Why does flushing one toilet pull water out of another?
Sewer gas odors and severe water siphoning across fixtures usually point to a blocked or inadequate plumbing vent system. When a vent is clogged, flushing a toilet or running another fixture creates suction that pulls water right out of nearby traps, leaving an open path for foul sewer gases to enter the living space.
What people tried
Every workaround mentioned in the threads below. We haven’t tested any of them — and nobody here is claiming they worked.
- 1Opening vanity drain cleanouts to create an air intake
- 2Running a snake down the vent stack
- 3Checking for partially clogged lateral lines
- 4Pouring water into the drain
- 5Putting a clean out cap on it
- 6Installing a studor vent
- 7Opening vanity drain cleanouts to test for air intake
- 8Consulting local plumbers
- 9Pouring water into dry toilet bowls to temporarily stop sewer gas smells
- 10Attempting to research standards or prepare a rebuttal for the property management company.
- 11Trying to DIY plumbing repairs using YouTube videos
- 12Living with the smell due to a lack of funds to hire a professional plumber
- 13Changing out the existing trap
- 14Searching online for DIY plumbing advice
- 15Trying to determine if the smell is toxic
- 16Manually refilling the dried-up trap with water
- 17Installing a one-way trap seal device
- 18Fixing sagging condensation lines to ensure the trap stays filled
- 19Pour water or mineral oil into unused floor drain P-traps
- 20using hot water and dishwashing liquid
- 21considering baking soda and vinegar or Drano
- 22using chemical drain cleaners
- 23using a plunger
- 24Capping or cutting back the open drain line
- 25Pouring water down the unused p-trap to block the gas
- 26booked someone to pump the septic
- 27Swapping an s trap for a p trap
- 28Adding an air admittance valve (AAV)
- 29Offsetting away from the drain
- 30Calling a plumber
- 31Replacing flex traps with regular tubular P-traps
- 32Checking if the trap has water
- 33Conducting a smoke test to check for a cracked vent pipe in the wall
- 34Having a plumber clean the trap and siphon
- 35Sealing the wall opening where the drain pipe enters with expanding foam
- 36Call a plumber or general contractor
- 37Find a cap that fits at the local hardware store and affix it with locktite
In their words
Unedited, most upvoted first, each linked to the thread it came from.
“Bought a house 3 months ago, the bathroom sink was a little stinky. Tried some hot water and dishwashing liquid which helped a bit but only temporarily.”source ↗
“My bathroom sink is smelly!!! 😳 but thank you for the info, I dread the idea very very much. 😁”source ↗
“We bought this place about 6 months ago. It's on a septic system and we have noticed a septic smell under the kitchen sink every so often, and it comes out of our bathroom REALLY bad whenever the barometric pressure drops during a sudden storm. But here lately I've been smelling it almost all of the time coming from our bathroom.”source ↗
“I'd love to just pay someone to come figure it out and fix it, but I just don't have the funds right now.”source ↗
“I’ve been dealing with a persistent sewer/drain smell coming from the cabinet under my kitchen sink and I can’t figure out where it’s coming from.”source ↗
“A plumber has already been here and cleaned the trap/siphon and the accessible drain parts, but the smell is still there.”source ↗
“We get a horrible smell coming from the shower drain on-and-off that makes me gag, and me are having issues with drain flies.”source ↗
“My wife and I have been smelling sewer gas in this bathroom since the day we moved in, and I’ve been trying to track down the source.”source ↗
“Whenever we flush the toilet in Bathroom B, the water in Toilet A gets heavily disturbed and pulled down.”source ↗
“I’m completely lost at this point. Why didn't the open air intake (open vanity) relieve the suction on that trap?”source ↗
“My wife and I have been smelling sewer gas in this bathroom since the day we moved in, and I’ve been trying to track down the source.”source ↗
“Whenever we flush the toilet in Bathroom B, the water in Toilet A gets heavily disturbed and pulled down.”source ↗
“I’m completely lost at this point. Why didn't the open air intake (open vanity) relieve the suction on that trap?”source ↗
“There is a drain pipe located behind the dryer and washing machine, and it is emitting a foul odor resembling sewage. What kind of pipe is this, and how can I deal with the smell coming from it?”source ↗
“That trap is most likely dry letting sewer gases in.”source ↗
“Sewer smell coming from this bathroom sink area (though I have not definitively confirmed it is the source).”source ↗
“Previous homeowners left this old pipe uncapped. Our basement intermittemtly reeks of ammonia, and on cool nights, the smell rises into the house.”source ↗
“When we turn on our whole house fan, we get a sewage smell in the house upstairs.”source ↗
“The smell is not particularly strong around either of the two toilets that were installed. But it’s very strong in other areas upstairs.”source ↗
“When we first moved in, there were a lot of drain/sewage flies, along with significant humidity and a musty smell in the basement.”source ↗
“I brought up the open sump pit with our property management company, but their director told me the gap needs to remain for airflow.”source ↗
“When I bought my house recently I began noticing a rotten egg smell coming from my laundry room in the basement when the detergent pump went off.”source ↗
“It could go weeks without smelling it but every now and then it stinks up the basement.”source ↗
“I also noticed I don’t have a p trap under my sink I’m not sure if that’s necessary or not.”source ↗
“Bad odor coming from this area.”source ↗
“if the floor drain is your smell issue, you can get a one way trap device that lets water in but not smells out.”source ↗
“The issue we've noticed is sometimes we smell a sewer type smell coming from under the sink.”source ↗
“yesterday we noticed the sewer like smell - and I'm wondering if it is coming from that open drain line.”source ↗
“for the last couple weeks I’ve noticed a strong sewage smell when my dishwasher starts.”source ↗
“Help!! -a poor homeowner 😭”source ↗
“My house has a sink that emits a sewer smell.”source ↗