Terrible Sewer Gas Smell From Bathtub Drain, Stumped 3 Plumbers

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smellyinmd

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I am getting this terrible sewer gas smell coming from my bathtub drain. It is practically blowing out of my drain and fills up the house with terrible odor if I leave it uncovered for more than a few minutes.

Generally, the smell is worse during the day than it is at night. The tub gets used at least twice a day. The house is a single story home on a slab. The smell started about 1-2 months ago, and was not an issue for the first few months that I lived in this home.

Troubleshooting that I have done so far:

  1. Checked the vent stack from the roof, it was clear.
  2. Plumber came by to check if P-trap was not filling, camera inspection showed trap is full of water. Tried turning on all the other faucets in the house and the water did not empty from the trap. Smell still came through even though trap has water (WEIRD).
  3. Tried bio-clean for a week, no luck.
My first theory was that there was a vent problem. But that doesn't seem to be true because the trap is holding water fine and I have no issues with slow drains in the house (including the tub). My second theory was that the trap was not holding water, so gas was free to come up. But, a plumber confirmed that the trap was full of water while the smell was coming through.

I am out of ideas and would prefer not to rip the tub out to fix this. Any ideas on what could be causing the smell? THANK YOU
 

LLigetfa

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My guess is you have a bacteria colony in and/or before the trap. I had that problem in a shower drain a few years ago. Pouring bleach down the drain would fix it for a short while but it kept returning until I ran a cleaning brush through the drain.
 

smellyinmd

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My guess is you have a bacteria colony in and/or before the trap. I had that problem in a shower drain a few years ago. Pouring bleach down the drain would fix it for a short while but it kept returning until I ran a cleaning brush through the drain.

Thanks for the reply. I poured bleach down last night and today there is no sewer smell! However, there is a bleach-y smell blowing out of the drain.

The trap is still full of water, any idea on why there is air blowing out of the bathtub drain?
 

Reach4

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The trap is still full of water, any idea on why there is air blowing out of the bathtub drain?
If you have air blowing out, there is a problem... probably a bad trap despite seeing water in the trap.

Try a little smoke from a wooden splint or extinguished match to make sure there is an exiting air flow.
 
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smellyinmd

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If you have air blowing out, there is a problem... probably a bad trap despite seeing water in the trap.

Try a little smoke from a wooden splint or extinguished market to make sure there is an exiting air flow.

I can definitely feel the air coming out of the drain. When you say bad trap, what do you mean? Like cracked or something? It's holding water fine so I dont know what could be wrong with it.
 

Jadnashua

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Definitely a conundrum...if the trap is actually full, you can't get air blowing out of it unless the tubing from the drain to the trap has a hole in it. With a flashlight and maybe an inspection mirror, you could probably investigate.

When you feel air coming out, double-check that the trap has still got water in it.

The last thing I'd try would be to remove the cover on the overflow, use a bottle brush or something similar, and try to clean that area out, too. Maybe soak it in bleach, and then scrub. Pouring bleach down the tub will not reach the overflow, or much of it, if any.
 
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Reach4

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I can definitely feel the air coming out of the drain. When you say bad trap, what do you mean? Like cracked or something? It's holding water fine so I dont know what could be wrong with it.
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A trap's job is to block air. There is a "water seal" of about 3 inches normally. If the pressure is high enough, the air could push through. But the pressure should not be able to get that high.

I think your trap has a problem, or it is somehow in the wrong place. Can you see the trap from the basement or crawl space?
 
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WorthFlorida

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You have something in the home causing a negative pressure and it's pulling air in from any source. You're in MD, you're into the heat of summer, is the AC on? Is the clothes dryer or AC on when you feel this air blowing out of the drain? Is it the drain or the overflow tube? They are one and the same connection wise, you should feel air from both. Is the bath tub stopper at the drain or a lever from the overflow. Put some water in the tub and close the stopper, any air then has to come from the overflow. Bottom line, you never should feel air coming from any drain.

