a terrible problem for me. I hate to admit that I have a new home of unknown tightness with no mechanical ventilation, so I wonder if a continuous exhaust would solve this problem.

        Now that I think about it, I probably should have my house tested for tightness. Anyone know of anyone with a blower door in the Oklahoma City area?

        1. I had to laugh this morning
          I had to laugh this morning as I was sitting in the space that has a rattling damper while reading this question. Tamarack has series of non-mechanical fabric dampers that is a perfect solution for your problem. They are made to slide inside of metal hard-pipe of various sizes. Take out the plastic rattler and insert some pipe and this damper if you can get to the flex anywhere in the run.

          1. Thanks Thomas, I will look
            Thanks Thomas, I will look into Tamarack. I didn’t realize that they sold this type of product. Most of my rattling problems are with the dryer and range hood exhaust. Can these dampers work reliably under these conditions? Would it really be necessary to take out the existing flappers (rattlers)?

          2. Roy, If you think about what
            Roy, If you think about what air movement is causing the dampers to rattle, you will find adding a quiet fabric damper in series with the same direction of air flow will not change the dynamics. Low pressure on the outside from wind away from the house and high pressure from wind entering the house somewhere else will cause the dampers to move in and out (rattle). That same air flow will occur with the additional damper installed. In my opinion, you will have to remove the offending dampers regardless.

          3. Thomas, you are probably
            Thomas, you are probably right about the need to take out the rattlers, but I think that I will try leaving them in first to see if the problem still persists with the Tamarack dampers. However, I am still a bit concerned about using “fabric” back-draft preventers on the hot linty outlet of a dryer or the dirty outlet of a range hood.

          4. Roy,
            Try using a small piece of mans best friend, Duct Tape.

          1. Yea, John, I confess. My
            Yea, John, I confess. My house is not 62.2 compliant. When I moved to Oklahoma, I looked for the “best” new house that I could find in terms of construction practices. The one I bought is the only that I could find with the range hood and gas log vented to the outdoors. The real selling point for me was the conditioned attic (spray foam roof deck). But it has no central mechanical ventilation and no HERS score. I really would like to get a blower door test, but I can’t find anyone who can do it in this state yet.

            The builder also advertised that it had energy-efficient lighting. It turns out that he put some cheap LED fixtures in the garage and a CFL in the tornado shelter. The rest are incandescent. He advertised high-efficiency air conditioning. It turned out to be 14.5 SEER. It has been an interesting experience.

  7. THis is such a perfectly
    THis is such a perfectly timed article (was planning to google your site for this topic). I got a Airthings radon detector for Christmas and I’m bouncing over and under the safe limit. I have a couple of whispergreen bath fans that run on low all the time in very airtight “pretty good house” and it seems to be pulling air up from my basement. I have caulked and sealed everything I can down in the basement, so I think I am going to have to switch ventilation strategies.

    1. Clay, is the air in your

      Clay, is the air in your basement worse than the air above?  If not, why would it make the IAQ worse?  If so, what are the problems you need to fix in the basement?  Or is the problem with the outdoor air being pulled in through the basement?  So many questions.

      1. Thanks for the reply, Allison
        Thanks for the reply, Allison. Yes, the basement has poor air quality. It is unfinished, has a dusty (not musty, but like chaulky drywall) smell, and the radon levels are pretty high (~14pCi). 1st floor IAQ gets worse when we run the range hood, bath fans, etc. I put weather stripping on the door down to the basement, which helped the dusty smell, but the radon levels still increase. So, I am assuming that the negative pressure strategy I have now is pulling outside air in through the basement, and up into the living space. Granted, with it being colder outside, I have more stack effect right now than at other times, but I still wonder if I need to make changes.

        1. Or maybe I just need to look
          Or maybe I just need to look into a basement exhaust system and/or a radon mitigation system.

          1. Radon gas comes from the soil
            Radon gas comes from the soil or groundwater and generally seeps into your house through basement cracks, drains, sump tiles, etc. That is why radon mitigation systems use a fan to depressurize the soil under the slab. So make sure your mitigation system results in a lower pressure under the slab than in your house.

          2. Clay — Yes you need to pull
            Clay — Yes you need to pull air out of the basement and exhaust it to the outside. The basement should be negative with respect to the floor level pressure above.

