3 ms·

I'd suspect it's more likely the opposite: as the pipe is dewatered it creates a negative pressure that pulls the water out of the p-traps which then temporaril

I'd suspect it's more likely the opposite: as the pipe is dewatered it creates a negative pressure that pulls the water out of the p-traps which then temporarily allows gaseous exchange with the building air. EDIT: Nevermind, I originally was thinking that a sufficiently negative pressure differential could overwhelm the capacity of the vent. But the more I think about it, the pressure range that I'd expect even with an unusual operation probably wouldn't be far enough outside normal range to do that. ...although there are definitely still some possibilities to do some stupid.

mikehotel 7y ago
It shouldn’t be possible to pull water out of a p-trap if there is a vent that isn’t blocked.
- jcims 7y ago
  If the low pressure is at the vent (e.g. Bernoulli + windy day) the higher ambient air pressure at the drain will push the water through the trap and up the vent pipe. This would only be temporary, but if gases have accumulated near the trap they could diffuse back through the trap and into the room. These would seem to be exceedingly temporary situations however and not likely a source of chronic exposure to pipe gas. The only exception here might be if some HVAC imbalance keeps the ambient pressure in the room at 3-5" of head pressure over the vent.
hinkley 7y ago
It's possible. On the top floor of a building I worked at long ago, there was a drain in the floor of the men's bathroom. It had a p-trap but on windy days there was enough vacuum from the vent stack to drain it out. I encountered a neighbor pouring a cup of water into it one day and he explained the problem. I started doing the same, and had to do it every couple weeks during windy periods. But I think that's a problem for the upper floors, both due to fluid dynamics and because the first trap that loses seal would mostly restore equilibrium.
- perl4ever 7y ago
  My parents' house had a guest room addition with a bathroom which wasn't used for long periods of time, and it became apparent that if the water wasn't run enough it would start to let odors back through the pipes. Anyway, everything uses PVC, right?
  - wahern 7y ago
    Not everywhere: https://www.pmmag.com/articles/97318-plastic-pipe-still-limited-in-plumbing-applications https://www.pmmag.com/articles/97318-plastic-pipe-still-limi...
taneq 7y ago
I worked in an office once that had exactly this problem. The sink in the second floor lunch room was fine generally but if you filled it and then pulled the plug, the vacuum behind the water filling the drain pipe was enough to suck the water out of the u-bend and leave the pipe open, after which all of the vapours of Hades issued forth until you ran the tap again for a few seconds.

mikehotel 7y ago

It shouldn’t be possible to pull water out of a p-trap if there is a vent that isn’t blocked.

jcims 7y ago
If the low pressure is at the vent (e.g. Bernoulli + windy day) the higher ambient air pressure at the drain will push the water through the trap and up the vent pipe. This would only be temporary, but if gases have accumulated near the trap they could diffuse back through the trap and into the room. These would seem to be exceedingly temporary situations however and not likely a source of chronic exposure to pipe gas. The only exception here might be if some HVAC imbalance keeps the ambient pressure in the room at 3-5" of head pressure over the vent.

hinkley 7y ago

It's possible. On the top floor of a building I worked at long ago, there was a drain in the floor of the men's bathroom. It had a p-trap but on windy days there was enough vacuum from the vent stack to drain it out. I encountered a neighbor pouring a cup of water into it one day and he explained the problem. I started doing the same, and had to do it every couple weeks during windy periods. But I think that's a problem for the upper floors, both due to fluid dynamics and because the first trap that loses seal would mostly restore equilibrium.

perl4ever 7y ago
My parents' house had a guest room addition with a bathroom which wasn't used for long periods of time, and it became apparent that if the water wasn't run enough it would start to let odors back through the pipes. Anyway, everything uses PVC, right?
- wahern 7y ago
  Not everywhere: https://www.pmmag.com/articles/97318-plastic-pipe-still-limited-in-plumbing-applications https://www.pmmag.com/articles/97318-plastic-pipe-still-limi...

taneq 7y ago

I worked in an office once that had exactly this problem. The sink in the second floor lunch room was fine generally but if you filled it and then pulled the plug, the vacuum behind the water filling the drain pipe was enough to suck the water out of the u-bend and leave the pipe open, after which all of the vapours of Hades issued forth until you ran the tap again for a few seconds.