Drains should always have a P-trap within so many inches of the drain opening. That prevents sewer gas from escaping from the drain. This is what should be there if it's a sink drain.

The issue with a vent 90/elbow in a sewer line is that they slow the water flow and can easily become obstructed. Normal elbows are curved to help prevent the water flow from slowing down.

[–] Buddahriffic@lemmy.world 1 points 3 months ago (1 children)

Thanks for the info!

Why does the p-trap need to be so close to the opening? As long as it's somewhere along the line, shouldn't it prevent gases from flowing? My own kitchen sink doesn't have a p-trap in the cupboard below but I assume there's one downstream because I've never noticed sewer smells in the kitchen. I figured it was the developers being cheap as soon as I noticed it, but they seem to at least have prevented the issue another way, so it's hard to say if they were cheap or just efficient (or maybe neither if it's just a p-trap in a different location).

My bathroom sink does have a p-trap under the sink though, so I'm still confused as to why it's done one way in one spot and another in the other.

Only other guess I have at this point is it was done that way to simplify fitting the drain pipes under the sink because the hole for the pipe is directly underneath the one sink drain and it just feeds directly down via a vertical pipe, with the other sink's drain feeding into that via a y-connection.

[–] jubilationtcornpone@sh.itjust.works 3 points 3 months ago* (last edited 3 months ago) (1 children)

One problem with a "central" P-trap is that if it were done that way, you're going to get nasty smells somewhere. Sewage frequently doesn't make it out of your house as fast as you think it will and running water down the drain changes the air pressure in the drain line to varying degrees.

The main reason it's positioned so close to the drain opening is to accommodate venting. Every drain opening has to be vented, typically with a pipe going through the wall to a vent stack in the attic, to prevent the P-trap from being sucked dry when you do something like flush the toilet or run the clothes washer.

Ever run the washing machine and hear a sink on the other side of the house gurgling during the drain cycle? That's because the washer draining changes the air pressure in the sewer line and the sink isn't properly vented causing air to bubble through the the P-trap.

[–] Buddahriffic@lemmy.world 1 points 3 months ago (1 children)

Ok, I don't ever hear gurgling from other drains, so whatever they did end up doing must be set up properly. There is what looks like a capped vent that ends just a bit above the cupboard floor, guessing it acts as a buffer for air pressure to allow it to fluctuate without venting the gases into my living space.

[–] jubilationtcornpone@sh.itjust.works 3 points 3 months ago (1 children)

Might be an air admittance valve. Those are usually used when venting to the roof isn't possible or practical.

Plumbing is one of those things that's sort of an art. You can do a whole bunch of shit wrong and have it still technically function properly. Nobody will know except you and the next person who actually knows what they're looking at.

[–] Buddahriffic@lemmy.world 1 points 3 months ago

It's surprising how much depth there is to plumbing. And alarming how easy it is to become overconfident that one has it mostly figured out.