  8. A fun project for a rainy
    A fun project for a rainy Saturday is to mask your fireplace or a door then cut about an 8” diameter hole in the plastic. Tape the open end of a lightweight plastic bag to the opening. One from the produce section works well. Run your HVAC and watch the bag. Is it sucking into the house or out? Start your dryer and kitchen hood and watch what happens. With no mechanical equipment running you can observe stack effect too.

    Pretty fun and a great science fair project for your kids!

    1. Mac, that’s a very clever

      Mac, that’s a very clever idea!  As I said in a reply to Karl Peer above, the same method would work with other openings, too, like the attic hatch.

      1. Thanks Mac, you finally gave
        Thanks Mac, you finally gave me something useful to do with a fireplace 😉

  9. This is a great topic and
    This is a great topic and loaded with overlapping issues. First, I build very tight hurricane resistant homes in humid coastal NC. I always aim for less than .5ACH at 50 Pascal. I have learned that a dedicated (non-HVAC associated) dehumidifier is a requirement here, even during the summer months as my HVAC has been sized for low sensible loads that reduces my latent load capability as well. We maintain 45% or less pretty much year round including the 4 swing months when little heating or air is needed. Now on to the make-up air issue. Leaks in a building’s envelope are only an issue is there is a pressure differential. Negative equals air infiltration and positive equals air exfiltration. I NEVER want my houses to be negative as I have no firm idea where that air might be coming in, along with whatever else it wants to drag along. Letting the house decide where it wants to breath is like having you nose placed near your lower exhaust port. God and nature intentionally placed every creature’s nose at the opposing end of their body for a good reason. That being stated, I provide a 6 inch duct to the outside with a one-way damper that only allows air to pass if the house goes negative, thereby giving the house a “nose” where I want it and can properly deal with it. But that does nothing for air quality and the need for .35ACH makeup air. Enter a new product that I just found, the Air King QuFresh 120cfm Air Exchanger with temperature control. I simply connected it’s 6 inch duct to my outside air duct and Voila, I can set the cfm needed, the temp and humidity ranges allowed, completely independent of my HVAC system just like my dehumidifier is. By the way, I fully condition all of my attic space with 3″ closed cell foam under the roof-deck, so I simply add the makeup air to that volume so it can be mixed and conditioned as necessary prior to introducing it into the living spaces through vents between attic and the living spaces. The The Air King only operates when outside conditions are suitable per my settings. I have now introduced a pressure device that provides the necessary air movement.

    1. In addition, if any are
      In addition, if any are wondering where that makeup air gets back out, the bath fan dampers are always there as pressure release if needed. Which helps with air distribution as well in moving stale bathroom air out.

  10. Can I create negative
    Can I create negative pressure in my entire home if I circulate air from one room to another? Basically, it would be a closed system with a partition between the two rooms. I want to exhaust on one end of the partition and intake on the other end of the partition.

  11. My residential condo building
    My residential condo building is 10 years old. When I open the windows strong wind rushes in; or when I open the front door (from my hallway) wind also rushes in.

    The good part about this is that the smell in my bathroom goes away when the windows are open. But it comes back when then windows are closed.

    I’ve been cleaning and disinfecting the drains in the bathroom for 1.5 weeks now to rule that out as a cause for the smell.

    And now, I am focusing on the Venting (the bathroom has a vent in the shower with no louvers) and I have learned it has vent pipes behind the wall.

    1. I am trying to narrow down the causes of this smell (which wakes me up in the morning as it comes back more strongly with the windows closed over night and has me feeling very concerned). Do you have any suggestions how I can determine if it is my Ventilation (and therefore not my drains)???

    2. Especially during COVID I’d like to determine the right type of contractor/ specialist to call in for help after doing my own investigation. I’ve done everything I can do. I initially thought I should for sure call a plumber (to clear the drains), but now I am not sure if I should call the HCAC contractor (to look into the ventilation here). Is the bathroom vent handled by the HVAC contractor too (testing function and professional cleaning)? Who handles testing the vent pipes behind the walls in bathroom?

    Your articles on negative air pressure and ventilation are so helpful as I always wondered about the wind rushing in but didn’t know much about it (and after spending 2 weeks and many many days in past years reading about how to unclog a drain, ptrap drying out, and how to know if sewer gas is in the home….which now I dont think this is?).