When you feel the air movement, turn off the AC and dryer and open a window to equalize the air pressure inside the home. If there is no air movement at least you narrowed it down. If there is then you have something not normal. Is this a private single family, condo or townhome?

upload_2018-7-11_18-58-55.png
 

MKS

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It's on a slab so verifying that there is a trap is a problem.
Have you had any other work done recently?
 

smellyinmd

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Definitely a conundrum...if the trap is actually full, you can't get air blowing out of it unless the tubing from the drain to the trap has a hole in it. With a flashlight and maybe an inspection mirror, you could probably investigate.

When you feel air coming out, double-check that the trap has still got water in it.

The last thing I'd try would be to remove the cover on the overflow, use a bottle brush or something similar, and try to clean that area out, too. Maybe soak it in bleach, and then scrub. Pouring bleach down the tub will not reach the overflow, or much of it, if any.

Plumber did not see any holes in the pipe, but I wouldn't rule out the possibility of something being loose. My next step might be to open the drywall and investigate.

index.php
A trap's job is to block air. There is a "water seal" of about 3 inches normally. If the pressure is high enough, the air could push through. But the pressure should not be able to get that high.

I think your trap has a problem, or it is somehow in the wrong place. Can you see the trap from the basement or crawl space?

I have to open up the drywall behind the bathroom to investigate. I doubt that the trap is in the wrong place because this was not an issue for about 6 months when I first moved into this home. I do see the water moving gently on the surface of the water seal though.

You have something in the home causing a negative pressure and it's pulling air in from any source. You're in MD, you're into the heat of summer, is the AC on? Is the clothes dryer or AC on when you feel this air blowing out of the drain? Is it the drain or the overflow tube? They are one and the same connection wise, you should feel air from both. Is the bath tub stopper at the drain or a lever from the overflow. Put some water in the tub and close the stopper, any air then has to come from the overflow. Bottom line, you never should feel air coming from any drain.

When you feel the air movement, turn off the AC and dryer and open a window to equalize the air pressure inside the home. If there is no air movement at least you narrowed it down. If there is then you have something not normal. Is this a private single family, condo or townhome?

View attachment 47549

This makes sense to me but I have to do to further testing. I do notice that the smell is worse during the day, especially while our attic fan is on. Right now, the AC was off and there was not much noticeable smell by the drain, but when I turned the AC on the smell was noticeable. Still, the smell was way more noticeable, and you could feel the air actually blowing out, during the day when the attic fan was running.

The home is a detached single story on slab, 1 bathroom, 950 sq ft. The return for the AC is right outside the bathroom door. The air blows out of the drain regardless of the dryer operating. I can feel\smell the air out of both the overflow and the drain during the day. I'll test out plugging the drain and feeling the overflow tomorrow when the fan is running. I'll try out the windows tomorrow as well. I will leave the chimney damper open as well. Thanks for the suggestion.
 
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SteveW

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When you say "attic fan," are you maybe referring to a whole house fan?
 

smellyinmd

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When you say "attic fan," are you maybe referring to a whole house fan?

It is a fan in my attic that blows hot air out of the attic. The roof also has 4 air vents to try and replace this air.

Looks like this from my attic https://upload.wikimedia.org/wikipe...-roof.jpg/220px-Attic-fan-underneath-roof.jpg

The tub trap will be below the floor.

Yea I probably wont be able to see the actual trap, but I might see something loose\cracked. Not sure! Hopefully it is just an air pressure issue that I can deal with without ripping the bathtub out.
 

ngmtl

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Check the vent in the stack is not clogged, this would be an air clog not a water clog. If there is positive pressure or gas that can't make it out to vent on the roof, it could be bubbling up and escaping through your p-trap water. This is my theory, but may want to vet it with Terry or another master.
 

Jadnashua

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It would take some fairly strong pressure to be able to push gas past the trap. IF there was a lot of pressure, you'd probably be able to see its effect in places like the toilet, where the water in the bowl would be moving. You might be able to see the water in the trap of the tub moving, but you may need an inspection camera as it typically isn't right underneath the drain (it's often underneath the overflow).
 

