I apologize for the length of this post, but since the sewer and HVAC guys I've talked to seem to be stumped, I'll try to explain my situation in as much detail as I can, and hope that someone out there has some answers!

For the past three winters (the four before that were fine), I have been getting a tremendous amount of sewer gas build-up in my second-story attic space. I say winters, because I do not have any issues unless daytime highs are in the 50s F or below. Also, the smells tend to start around 8pm, peak at 10pm, and fall off after midnight.

To explain the inconsistency: my understanding is that on warm days, the ground is warmed up enough to allow convection to carry sewer gases up and away from roof vents, but on cold days gases can become "trapped" closer to the ground due to temperature inversions (colder air below warmer air).

So, given the days/times I'm having problems, I have reason to believe that the sewer gases are mostly coming from neighbors' plumbing vent stacks and entering my attic through soffits and other roof openings. On these cold nights the sewer gas odor is often present outdoors as well, but seems to depend on what the wind is doing. Sometimes I can smell it in my back yard, other times in the front, and often I can smell it in other parts of the neighborhood.

In the part of the attic where the gas build-up is strongest, the gas is seeping into the upstairs bedroom below it making it basically unlivable. Also, My HVAC/Furnace units are in the attic, so any time the heat runs, it distributes some of the gas throughout the entire house.

The city has already been out multiple times to flush the sewer main and smoke tested for leaks etc. but nothing has helped. However, my neighbors have not been complaining about this issue, which tells me there may be some things I can do myself to keep the sewer gases from building up in the attic.

Here are some things I'm considering:

1. Have my next door neighbor install some type of filter on his plumbing vent stack (I already installed a charcoal one on mine and it didn't help). That neighbor is in a one-story house next to my two-story, and the gases seem to be worse on that side of the attic than the other side (my other neighbor has a two-story house). I doubt this would resolve the issue completely, but maybe it would help?

2. Run an ozone generator in my attic on cold evenings. Based on what I've read, this could be pretty effective, though it comes with drawbacks of its own, and does not address the real issue of why the sewer gases are building up in my attic in the first place.

3. Have an HVAC company check for proper system balance. Maybe I have a negative pressure situation in the house/attic that is drawing more air into the attic than normal?

4. Give up and move somewhere else

Thanks in advance for any advice.

 

It sounds like you've been pretty thorough with your investigation so far.

You mentioned that the city did a smoke test. I assume they were only testing their mains and did not smoke test your home. I tend to think the likely source is somewhere in your home. You could have a broken pipe in the walls somewhere. A gas detector can detect far below what your nose can smell and it can provide an objective readout that you can use to help pinpoint the source.

1. I was about to suggest having yours and your neighbors vents extended like little smoke stacks to get them high enough to get the gasses up and away better but when you mentioned that your house is 2 story while your neighbors is a single it killed that idea. A filter on your neighbors house might be a good try. You might even move the one on your house to theirs if you think they are the source and not your vents.

Have you been on your and your neighbors roof to smell for the gas? Are you able to almost certainly confirm that your neighbor is the source? If it's not easily detectable outside as a stream heading for your house I can't figure how your attic could be collecting and concentrating it. You might also be able to borrow a gas detector from the gas company or local fire department. They might not let you borrow their detector but might come over and operate it for you. I don't know if there is any way your attic can collect and concentrate sewer gas so I'm looking for a stream outside that has a higher concentration. Since sewer gas is heavier than air it's not natural for it to go uphill and into your attic. You should be able to find a definite, concentrated stream carried by the predominant air currents around your homes.

2. Ozone can work on smells but I've had best luck at concentrations that are hazardous which can only be done when a house is vacant. You need a longer term solution so I would NOT recommend generating ozone as a long term solution. It's a toxic, highly oxidizing gas that's not good for humans or pets and it also attacks other things like some plastics.

3. I think this is a good next step. Even if you don't have a serious negative pressure situation you could discuss options for creating positive pressure inside your home. Hopefully drawing air in from a clean location. Sewer gas is heavier than air so it will tend to sink. You might need an air intake up high on your home or at least on the side away from your neighbor.

 

All great ideas from Pilot Dane & great efforts you've already made but remember on any fix it start with trying the simplest & cheapest first - a filter on your neighbor's one story house. Then one morning try flushing strongly concentrated ammonia in your toilets and nothing else and see if the smell at night is noticeably more of an ammonia only smell vs a typical sewer gas smell of ammonia & feces. I would suggest bleach instead of strong ammonia if your not sure you can distinguish ammonia from sewer gas but straight bleach could def hurt your pipes more. You can also consider vinegar instead of ammonia if you are afraid the ammonia smell will be unbearable. I expect that you will notice the change in smell type & therefore be able to eliminate your neighbor as the cause and prob have to tear down some walls or flooring to track down the pipes for leaks which I know is what you were trying to avoid.

 

Thanks Pilot Dane and DavidK7 for your detailed responses. Before I bother my neighbor about this, I think I will have an HVAC company come test my systems for proper balance, leaks, etc. So far the ones I spoke to over the phone were unsure what to do about this, but maybe once they come out the root of the problem will become evident.

 

I'm betting the source of the smell is something in your house, possibly a cracked or leaking pipe.