    3. If I have alot of wind rushing in my windows as I leave them open now all day to let the smell vent or dissipate (??) (I believe this is negative pressure), will be Ptraps dry out quickly within hours due to the wind. If so, should I cap my drains for now until covid has tempered and deal with this in a few months.

    This is all taking place in a 2nd bath which does not get use but I have pouring water down drains daily for weeks to rule out the dry p trap, and cleaned drains, water runs down smoothly. For this reason I could cap the drains and focus on the ventilation only? Is this a sound idea to you?

    P.S. I am praying for your recovery and well being as I read your personal blog post. Stay positive.

    Thank you.

    1. Robin How many stories is
      Robin How many stories is your building and what floor are you on? Does it smell like sewer gas?
      Does anyone in other condos have the same problem? Do the exhaust vents in the bathroom have fans that you control. My initial thoughts are that it is not the ventilation system unless the vents in the bathrooms are bringing air into the condo. Put a piece of toilet paper up against them — does it stay or does it fly away? Sorry to use up your toilet paper that way.

      1. Thanks John,
        Thanks John,

        My building is about 30 stories and I am on floor 10.

        The tissue paper sticks to the vents. Though I tried a half piece of paper of paper towel last night and it stuck but by morning it had fallen?

        I don’t have fans in the bathroom.

        I finally called in a handy man from building late yesterday and with all windows open, he didn’t think it was sewer smell.

        He will come back today to smell again with all windows closed over night.

        The smell comes back by morning.

        Any ideas of how to improve an individual condo units/apartments ventilation system so that I dont have to always rely on windows with fresh air rushing in? I have never had my a/c ducts cleaned (just filter changes).

        Thanks alot.

        1. First, it is highly unlikely
          First, it is highly unlikely (I would say impossible) that cleaning you AC ducts would have any effect on the problem you are mentioning. Second, I am still inclined to think it is sewer gas. You are not complaining of smelling what someone else is cooling (a common complaint in buildings such as yours). While the ventilation system may be of some assistance, the best way to get rid of pollutants (smells) is at the source. Third, we have now established that all the vents in your condo are exhausting air from the space and therefore contributing the the negative pressure of your condo.
          With respect to the ventilation system, there is a solution that can be implemented if the source of the smell is not removed. If your condo were at positive pressure with respect to the source (locations other than your condo) then the smell would not enter your condo. This can be accomplished by a combination of sealing air pathways into your condo and introducing outside air directly into the condo.

          1. John,

            Thank you again, your further reply is helpful.

            A question about part of your reply to help me to understand it (I pasted your text below, for reference):

            Pasted from your reply:
            Third, we have now established that all the vents in your condo
            are exhausting air from the space and therefore contributing the the negative pressure of your condo.
            With respect to the ventilation system, there is a solution that can be implemented if the source of the smell is not removed. If your condo were at positive pressure with respect to the source (locations other than your condo) then the smell would not enter your condo. This can be accomplished by a combination of sealing air pathways into your condo and introducing outside air directly into the condo.

            My follow up questions to this:

            A. When you said this: “we have now established that all the vents in your condo are exhausting air from the space and therefore contributing the the negative pressure of your condo”…

            1. Are you referring to following 3 Vents: a) vent in each of my 2 bathrooms, b) as well as the vent on the ceiling in my kitchen?

            2. And if so, is it “normal” and/or “good” to have “negative pressure” in the apartment? I would think yes, but want to be sure I understand correctly.

            3. Also, please note, the tissue does not stick to the vent in the ceiling when the windows are closed. Is that ok? But when the windows are open I can hear this vent pulling air.

            4. I also want to clarify that you are not including in these “VENTS”, the vents in each of my 3 rooms (1 vent in master bedroom, 1 vent in second bedroom, and 2 large vents in living room which is open to kitchen and hallways. These vents, I assume are part of the Ventilation System and blow warm/cool/fan air depending on how I set the thermostat….and are not part of your answer pasted about about “all vents” in my condo are exhausting air from the condo and contributing to the negative pressure. Is that correct?? I had not until just now used toilet paper to test each of these Vents….the toilet paper did Not stick to any of these vents…a very gentle warm air comes out, because my heat is on auto and low.

