Well, this stinks! Paging those with plumbing knowledge...

In the spirit of being genuinely very far off topic and drawing on this community's vast knowledge base, I have a question for any of you who have plumbing experience or septic tank experience.

I live in a single-story ranch-style house built around 1960, where we have been for about five years. The home is "in the country" -- or at least outside city limits, meaning there are no city sanitation services. We have septic tanks, which I assume are original to the home. They are concrete and have passed a couple of inspections, so I think they're fine. There are two main tanks in series which service the toilets, sinks, showers, and so forth. The kitchen and laundry have their own grease trap and tank with a separate set of field lines. Everything seems to work as it should with stuff going to the tanks. No leaks, nothing backs up, everything goes down, all good.

Our problem isn't with solids or liquids, but with gases. Periodically (but not constantly at all), our master bathroom gets quite the unpleasant whiff of sewage. It's bad, really bad, and I will spare you further details. It doesn't seem to matter whether the tanks have been pumped recently or not, and it does not seem to depend on the time of day, the season, or anything like that. The only trend I have been able to sort out is that it's most likely to stink when water is running in the master bathroom (shower or sinks, but especially the sinks) at the same time the central air conditioning system is running. Flushing the toilet does not seem to cause it, and no other rooms / bathrooms have this issue.

My only guess is that we have some kind of a venting problem, and the septic system is somehow venting into my bathroom. The sinks both have standard p-traps on them, so I don't think it's as obvious as sewer gas coming back up the pipes -- but I don't know how else it would be getting in the house. Also, I don't hear bubbling or gurgling. I'm not sure whether it's relevant, but the master bath is the furthest point of the house from the septic tanks.

What sage advice can you guys share? Is there some obvious DIY repair (that isn't obvious to me) to deal with this, or is it time to bust out the yellow pages and call a plumber?

Thanks!
Cheers, John

It sounds like no one way valve was put on the main pipe between the house and the tank. Or at least none at or near the master bath. You will find that the smell if most likely coming up through the sink.

The whole point of venting is so that some water stays behind in the associated trap after draining.

Envision that the trap and water works as a one way valve of sorts.

Assuming that everything worked OK at some point in the past my money would be on a venting system blockage.

Make sure the P-traps are not getting siphoned out, if you have a pipe drain that is partially plugged, you could end up with a "full" pipe situation that creates a siphon from the P-traps, not allowing the siphon to break, and leaving the traps empty. I would say after encountering the smell, check the P-traps for water, may be able to narrow down a venting issue or clog issue in the general piping run. But a full pipe could result from a long down stream clog, filling up the drain pipe, but not overflowing any drains.

Just noticed the note about the AC running, maybe an imbalance in the bath air flow is pushing out the p-trap, try shutting down the air supply to the bath, put a taller p-trap on the sink.

Do you have a tub in the master bath that rarely/never gets used and might have a dried out trap?

Quote:

---

Originally Posted by Hoooper

Do you have a tub in the master bath that rarely/never gets used and might have a dried out trap?

---

I have found this ^ to produce the same smells with our system. We have a tub that rarely gets used and the trap dries out and lets the gasses in.

Another +1 on the dry trap. Have you ever noticed when it gets super windy that the water in your toilet bows will move up and down in level? If it gets pulled too low, your p traps can let gas in. Likewise, if a trap isn't fed regularly, the water can simply evaporate and let gas in.

Wow, thanks for the quick input, guys.

The master bath has a tub/shower combo (not a fancy or updated house!), which is used every day, morning and night. There are his-and-hers sinks, both used daily. There is no way the traps are dry from non-use, but I suppose it is possible they are siphoning, which would have the same effect. I don't know a good way to test that other than just disassembling the p-traps and seeing if they are holding water or not. I wonder if they are "partially siphoning" while water is in use, but then somehow breaking siphon and retaining enough water for the trap to work correctly once the spigot is turned off. That doesn't make sense from a siphon perspective, but it would explain why the stink is intermittent.

I assume that the whole septic system is vented somewhere to the atmosphere, through one of the pipes coming through the roof. I really wonder if something in that main vent system is plugged or clogged, which is forcing the gases to vent to the house/bathroom. The fact that the stink is intermittent and seemingly tied to water use + AC running is what confuses me -- along with the fact that it is only in this bathroom. I guess that all depends how the venting system is laid out, and there's no way to know that without getting up in the attic and tracing pipes. It's way too hot for that right now. I'll check the p-traps on both sinks this weekend, and if they are holding water like they are supposed to, I'll probably call a plumber to scope the vents on the roof or whatever else he feels will take care of it.

