If the one sink drains and the other does not, I would say there is a problem with the slope or a clog at the sink, A picture of the sink plumbing would be nice. Otherwise, yes check vent is clear and drain is clear. Snakes dont always fix a clogged drain btw, sometimes there are grease clogs that the snake goes right through. Grease clogs typically happen when a kitchen is involved, bathrooms usually dont get them. Start with a snake then follow with a blow bag if still draining slow. http://www.acmehowto.com/plumbing/clogs/how-to-use-blow-bag.php

You should put water in the shower drain monthly to prime the trap so you dont get sewer gases in the house. The water leaves the trap due to evaporation and the water seal the trap provides is broken, allowing sewer gases in the house.

Hi again. Sorry I am so late getting back. Life happens. Okay...here's a pic of the under-sink plumbing. 2 sinks. I use the left one to wash dishes and the right to rinse. So when the left sink is full of water/dishes, the one on the right backs up as I am rinsing. I have a small gauge tube that I've put down the drain a bit, and that seems to help. (the tube comes up and is hanging on a curtain thing). I was thinking I needed an "Air Admittance valve" kit......but those go on the distal side of the trap, so I was thinking that wouldn't help. We developed this problem after my husband removed an old garbage disposal that was connected to the right/rinsing sink. We didn't have this problem with that old set-up very much. So here's a pic of the plumbing under the sink. I don't think it has vent anywhere close by.

Thanks for you help!

First, you should take apart the horizontal piece and see if there is some build-up.

I am hoping there is a vent behind the wall there, if your horizontal piece is not clogged, a lack of a vent is probably the source of the problem. A correct vent to the roof at the kitchen or an air admittance valve would probably fix your problem. If you dont have a vent or are unable to install a proper vent in the wall you should put the AAV after the trap on the sewage gas side.

As far as the utility sink in the bathroom with a vent (too far away to be usable as a vent for the kitchen, unless they share the same wall with the plumbing in it) I would suspect there is another clog, either in the trap, drain, or vent.

Thanks Vith. I can't imagine there's anything in that horizontal pipe. It was put in around July and that's when the problem started. When I have the left sink filled with water, and the right sink (for rinsing) starts to back-fill, if I just open up the left side for a second or 2, then the water in the right sink drains out quickly. Would that suggest it needs an AAV? If there was a vent behind the wall there, under the sink......shouldn't there be a pipe coming out of the top of the house around there? (which there isn't).

The utility room (with washer, sink, toilet) does have a pipe coming out of the roof....but it's not very close to the kitchen sink.

We don't ever smell sewer gas in the kitchen. Would an AAV help to prevent the suction that seems to be happening with the sink on the right? I think the same thing would happen if we reversed how we use the sinks, but I'm not sure.

I'm also not sure I understand about an air-lock......which sort of seems to be what might be happening. I really don't think it's a clog. Both sinks drain great when they're both open. And my husband cleaned all those pipes out in July......and most of them are new........so a clog, at least under the sink, is highly unlikely.

We do have a very old dishwasher that we have never used. (We use it to store stuff in). It isn't connected to anything under the sink........but it must have a drain somewhere......... Would that be causing this funky problem somehow?

Any more ideas?

Thanks for your help!

I would want to know if that black piece where they join is just a plain 'T' or the part with an internal baffle for directing the right pipe toward the bottom. Or is it designed to drain to the bottom of the 'T'.

Hmmm.....Hubby couldn't remember for sure. Can I tell by just looking at it externally?

Even if it were or weren't an internal baffle, I need to add this: Each sink drains perfectly when both are open. The left one drains fine, even when the right one is closed. And the water that backs up on the right side is only a back-up of the water that is, at that moment, rinsing off the dishes. It does eventually empty within a short period of time........but I don't like clean dishes sitting in that water. I don't know much about plumbing, but it seems like there becomes some sort of air lock when I'm doing dishes and rinsing them (left sink full of water, right sink open). I think I mentioned that I have a piece of tubing (like from an oxygen mask) that I can put down in the right sink a little ways, and it keeps water from backing up. If it should start backing up, I just blow a little air into the tube and the water goes down. (Don't make fun! haha).

My initial post mentioned a "clean-out" we have in the basement, which feeds into the drains from a small bathroom. But it's about 15-20' away from the kitchen sink. I didn't know if opening up that clean-out and cleaning it would help the kitchen sink?

I don't quite understand venting, so I'm at a loss to think this through very well.

Let me know if you need to know anything else! I appreciate everyone's input!

That tee is standard for a double sink tailpiece. You find those commonly. I am guessing they design it that way due to the fact the drain assembly is meant to be taken apart and cleaned, unlike your houses plumbing.

Lack of a good vent would cause your problem. You probably have an S trap, and you need air behind water for anything to drain properly. I am guessing the sinks drain fine when empty because one sink acts as an air relief for the other. It seems to become a problem with the right sink due to the horizontal run before the trap.

Another thought, what kind of slope is after the trap? I cannot tell if it slopes upwards into the wall. That could possibly be working together with the lack of vent to cause the problem.

Hi Vith,

The piece that is after the trap (that goes into the wall) has a very, very slight incline. When you say I must have an S trap.......would that second turn be behind the cabinet? I don't see one in the cabinet.

Okay.....I was preparing dinner and doing some dishes and decided to do this test. As I said earlier, I have a long oxygen tube down in the rinsing sink. It seems to keep the water flowing. It's only down the drain about 5". I decided to pinch off the end of the tubing that was open to air........and the water backed up. As soon as I unpinched it, the water flowed down the drain. I did this several times and got the same result. Does that help with the diagnosis? :)

Air lock due to no proper vent. Not sure what more you are looking for, that is the problem. There must be air behind water after the trap for it to drain properly.

That's what I was thinking. But I didn't know enough to be sure it wasn't something else.

So an Air Admittance Valve would go just between the trap and the wall?

That is the normal location for it, yes

You should also adjust the drain assembly so that the slope after the trap is correct, that is probably making this problem worse. You can take the horizontal piece and the tee off and cut the tail pieces higher (remove burrs with sandpaper or knife).

Thanks for your help Vith!