            5. Right outside the door to my master bath (which has the smell) and my master bedroom gets the smell more than the other rooms….I have what I think is called the Return (a large floor to waist high Vent on the wall directly outside the bath with the smell). I just tested it with tissue and see that it Blows the tissue. Is this wrong? I would think it would “return air” into the ventilation system and not blow air? I have my HVAC in that room set to heat, with auto fan (med speed). Should this always have air coming in or blowing out or does it vary?

            Lastly, the 2nd part of your answer, I am not able to follow this. Any chance you can explain in laymen terms, so I can think about it, I would appreciate.

            For reference, I pasted the 2nd part of your answer below:

            “With respect to the ventilation system, there is a solution that can be implemented if the source of the smell is not removed. If your condo were at positive pressure with respect to the source (locations other than your condo) then the smell would not enter your condo. This can be accomplished by a combination of sealing air pathways into your condo and introducing outside air directly into the condo.”

            Thank you very much.

        2. John, I read your 2nd reply
          John, I read your 2nd reply and thank you very much, your clear outline and explanation summarize what is going on and I understand it based on all the reading I’ve done these past few weeks. Thank you so much for helping me narrow this down.

          Two additional pieces of info:

          1. Yesteday when I finally called my super, my handyman confirmed that the neighbor next door to this bathroom and the neighbor below this bathroom both are not dealing with sewer gas smell. So, technically, that was ruled out just yesterday although, I had the same thought as you initially since many people in my building are not here now. So I covered my bathroom vent with a garbage bag to see if it prevented the smell from coming in. It didn’t the smell was worse. That’s when I tackled the drains.

          2. And the tissue paper held. As mentioned in my reply earlier, last night I put a half a sheet of paper towel on the bathroom vent and it stuck. But by this morning, it was fallen down.

          3. My building said 1 day last week a person in my line complained of food smell in their bathroom and he said a belt (I believe) required replacement in my line and that the bathroom vent was not working for that 1 day. But he said it worked before that day and after. And the tissue paper stuck (except the half sheet of paper towel fell by morning today). This morning, my super emailed me that he increased the speed of the fan yesteday. To give that a try. But I left the door closed to the bathroom last night and the smell (of some sort) comes back.

          4. The reason I am also focusing now on how to improve (or fix and also understand ventilation and vent pipes) is because the smell (more subtle but still the same smell) appears in my bedroom which is next door to the bathroom and by morning with the windows closed, I smell it….i figure it is coming slightly through the HVAC vent because that is the only opening in the bedroom. Also yesterday, I opened windows in my bedroom and 2nd bedroom but did not open a window in my living room (because was too cold and windy) and I then smelled the same smell (subtlety) in my living room. Again, the only reason I could think was it was seeping into the living room through the HVAC vent. I then opened the window just to air the whole place out. By the time my handy man came, the smell was not noticable.

          He did say that opening windows is necessary to move the Fresh air through the HVAC system (in the return and out the vents).

          5. Today with windows all closed, I did the tissue to the vent in the ceiling in my kitchen (which is open to the living room) and it did not stick. It did blow away. It did nothing. Is this normal?

          6. I do tend to agree with you that the smell may be sewer smell that is coming from another apartment that is empty….maybe not my next door or below me neighbor… so I will not rule that out. Otherwise why would my be “worse” during covid (while many are not here in the building). I thought possibly it was worse because I was cleaning and running water, added moisture, used vinegar in drains…so I tried to use fresh air and flush with water to rid the bleach and vinegar/baking soda smell. But yes, the smell comes back daily with the windows closed, even when I have not added anything vinegar or cleaning products into the mix. So I no longer think the smell could be due to my cleaning efforts. Its has to be a drain, vent or recircuating hvac system.

          7. I ordered screens for my windows so I can more comfortably let fresh air in.

          8. If windows are open, do ptraps dry out faster? I was told the ptrap should usually stay wet for 2 weeks just by running the faucet for a couple minutes. I’ve been putting water in daily these past 2 weeks. But now that learned that fresh air is helpful to circulate air through my apartments HVAC system….and I will leave windows open more often, I wonder if I will then dry out my traps more frequently?

          Thanks alot for your thoughtful replies. I am trying to learn so I can make wise decisions and for peace of mind too…


          1. Robin: I have to run and so
            Robin: I have to run and so it will be a day before we can continue this discussion. One quick thing is there is essentially no difference in how long traps stay with enough water in them to block sewer gas whether the windows are open or closed. Another item Are you saying this started when COVID19 got traction and people left their condos for other locations?

