We've had similar experiences in several homes, but our findings/solutions won't necessarily be right for you.

In house one, the problem was found to be an errant screw. The screw was from a light fixture installation over a sink, which punctured the vent stack, allowing sewer gas to escape. The builder tried at least half a dozen different fixes (including tearing up our shower) before considering that.

In two other homes, a definitive cause was never found. We had air vents cleaned, sprayed them with disinfectant, etc., after checking everything plumbing related. One builder even paid to have a plumber run a camera scope through the pipes to see if there was hidden damage (that house was also on slab.) Nothing. I bought Roto-Rooter's Pipe Shield and began using it weekly in the master bathroom. The odor disappeared, but I cannot swear that the Pipe Shield did the trick.

At our current house, a plumber suggested that I pour a little mineral oil into each drain, especially the jacuzzi tub that we rarely used. He said it would help prevent the water in the trap from evaporating quickly in dry weather (or when the A/C ran non-stop in the summer.) I now use Biokleen Bac-Out in all drains once a month, too.

Good luck! I understand the frustration your problem causes.

Is this a new house or an old one?

There was a poster on here a while back whose vent pipes were leaking onto his ceiling. Turned out they were never glued together. Newer construction.

Thanks for the replies. The house is 12 years old, but this is not the first time we have had the problem. Previously we thought we were negligent in keeping the traps wet in a seldom used tub and shower, although I could never prove this conclusively, and the odor always eventually disappeared. This time the problem is much more persistent and obnoxious. Unfortunately getting to the vent pipe in the attic is a real problem because of the low ceiling/roof clearance in this alcove area and the maze of ductwork restricting access. Would a camera inspection inside the vent pipe be a suggestion for checking the joint connections, or could a pressure check be done on the vent plumbing? The mineral oil in the traps and the Bioklean suggestiona are good ones and we will try them. Maybe the evaporation issue is the culprit because we only have the problem in the summer when the AC is running, but I am still really confused by why the odor ONLY occurs in the evening. Thanks.

A camera in a vent pipe will not detect that it is glued together.

What type of heat do you have?

AC blower will cause negative pressure in the house, drawing in air from wherever it can.

We have two furnace/AC units. Gas forced air furnaces with AC using same ductwork. The blowers are dual speed with the higher speed mostly used for cooling. As mentioned in earlier post we have never experienced the odor except during the summer. However, the odor has occurred when the AC has not been in use and no blowers were on.

Is it a "Rotten egg" smell? Where is the gas pressure regulator located? Near a window, intake,.... The regulator has a vent on it which may burp a small amount of gas on overpressurization.

Hope this helps,

Dan

Thanks for the reply. This is not a rotten egg smell as would be the case with a natural gas leak, and the gas line and regulator are on the far side of the house from this bathroom area. I'm pretty convinced this is a plumbing vent problem, but am puzzled by its "scheduled" occurences. Only appearing in the evening. I asked in an earlier post whether it is possible to do a pressure check on the vent lines but no-one has responded to this question. Any other suggestions out there?

Is it still happening? Is it possibly bats?

Pressure testing is typically done before plumbing fixtures are set in place. In order to test an existing system, I believe you would have to pull each fixture and place a test plug at each opening, then you can test the entire system. I don't know of any other way.

"Pressure testing is typically done before plumbing fixtures are set in place. In order to test an existing system, I believe you would have to pull each fixture and place a test plug at each opening, then you can test the entire system. I don't know of any other way."

Plus, you would have to somehow plug the connection to the sewer. Typically, pressure testing is done before the permit to connect to the sewer is approved. I don't know, maybe an inflatable bladder on a line snake, sent down past the last wye connection to the main sewer line. I don't think it is a good idea to separate the pipes.

Thanks for the replies on the pressure check question. I currently have all the drains and overflows blocked off in this bathroom. I also have the exhaust vent sealed off in case gas from the plumbing vent is finding its way back inside through the near-by exhaust vent. These steps have almost eliminated the odor in the house. We had one evening with a slight smell inside, plus a stronger (same) smell outside the house. I'm giving it a few more days and then I will start un-blocking things one-at-a-time to see if I can locate the source.

Are you on a septic system or city sewer?

City sewer.