Reach4

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Hopefully it is just an air pressure issue that I can deal with without ripping the bathtub out.
How motivated are you? You could measure the air pressure in your drain lines. This can be done very cheaply. The procedure would be to run some clear tubing through a trap into the drain. Blow to clear any water. A toilet trapway would be one such trap that could be used. That would keep that toilet from being used during the monitoring. Another trapway could be used.

Take the near end, and put it into a glass of water. Normally the level of the water in the tubing should be about the same as the other water in the glass. If there is pressure in the drain air, the water level in the tubing would drop. An inch, at any time,would indicate a lot of pressure. What you have made is called an open air manometer.

You can buy clear tubing easily and cheaply enough. You can also get it free from somebody who uses an oxygen machine at home and throws away a 30 or 40 ft length every month. Cut off what you need.
 

WorthFlorida

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Depending how old the home is, the drain is either galvanized pipe (they still rust) or PVC (cracks or a glued joint broke loose). On a slab the drain for the tub is usually a square hole in the concrete, maybe a 12" square. If there is some kind of leak, the odors are coming from the ground itself and not the drain. SInce you cut the wall out from behind the tub you do have a way to check this theory out.

This is not difficult to do but remove the tub drain from the top and replace the gasket that sits between the drain flange and the tub. They can deteriorate causing water to leak into the ground. Also check the overflow gasket that sits between the tub and the overflow flange. These can work lose. The one or two screws on the overflow cover work lose or if they were removed the gasket can easily have fallen. But you would could have seen that from behind the tub. When you're satisfied that these gaskets are good, use spray foam to seal up the hole in the concrete floor.

Another area to look at is the wall cavity where the vent pipe is. It probably goes up into the attic space and the hole cut in the top plate should be sealed with fire proof foam seal. If not the attic fan is pulling air from that cavity thus the drain area of the tub.

As jadnashua stated, it does take a lot of air pressure to pull water out of a p trap.
 

smellyinmd

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Thanks for all the tips guys.

I did some testing today and have found a correlation with the air\smell with my ac\attic fan. In the morning, AC was off, attic fan was off, there was no smell and no air blowing out (that I could sense). Later, turned on the AC, attic fan still off, I could notice a smell from the drain but not sense air blowing out. With the attic fan on and the AC on, the smell was there and I could also feel the air blowing out of the drain. Seems to be more of an HVAC issue with pressure in my house. However, I still have not determined where the smell is exactly coming from yet. I also have my doubts now that it is sewer gas being pulled through the water in the trap.

Any suggestions on how to solve that pressure problem? I am thinking to either add more vents in my roof or install some sort of pressure equalizer to my hvac. Thank you once again.
 

Reach4

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However, I still have not determined where the smell is exactly coming from yet. I also have my doubts now that it is sewer gas being pulled through the water in the trap.
Your observation should increase your suspicion that air is being pulled through the water in the trap. If you measured the relative pressure in inches of water, you could tell.

With the attic fan on and the AC on, the smell was there and I could also feel the air blowing out of the drain.
You could isolate the problem further by turning on only the attic fan or only the AC blower.


Any suggestions on how to solve that pressure problem? I am thinking to either add more vents in my roof or install some sort of pressure equalizer to my hvac. Thank you once again.
For the attic fan, you would normally want to add more/bigger soffet vents, or clear the insulation etc from what you have now. You may need baffles/chutes to keep the air path from the soffet vents from being blocked.
 

smellyinmd

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Your observation should increase your suspicion that air is being pulled through the water in the trap. If you measured the relative pressure in inches of water, you could tell.

You could isolate the problem further by turning on only the attic fan or only the AC blower.


For the attic fan, you would normally want to add more/bigger soffet vents, or clear the insulation etc from what you have now. You may need baffles/chutes to keep the air path from the soffet vents from being blocked.

I will have to try the manometer experiment you suggested to see, thanks.
 
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