          2. Hi John,
            Hi John,
            Thanks for your always organized and informative answers. You hone in to the most important things I mentioned, your support is very appreciated.

            I have been reading up a bit so when you return in a day or so, I will be better able to understand your answers.

            Thank you, I now understand that when I leave the window open, I dont have to be concerned that the traps will dry out faster. I appreciate that info!

            Regarding did this smell start as covid got traction and people left the building. Its hard to say. Uniquely, I wanted to tackle a “smell” in this bathroom for a years. It’s my master but I dont use it because there was always a subtle smell and at times it was stronger than others. I was never 100% sure if the smell was from dirty grimy drains left by previous resident and/or by dried ptrap (because was the bathroom I dont use daily, I left it to the women who helped me clean weekly to handle drains and also pouring water into each ptrap in the bathroom. In past, she would tell me there was a very bad smell (she said “too much” and the clean clothes she hung in there to dry from the week before would need to be rehashed). But since I was not the one pouring water in each ptrap, I could never be 100% certain the smell she described was because she let the Ptrap dry out. However. the past few months it has my responsibility to clean…I do smell something so I would leave the door open and sometimes closed. But
            during covid, I decided to clean the drains and tackle this so that I could have peace of mind. And I can say 100% that during Covid, I have poured water in each ptrap daily and the smell is consistently there daily and yes, it is stronger when the door to bathroom is closed (like right now). Before covid, I rarely smelled it from the bedroom or living room…but now I do, however, I am beginning to think that is because the Return for my HVAC is directly outside this bathroom door and I have covered the vent in the bathroom and kept the door closed and I also opened all my windows which I believe has caused this smell to go into my Return (right outside the bathroom door as I try to air the room out by leaving the door open sometime) and from the Return, I think the smell has been redistributed through my HVAC vents into my rooms. That is the only Return in my apartment. I am not 100% sure the answer….maybe there are 2 issues going on at once….pre covid scent and during covid.

            Its definitely something though. Since I wasn’t overseeing the ptraps myself 100%, I was always unsure if dry Ptraps were the underlying cause then left a lingering scent (I rarely aird out my apartment in past because we have no screens and I wasnt sure if outdoor air brought in dust etc). And so, I was lax on bringing in plumbers or HVAC contactors to help me identify the smell. My handy man came in the past and said he didnt smell anything. When he comes Monday he will smell it but I still don’t know if it is sewer gas…I think he will know that for sure since he is familiar with the smell of sewer gas.

            Thanks again and I hope you’re having a nice day.

          3. If nothing else seems to help
            If nothing else seems to help, and you are still pretty sure that it is sewer gas, I would sniff around between the toilet and the floor, especially if it has not been caulked there, to see if the wax ring under the toilet might have lost its seal.

          4. Hi Roy,
            Hi Roy,
            Thank you for your suggestion. I had done that last week but didn’t smell anything (admittedly, I only smelled in the front). Also, my handyman who came on Friday said he didn’t smell anything around the toilet and he felt whoever installed it did a very good job (he didn’t think it could be the toilet).

            But yesterday 2 different handy men from my building came back and confirmed this is definitely sewer smell. I had the bathroom exhaust vent covered/sealed off and the door closed for 2 days, so they would have no way to miss the smell. So that smell is sewer gas and John’s hunch all along, was correct.

            And seems the source is, in fact, my toilet which they said is not properly sealed. They were not able to lift the toilet to check the wax ring (seems they require a plumber in fear they could break something-it is stuck on very well?) but they noticed an air gap where the screws are located and when they put their nose to that area, they smelled the odor more…so they caulked around the screws as a temporary fix. Until a plumber can come and remove the toilet to then properly re-install it. For now, I have my windows open.

            The other handyman who said he didn’t smell anything must be desensitized to the smell, because although I had all the windows open the day he came in last week, a subtle smell is always there. He smelled along the base of toilet, including the back and said he didn’t smell anything.

            I am hoping the caulk keeps the smell contained. I kept my windows open overnight now knowing it was sewer gas to be cautious. I’ll make decisions on when the best time to call in a plumber….
            And I will look into ways to bring fresh air in without bringing dust into the HVAC system….

            And I’ll mention to my building that all residents who have left should allow the staff in to fill their ptraps to also avoid sewer gas smells from empty apartments leaking into other apartments where people are residing.

            Thanks so much for all your help.