Just a thought...considering the consistency of time of day which also corresponds with cooling of the air. Is it possible the cooler and heavier air is causing the vented gases to drop and follow the roof line and find it's way back into the house, perhaps through the vent or eaves? You said you had the vent sealed off, how about sealing from the outside too. Also, temporarily extend the sewer vent higher on the roof.

How about a neighbors proximity, could their vent gases be heading your way? If you live near or down wind from the sewage treatment plant or pumping station, it's odors cold drop lower in the evening and drift your way. This happens a lot in areas that do not properly pump and treat their sewage.

Perhaps the odor is getting into the house in a diluted manner and the chimney effect of the bathroom exhaust vent draws it into the bathroom in a more concentrated manner.

If you use gas, it's possible you are getting a down draft through the exhaust chimney from the cooler air and it concentrates into the bathroom.

What effect does the bathroom fan have on eliminating the odor and if it does, is it a long term effect? The exhaust gases from burning LP does have a similar odor as sewage gas. Also check your toilet to see if it fits loose to the flange, allowing gases to escape into the room. Check your drain pipe, at and anywhere after the traps under the sink and tub also.

Just trying to determine if the problem is indeed in the bathroom and not somewhere else and then concentrating in the bathroom.

Thanks maryland irisman. You're on the same trail I'm on. I strongly suspect we're getting a back-draft effect down through the exhaust vent. When I disassembled the exhaust fan housing a couple weeks ago I noticed the "el-cheapo" plastic flapper didn't seem to be closing properly. That's what led me to seal off this exhaust vent. Also, when I went up on the roof to inspect the vents for bird nests, etc., I did notice the odor right at the eave just below where the plumbing vent exits the roof. This plumbing vent is about 2 feet further up the roof from my exhaust vent....so if the lower evening temp could be allowing sewer gas to drop down toward the exhaust vent housing maybe that's where the odor is getting into the house. I am about ready to start unsealing my drains, overflows, etc. Once I am able to determine the exhaust vent is the culprit I will experiment with raising the plumbing vent stack. Thanks for your help, and I'll post the results.

The el-cheapo plastic flapper might just be letting the smell back in. Yes, I'd like to know what the final solution is, this topic comes up every year or so, it would be good to reference people back to this post.

Any update to this? I am having a similar problem, but I believe the source is my shower drain.

This is so strange! I have the same oder in the master bath. Same hours late evening

We are experiencing same problem in our Master Bathroom/commode area. It is a small (4x4) room that we can close off and know that it's definitely coming from there and not a sink, tub or shower. As others on this thread, I suspect it's coming either from exhaust vent backdraft or from the swear vent pipe. However, the smell is not sewer gas but rather more like urine/bird droppings. The suggestions above will lead me to check the roof for a nest and/or the flapper on the exhaust vent. Another thought is the toilet seal but I don't see a water leak in the ceiling directly below the toilet so....I think not. I'll re-post if I can find and correct the problem. Thanks all...this thread had helped much.

I am in Phoenix, Az. & also have this problem in my master bath & only when the AC is running. We also have evap cooling & gas heat & do not have this problem when either is running. I replaced the toilet wax seal & still have the problem. I rerouted the AC condensate line which was in a vent pipe by the bathroom but not the vent pipe area where the smell is. I also put a 3' piece of ABS pipe to extend the vent pipe. I opened up the wall by the floor where the vent pipe is & a draft comes out when the AC is running. How can that be as this is in the wall? Running out of options.

Interesting. I am also in Phoenix and have this problem also. Only when the AC is running, yet the AC vent itself in the bathroom doesnt stink. We are not very handy and have no idea what to do, but the smell is making me nauseated. I keep pouring bleach down the various drains in there just to drown out the bad stink if nothing else.

From my own experience, it could be nothing more than a toilet that needs to be thoroughly cleaned. Not just in the bowl, but every surface of the toilet. Look for spattered dried "waste" under and behind the seat, and the top, inside, and outside of the rim. Remove the seat and clean between any small gaps. Also wipe all the outer surfaces, all the way to the floor where something may have dribbled and dried.

About a week ago we had the vent pipe in the bathroom sealed & dropped a smoke bomb & very soon found our problem. The vent pipe was broken. We replaced the pipe & the smell went away. Still have no idea why we only smell this when the AC was on.

I just recently bought a house in maricopa AZ and my toilet in my master is in a separate room of its own and there is always a smell even if we clean that room thoroughly. Driving me crazy can anyone help?