Vent directly between trap and drain, or not?

wvphysics

The photo shows the proposed plumbing diagram. Similar to an earlier post of mine, but a few differences. My question is about the sink drains. Is it preferred that the vent branches be between the trap and drain as in the picture or is it preferred to put in a tee(or wye) and have the vent "behind" the connection. By behind I mean in flow direction, so on the diagram the vent branches would be to the right of the sink connection rather than the left.

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benjamincall

After your p-trap arm, you shouldn't dip below the crown weir of your trap outlet before you connect to the vent, i.e., you shouldn't turn down before you vent. If you don't observe this rule, you run the risk of siphoning your p-trap. The best practice is to run a vent out of the top of a sanitary tee that connects to your p-trap arm. An added benefit is that you won't use as many fittings and you won't have to drill as many studs.

HARRY 65

why on earth would you want to vent drains , sorry for asking but i'm from britain and this is never done anywhere here

benjamincall

We vent drains to avoid siphoning. Dry traps allow sewer gas to enter a home. Not every trap will siphon on a regular basis, but the idea is to create a largely fail-safe system.

stuart45

They are vented here for the same reasons.

HARRY 65

ok i think i understand you , but if you have a "p", "s", or straight trap it is sealed against any sewer smell

benjamincall

Yes, these traps form a seal unless a fixture discharges sufficient water to fill the pipe, creating a vacuum that pulls the water out of the trap.

HARRY 65

ok thank you , its just strange the different plumbing set ups for the same job

Proby

Is that the only reason?

I was once told that venting is also to increase the flow, and if your venting wasn't done correctly that a bathtub or sink might not drain adequately. Is that true?

wvphysics

Think of it like a bottle of water. Turn it upside down and flow chugs as it tries to suck air as water pours out. Poke a hole and it sucks air from the hole and flows.
So yes, venting helps prevent slow drains.

Proby

Thanks for the response and confirmation.

I'm curious about something (I like to understand things fully because I am a geek). Your bottle analogy is confusing me because the bottle would be the tub or sink, but it already has a "hole" in it since it is open to the air and not sealed like an upside-down bottle.

You follow what I am saying? I'm still a bit confused.

Thanks for any help.

HARRY 65

it amazes me how we british and those in france , spain , etc; our plumbing systems have worked and worked well without vents all our baths , wash basins, shower's , etc; empty completely with no problem's , isn't that funny

from a stupid old plumber who will have to learn all about venting

Alan

The problem most times is that the amount of water you have in your sink, bathtub, or what-have-you is too deep for air to be drawn in as water is drained out. You notice as you get shallower and shallower, you start getting the whirlpool effect where both air and water are being sucked into the drain. The slow drain problem is less noticeable on tubs with overflows, and lavatories with overflows as it is on showers, and single bowl kitchen sinks, or even double bowl kitchen sinks when one side is plugged.

__________________
Journeyman Plumber

HARRY 65

ah!! an apprentice ok , just having a bit of fun with my american cousins across the pond

but a question for you , when we pull the bath plug in britain ( north ) the water rotates anti-clockwise , in austrialia ( south ) it rotates clockwise , can't be gravity or the pull of the sun or moon , so why doe's it happen . what happens in america if you block the vent will it go straight down without rotating

wvphysics

The rotation is due to coriolis effect, which is due to rotation of the earth and slightly differing linear velocity as you go away from the equator. If you have turbulence from sucking in too much air it can disrupt the vortex, but it rotates the same in the entire northern hemisphere.