Sewer smell from wall cavity with fan on, a big problem?

[Coherent]

We had bath fans installed in our 100 year old home and I now notice when the door is closed and the fans are on that it will pull air through a 1/4" hole in the medicine cabinet from the wall cavity. It has a sewer gas type smell. You can only smell it when the fan is on.

My question is how big of a priority should it be to fix it? The bathroom is tiled up to about 5 feet which makes creating an access hole to investigate challenging. We have aspirations at some point in the future of renovating the bathrooms, but it certainly isn't anything that is on our radar now. I suppose this question can be broken down into first if acute spritzes of sewer gas small are much of a health concern and the second is if having a volume of sewer gas building up in the cavity behind the mirror causes any structural damage over time to the wall.

What do you think? We had two plumbers out and one thought its not an emergency situation but should be resolved in a few years. The other couldn't even really smell the sewer gas and thought it might be the smell of old wood (he could also not want to go on a fishing expedition though). Thank you!

 

[Tuttles Revenge]

We pressure test drains and vents for a reason, to keeps sewer gases and germs out of our living space.

Sewer systems can have negative or positive pressure depending on a bunch of factors like your topography, sewer vs septic, time of day etc.. But adding the fans creats negative pressure in that area and draws air out of whatever is open.

I would pull the medicine cabinet out. But that may not give you a clear shot if its one of those old ones with the razor blade slot as they blocked off the stud bay usually 18" down.

A smoke test may be the way to locate the leak before opening walls.

 

[Sponsor]

 

[WorthFlorida]

If the plumbing has never been changed from cast iron, somewhere a hole has opened up. Sometimes CI will crack or corrode right through. If the bathroom is on the top floor with attic space above, look in the attic for the roof vent and check the pipe.

As you suggested, old homes can have a musty odor. If one plumber scented it, he would definitely know if it is sewer or not. As Tuttles suggested, a smoke test may be needed. It's a sure fire way.

 

[Jeff H Young]

I think its a health concern but I like living dangerous and take chances most people wouldnt . Also I think it be a lie to say sewer smell is always harmless. kinda like mold , I suppose rather than just take a chance (ignore) or go all out trying to find source might want to further study the risks .

 

[Coherent]

One of the plumbers did go in the attic to look at the stack and said he couldn't see anything wrong there. I previously switched out a toilet and that smell when I had the open sewer does resemble the same smell but perhaps how bad it smells from the fan varies from day to day.

This is a great suggestion of taking off the medicine cabinet to run the smoke test. I wonder if I'd need to get a separate handyman for that, it sounded like the first plumbers just wanted to open up the wall.

Thanks all!

 

[WorthFlorida]

Is the drainage cast iron pipe? Is the med cabinet over the sink? If both are yes, I would suspect the p-trap connection at the wye inside the wall is where there might be a leak from corrosion. It's only a guess.

 

[Jeff H Young]

One of the plumbers did go in the attic to look at the stack and said he couldn't see anything wrong there. I previously switched out a toilet and that smell when I had the open sewer does resemble the same smell but perhaps how bad it smells from the fan varies from day to day.

This is a great suggestion of taking off the medicine cabinet to run the smoke test. I wonder if I'd need to get a separate handyman for that, it sounded like the first plumbers just wanted to open up the wall.

Thanks all!

I think a lot of Plumbers havent done smoke tests and dont want to . I think you are better off with a plumber, most handymen I dont think have done smoke tests , unless you have a guy that you trust but I feel its outside of what a handyman might be proficient with. There are exceptions