            John, your support and answers to even my small specific questions, has been so appreciated as I was home trying to educate myself. And rule things in/out. I can’t thank you enough.


    2. Robin:
      Here are my assumptions — stop me where I am wrong:
      1) Since the “wind” rushes in from outside and from the hallway, you are on the lower floor of the building. Since you are on a lower floor your condo is at a negative pressure with respect to outside and with respect to the condos below you as well as some of the condos on your floor as well.
      2) Since you have dealt with all the drains in the bathrooms, it is not sewer gas from your drains.
      3) It smells like sewer gas since that is what you thought it was, therefore it probably is.
      4) Since these buildings are generally built with the bathrooms back to back with the neighboring condos and since these building usually have the bathrooms stacked above each other between floors, the most likely sources of the smell are the condo right next to you on your floor or one of the condos below you in the stack. If any of these are empty then it is likely that their traps are dry. Worse than that would be there is a break in the sewer line or sewer vent somewhere in the building.
      5) The ventilation system can make the smell worse or lessen it that all depends on whether the vents are moving air into your condo or out of it. Use the toilet paper test I mentioned in my previous post to determine what each of those vents are doing.

  12. Actually John, after writing
    Actually John, after writing my last reply, I turned my thermostat up a few degrees to initiate the Heat and I tried to toilet paper on the Return and the toilet paper sticks the Return. I was mistaken before saying it was blowing air out. I thought it did but now it doesn’t. When the temp is turned back down, the toilet paper doesn’t get sucked into the Return or blown away. It’s nothing. No flow either way.


  13. I was wondering if a simple
    I was wondering if a simple fix to the pressure difference might be to but a small battery operated fan blowing directly up the chimney to force air movement in that direction. We have the smoky smell problem occasionally. We use the candle in the fireplace method (a 3-wicker) which seems to help on those bad-smell-days. But just wondering if the fan idea would be a quick fix?

  14. Hi! This is a very
    Hi! This is a very interesting article, and a topic that we’re dealing with. We have a late 60s colonial in New England and have been dealing with a fireplace smell and draft in our house. I can feel air coming in through our (original) windows and it comes down the fireplace, leading to smells in our living room.

    We have a direct vent gas furnace/water heater and a radon mitigation system in the basement. We have five heat zones in the house (2 basement that are mostly kept low), 2 on the main floor and 1 upstairs (also that we keep off during the day when we’re not upstairs).

    The fireplace smell comes and goes, but definitely when the dryer is running. We’ve taken to opening some windows by the dryer when we run it, and that helps. Our damper is closed, too, so maybe it’s leaking? We’ve been recommended to get a top closing damper at the top of the chimney as well. I’ve told my husband (half-joking) that I’m ready to just take the whole thing out LOL.

    Anyway, I’d like to figure out what we can do to stop the fireplace smells from coming in. Frankly, it’s pretty annoying, and I’d like to not have to open a window during the winter when we run the dryer.

    We have not checked the top floor for leaks (though based on the comments here, that should be something we do). We were on the schedule for a blower door test before COVID happened, so we’re in a bit of a holding pattern right now. Any other thoughts as to what we can try?

  15. My house is under negative
    My house is under negative pressure all of the time – and I have had a few HVAC people out over the years to look at our air exchanger – thinking that must be the culprit? No one can figure it out. Would negative pressure in a house cause ear problems? Or is that just a weird coincidence 🙂 Thank you for your help!

  16. Very interested in finding
    Very interested in finding out more about negative pressure causing sewer smells. Specifically using a portable AC. I believe that is the issue I am having in my home. Besides shelling out a fortune to install central air, or always leaving a window cracked, are there other methods to stop this?

  17. Does negative air pressure
    Does negative air pressure apply to home sewer drainage systems? It would seem it shouldn’t because, with the exception of the soil stack/vents, it should be an enclosed system. That being said, after 3 plumbers and huge $$$, we are still dealing with a sewer gas issue in our home and the intensity seems to be adversely affected according to wind speed and direction. One plumber said it was due to negative pressure, but if the system is enclosed (except for the designated external vernting), how could that be possible unless there’s a leaking drainage pipe or leaking connection in the venting system. It is bad enough that e have been displaced from our home. And we cannot find anyone that apparently has the ability/knowledge to help. And my wife is physically suffering horribly to her nervous system.

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