How important are vents, really?

[Zimm0who0net]

So a little over a decade ago I remodeled my house. The kitchen moved and the plumber decided that rather than saw cut a huge swath of slab he would go through the stem wall and underground to connect with the main sewer line running to the street. Inside, the line came through the stem and straight up through the bottom of the cabinet.
I hooked up the last part and knowing nothing I just went to the big box store and bought the fittings to connect A (the sink) to B (the vertical drain), which ended up being an S-trap (seriously, how can stores still sell those things???). I now know a bit more and realize the plumber intended this line have a P-trap with an AAV installed, but that was never done.
So here I am running the main sink in the house with a disposal and dishwasher without a vent in sight for over a decade! Not a single clog, no noticeable gurgling or smells.
So I’m planning on ripping the S-trap out and plumbing this correctly, but it’s interesting to me how resilient plumbing can be even when you do everything wrong.

 

[Terry]

It may be that the smells you have been used to will change when you add the AAV and p-trap.
I remember helping someone in the kitchen and standing by the kitchen sink I realized it didn't smell like my home. I looked under the counter and saw why. I wound up changing the plumbing for her, and helping in the kitchen smelled so much better after that.

A lack of a vent doesn't affect how it drain, it's sole purpose is to break the siphoning of the p-trap, which is your golden seal against odors from the sewer system. And odors may or may not be noticeable, but even odorless gas is still gas.

 

[Sponsor]

 

[Zimm0who0net]

Thanks! So maybe there's been sewer gasses entering the kitchen all this time. Glad I finally caught this. I'll finish the fix and give an update a few weeks after

 

[Reach4]

If you release a sink bowl of water, there is a good chance it could suck the S-trap dry. If you run the water, and especially if you turn it off slowly, the trap would probably not suction dry.

 

[hj]

Actually, the lack of a vent does affect how the device drains. It usually makes it drain faster because you have gravity AND air pressure differential working on it. that is why the lack of vent causes the loss of the trap seal. Codes do not say what WILL happen, they are concerned about what CAN happen, which is why the vent is required. You can drive a car with bald tires for a long time, but you CAN get into a situation where a tread is required, which is why most people keep good tires on their wheels.

 

[Sylvan]

Picture taking a large plastic bottle full of water and turning the bottle upside down

Notice how the liquid pulsates starving for air plop ,plop, plop

Now take the same plastic bottle fill it with water turn it upside down with a cap on it and make a small hole on the bottom of the bottle and remove the cap

The water will flow as there is no vacuum being created it will flow freely like a funnel

 

[DIYorBust]

There is a small hole at the bottom, and that's your drain. So the make up air must bubble through the trap taking water with it, or bubble through from the other side as you describe. However, I wonder if a potential solution would be a trap primer that detects when water has moved through the drain. The AAV with p-trap seems to be the preferred solution. Another system I've wondered about is a super deep s trap. If you had a 4 foot deep S-trap, would it really siphon out?

But I think if you're a DIYer, and it's your sink and you're tight on cash, an S-trap is not a huge deal. Yes if you fill the sink to do dishes and drain it, it could empty the s-trap. That's a big deal for a plumber who could be responsible for it, but a homeowner can just add a little water to re-prime the trap if he knows what's going on. One of those clear traps might make it even easier. I'm not recommending this, but if you're looking at an unaffordable bill to plumb a proper vent, I'd say the s-trap is manageable. However an AAV ie cheater valve would be a big improvement and fairly cheap.

 

[Terry]

I've wondered about is a super deep s trap. If you had a 4 foot deep S-trap, would it really siphon out?

That sounds like a very bad idea. We give advice here that works, and will pass a plumbing inspection. Why do you think any of what you posted makes any sense?

 

[Reach4]

Another system I've wondered about is a super deep s trap. If you had a 4 foot deep S-trap, would it really siphon out?

If you want a trap that will work, despite not being approved by codes, consider the drum trap. I think it should be allowed for water softener standpipes and other drains that are not likely to handle solids. They were the norm for bathtubs in the past. The concern is they could clog with solids and cannot be readily snaked. Codes today specifically exclude them.

Output is the higher port.

 

[hj]

If you had a 32 foot deep "S" trap it COULD NOT siphon out. Anything less WILL siphon out under the right conditions, although the water "backfall", when the siphon is broken, might be enugh to establish a minimum trap seal until some water is used again.

 

[Gsmith22]

hj has it correct in post #5. the fact that something happened to work, even working for a long period of time, doesn't make it correct, proper, or even recommended. Codes are documents that specify minimum requirements for safety. No one stops you from doing something better "than code" and it doesn't mean something won't work if you install it "below code". Code is the cross section of "this works" and "its safe". Most building code requirements stem from some combination of common sense and "we had a failure not doing it this way in the past."

In theory, anything without a dedicated vent, is "wet vented". ie the air gets behind the water moving above and in the opposite direction as the water through the same pipe (air traveling above the water upstream as water travels below air downstream). It will work everywhere, always to varying degrees depending on amount of fluid draining, the size of the pipe, and distance air has to travel from remote vent among other things. In practice, it works very well in bathrooms (toilet, sink, and tub/shower tend not to be on at the same time) so code allows it there and doesn't' work so well elsewhere (too many variables/combinations to predict "safe working"). So even though you didn't have a dedicated vent, you had a vent in some fashion or else water would have never drained out. It might have been an incredibly tortuous path that no code would allow or human could discern but it obviously existed to some extent. Modern plumbing can be quire robust, dare I say fail proof (when it meets code). Which is why things like cholera don't occur anymore.

None of this is a reason not to correct for the lack of vent or marvel at your apparent good fortune in avoiding a clog, backup, gas buildup, or other disaster.

 

[DIYorBust]

That sounds like a very bad idea. We give advice here that works, and will pass a plumbing inspection. Why do you think any of what you posted makes any sense?

Sorry Terry! I'm not advising anyone to do this, just speculating about what might work from a physical perspective. But absolutely this is not what I would do or advise anyone to use in a plumbing system. In fact I just said I was wondering if it would work. I did say a p-trap with an aav was the best solution, but that would not pass inspection in many places. However it would work.

 

[DIYorBust]

If you had a 32 foot deep "S" trap it COULD NOT siphon out. Anything less WILL siphon out under the right conditions, although the water "backfall", when the siphon is broken, might be enugh to establish a minimum trap seal until some water is used again.

So there you have it, a 32 foot s-trap would work. I don't recommend this. But I think if we talk about it and think about why things work, we can probably learn more than if we just cite code. The engineers that wrote the codes may not have considered every situation, and when new solutions are discovered the code can be updated. I don't think a 32 foot s-trap should be considered though.