IâÂÂm experiencing an intermittent issue with the sewer gases (rotten onion smell) entering the flat through the bathroom. Very annoying as you can imagine. Bizarrely it âÂÂonlyâ happens 2-3 times a week, usually between 6pm and 8pm and it lasts for about 0.5-1h but it then goes away. It seems to happen more frequently in cold weather conditions⦠but there are exceptions to that. Some extra detail: WeâÂÂre on a top floor of a four storey refurbished office building. The neighbours directly below us are not affected by the issue but people opposite sometimes are too. It only ever starts in the bathroom and comes from behind the wall where the vent pipe is. The plumber sent by the developer came earlier this year, did the smoke tests and checked the installation in our flat. Apparently itâÂÂs all fine and the issue must be outside our flat. He mentioned that we share the soil vent pipe with the flat directly below so he went downstairs and âÂÂsealedâ something in the shaft holding the vent pipe (apologies for the lack of technical know-how and wrong terminology). Then it was fine for four months (summer) and itâÂÂs just started again! IâÂÂve done some research and due to the intermittent nature of the problem it shouldnâÂÂt be related to the toilet wax seal or t-trap or air admittance valve for that matter (donâÂÂt have any in my flat). I might be wrong though so I donâÂÂt want to exclude that just yet. ItâÂÂs definitely not cooking smells either. No smell on the corridors or in the basement. The plumbers seem to be out of options (!?) but the issue persists so we need to find the solution. Has anyone come across anything similar and managed to fix it? Could this be related to the usage of appliances/water/waste during the peak times? Please help.

Obviously this is an older home that most likely did not originally have indoor plumbing, therefore they ran the vent stack up the exterior of the structure. Most codes no longer permit any vents or waste stacks to run on the exterior of the structure, however in this case it would be grandfathered in. Having said that, even though the vent may be permitted to run on the exterior of the building it still must meet the code restrictions concerning how and where it may be terminated. No vent may be terminated closer than 12" from any vertical surface. In urban areas where structures are built close together or where a structure is physically located close to the property line it must be noted that NO VENT may be terminated closer than 10' horizontal from the property line. All vents must terminate 7' above any flat roof that is used as a deck within 10' horizontal from the vent stack. Under the Uniform Plumbing Code (UPC) ALL VENTS must be terminated a minimum of 6" or average snowfall plus 6" above the roof line, no exceptions. Under the International Residential Code (IRC) or the International Plumbing Code (IPC) all structures are required to have one "Main Vent" which must terminate through the roof and extend 6" above the roof line (average snowfall plus 6" in areas subject to frost). Main vents may not have a horizontal offset. (In plumbing any pipe which rises at an angle of less than 45deg above flat horizontal is said to be a horizontal pipe, whereas any pipe which rises at an angle of 45degrees or greater is said to be a vertical pipe.) Keep in mind that the stack in question is a "Main Vent". You could use a pair of 1/6th bends "sanitary 60deg elbows" or a pair of 1/8 bends "sanitary 45deg elbows" and offset the pipe from the wall to the edge of the roof at a 45 or 60deg rise, then extend it above the roof line. Under the IRC when additional auxiliary vents are required they may be terminated through the roof, in the attic space, or through the wall under some rather strict rules. Through the wall vents: a. may not terminate under a vented soffit. b. Must terminate 2' above any opening into the structure within 10' horizontal. c. must terminate 4' below any opening window within 10' horizontal. d. sidewall vents must be fitted with screens to prevent birds from nesting in the pipe. many local codes simply prohibit sidewall venting completely. As a personal note: While the IRC does permit terminating a vent in the attic space I personally refuse to do so and I would highly caution anyone to reconsider before doing so. One of the biggest problems with terminating a vent in the attic space is water vapor from the bath, shower or laundry discharge will eventually be carried into the attic space where it condenses with the cooler air and settles into your insulation. This not only renders the insulation ineffective, it also provides a perfect medium for mold growth. In addition, any heavier than air sewer gases that were carried into the attic space will eventually migrate through the ceiling materials and you end up with sewer stink throughout the house. As if that were not problematic enough, most homeowners and sadly many insulation installers are unaware that DWV vents may be terminated in the attic therefore they inadvertently cover the vents when installing additional insulation. In order to understand venting we must understand the nature of sewer gas. Sewer gas is a combination of a number of gases that are produced by the decomposition of organic materials. If the drain lines are pitched correctly there should not be any major accumulation of organic materials standing in the pipe, however the pipe walls are unavoidably coated with a residue of organic waste material so a minor amount of decomposition is taking place in the pipes constantly. Sewer gas is primarily composed of methane, nitrogen, carbon dioxide, sulfur dioxide, hydrogen sulfide, nitrous oxide and a lessor amount of water vapor. Some of these gases are heavier than air, while others are lighter than air and the proportions of the gases will vary constantly therefore we can never be sure if the gas is naturally rising up the stack or settling down to the main drain line. As liquid flows through the pipe it displaces the volume of the gases, therefore under some circumstances the displaced gas may rise and go out the stack or it may be pushed further down the pipe. The only thing that is certain is that it does move. In addition, as we discharge warm water such as from a shower, tub or laundry discharge the air and gases in the line are also heated and under these circumstances even gases that are basically heavier than air can and will be carried to the top of the stack by the warm air and gases. Once they leave the stack if the outdoor air is still the heavier than air gases will immediately begin to settle towards the ground ergo we smell sewer gases when outside on a cold still day. I would be interested to hear the theory as to why a 180deg bend on the top of the vent stack would prevent the stink from coming out, but let me suffice it to say that it will not work. If you don't believe it, take your shop vac and suck the water out of a toilet bowl, then close the bathroom door for a couple hours and come back and see what that bathroom smells like.

kirkhall, I don't see any floor drains down there. I think the shower drain and washer pipe are connected though (they're about 5 feet apart). A friend told me to pour water into the shower, which I did, and the smell got better in the bathroom. I still smell it in the washer pipe. It did get better after I washed a few loads of clothes. At least it's not stinking up the entire basement now.

If "gas meter thing"plumber brought is for detecting natural gas I believe you are on the right track. The utility companie's line might be leaking with gas following your servce line into dwelling. Adding propane to raise line pressure during coldest part of day is common pratice. If you feel compelled to help plumber locate the problem try this. Fold a clean cotten cloth several times and hold it against outlet of grill tank. Make certain to keep tank upright then open valve for 2 or 3 secounds. Does cloth smell similar to odor in basement?