Cheers, John

Do you know where the AC condensate ties into the sewer system, if it does? If not trapped and vented, and not connected to waste correctly that could possibly cause air to push out of the system.

Quote:

---

Originally Posted by Hoooper

Do you know where the AC condensate ties into the sewer system, if it does? If not trapped and vented, and not connected to waste correctly that could possibly cause air to push out of the system.

---

I do, and I hadn't thought of the condensate drainage line -- and with the heat we have had this summer, that AC unit has been working hard. The air handler is in a hall closet which happens to be adjacent to the master bedroom and bathroom. It should be relatively easy to check on that. Thanks for the idea!

Cheers, John

I also live rural and have a septic system.

You should have one or more vent pipes for the bathroom that extend up through the roof. If that thing is plugged, it could siphon the water trap (p-trap) dry.

One way to find if it is plugged is have someone go up onto the roof with a garden hose and see if they can "fill" the pipe such that water is flowing out of it.

Alternate problem, your vent piping has broken in the wall and isn't venting above the roof-line.

A third possible problem, some AC setups with the evaporator in the attic were set up to tie into the drain under the sink. If this gets decoupled, you can get fun smelly problems too. (BTDT at previous house). Generally when this happens you get AC condensation water standing in the bottom of your vanity, running out onto the floor and all that mess.

I live in a house with three bathrooms and its only me and my wife and we dealt with that problem of sewer gases in the house only to find out the P traps were evaporating over a months time and letting the gas into the house. Once a month she runs a bit of water in all toilets and shower drains. Problem solved.

Thanks again, guys. Brief update. It is definitely not dry p-traps. Between the four of us, every drain in the house is used on a daily basis.

Last night I took a look at the HVAC unit's condensation drain line. The air handler drain line has its own P-trap of sorts made from PVC, to accommodate a wired sensor to shut the unit down if water backs up in the drain line. It was dripping normally while the AC was running.

That outlet then drips into a large open PVC pipe -- I didn't measure, but I'd say it's a 4" pipe. That large pipe also receives the pressure release drain line on the water heater. The open 4" pipe runs down into a reducer to what looks like 2" pipe. There is a large 2" p-trap before it goes into the horizontal run out. I could see that water was standing at the bottom of the 4" pipe by the reducer, which tells me that the p-trap is working. I also flushed that line really well with hot water -- easily done since the water heater's drain line was there and I just had to open the pressure release valve to send hot water down the drain system. After a good flushing, I saw the same level of water, again suggesting to me that the p-trap is working correctly. There were no smells in the utility closet with that equipment.

I also know that the problem wasn't there because after my work and flush-out, the bathroom still stank something awful last night as we brushed teeth and got ready for bed.

That leaves checking the p-traps under the two master bathroom sinks to make sure they are working correctly, and possibly checking the vent pipes that exit through the roof. P-traps are cheap, so I may just fashion a couple of extra-deep ones and install them while I already have everything apart. Another friend of mine suggested putting secondary air admittance valves under the sinks as well, to let air in but not let sewer gas out -- basically a backup and/or bypass of the main vents if they are not working or stop working in the future. Any thoughts on those? Thanks again for the tips, and I will update again in a few days to let you know what I find next.

Cheers, John

after retiling a bathroom floor, I reinstalled the toilet with a flange extender and new wax ring. We noticed a rotten egg smell every once in a while. I thought it was due to a bad wax ring seal, but discovered that there was some dried thinset that was keeping the flange extender from sitting flush on the flange, leading to some sewage gas coming out without a leak from the toilet.
don't know if this is your problem, but something to consider if you notice the smell stronger around the base of the toilet.

Quote:

---

Originally Posted by dukegrad98

Thanks again, guys. Brief update. It is definitely not dry p-traps. Between the four of us, every drain in the house is used on a daily basis.

Last night I took a look at the HVAC unit's condensation drain line. The air handler drain line has its own P-trap of sorts made from PVC, to accommodate a wired sensor to shut the unit down if water backs up in the drain line. It was dripping normally while the AC was running.

That outlet then drips into a large open PVC pipe -- I didn't measure, but I'd say it's a 4" pipe. That large pipe also receives the pressure release drain line on the water heater. The open 4" pipe runs down into a reducer to what looks like 2" pipe. There is a large 2" p-trap before it goes into the horizontal run out. I could see that water was standing at the bottom of the 4" pipe by the reducer, which tells me that the p-trap is working. I also flushed that line really well with hot water -- easily done since the water heater's drain line was there and I just had to open the pressure release valve to send hot water down the drain system. After a good flushing, I saw the same level of water, again suggesting to me that the p-trap is working correctly. There were no smells in the utility closet with that equipment.

I also know that the problem wasn't there because after my work and flush-out, the bathroom still stank something awful last night as we brushed teeth and got ready for bed.

That leaves checking the p-traps under the two master bathroom sinks to make sure they are working correctly, and possibly checking the vent pipes that exit through the roof. P-traps are cheap, so I may just fashion a couple of extra-deep ones and install them while I already have everything apart. Another friend of mine suggested putting secondary air admittance valves under the sinks as well, to let air in but not let sewer gas out -- basically a backup and/or bypass of the main vents if they are not working or stop working in the future. Any thoughts on those? Thanks again for the tips, and I will update again in a few days to let you know what I find next.

Cheers, John

---

I added one of these under the sink in the room I was having problems with sewer gas smells. No issues since.

Oatey 39018 Sure-Vent Air... https://www.amazon.com/dp/B000H5SLWM...p_mob_ap_share

Quote:

---

Originally Posted by dukegrad98

The only trend I have been able to sort out is that it's most likely to stink when water is running in the master bathroom (shower or sinks, but especially the sinks) at the same time the central air conditioning system is running.

---

I just had a thought...

dead mouse in the HVAC air tubing/ducting?

I wanted to say thanks again for the tips and suggestions, and close this loop now that we finally have a solution. There was a delay that I will attribute to supply chain issues -- the local big box stores did not have the parts I anticipated needing for a week or two (despite what their website inventory would have you believe!), so I finally gave up and ordered them online, then had to wait for shipping, etc.

Once the parts arrived two weekends ago, I disassembled the drain plumbing on both vanity sinks. Everything was clear -- not to say sparkling clean, but definitely no clogs in the p-traps, drain lines out, or anything like that. Confident that the issue was with the venting, and since the primary culprit / trigger seems to be my sink, I reassembled it including an air admission valve like the one Papa mentioned above. Before doing my wife's side, we decided to see if there was any change from that modification alone.

It turns out that has proven to be an adequate fix. For more than a week there hasn't been so much as a whiff of stink in there -- well, at least from the plumbing system.

As I was doing this project, my wife reminded me that it wasn't that long ago (several months) that we replaced the whole HVAC system for this portion of the house -- new outside unit, new inside air handler, etc. I don't think we immediately had the problem of the smell, but I am confident that the new indoor unit moves a lot more air than the old one did. I also had changed the indoor filter elements somewhere around the same time we started noticing the problem with the bathroom smelling. I've just about convinced myself that the new air handler was creating enough of a vacuum/pressure differential to clear the p-trap and allow septic fumes into the house. (Depending on how we leave the doors in the hallway around the air handler return grate, it will pull/push them as much as thirty degrees when the system turns on.)

At least for right now, this gets a "definitely good enough" rating from the wife. It's great to be able to brush my teeth without a sudden smell that makes me want to lose my breakfast. I suspect there's still something going on with the primary vent, but I haven't been up on the roof or into the attic to investigate. I'll settle for the bathroom not stinking, and the next time I have a handyman out I will add checking the primary vent to the list for him.

Thanks again for the input --
Cheers, John

I wonder if the sink you put the air admission valve on was added after and depending on how the vent pipe from the other sink's vent was plumbed if that wasn't adequate for the other sink. Just a thought.

I was suspecting an intermittent dry trap or bad air admittance valve hidden somewhere but I'm glad the problem is fixed.
Depending on how it's plumbed a dual sink can cause one trap to siphon dry at times as well.

FWIW the curious thing is the way it got worse with the AC on. All HVAC systems are supposed to
be designed with a supply in the bathrooms but no returns in there so you don't suck up enough moisture
in the ductwork to cause issues. So in short the problem was worse when the room was under pressure
instead of the vacuum scenario someone mentioned. I also suspected a dry floor drain somewhere because
that's essentially a trap device as well.
Glad you have it fixed
Dale