We recently bought an old-ish house. Our kitchen has a double sink. A dishwasher is connected on the left and there is a garbage disposal on the right hand side.

When the left sink is closed off (e.g., with dish water in it), water in the right sink will not drain. (Actually it trickles out, but the sink will fill up with water). As soon as we unplug the left side, the right side drains normally again. What is wrong here?

Does the right side drain at the same rate when the left side is empty and open vs. when the left side is draining? It might just be that the water rushing down the drain from the left side pulls water down the drain from the right side. Can you measure the rate of flow of water down the right drain in each of 3 cases:
1. Left sink is dry and drain is open.
2. Left sink is full of water and drain is plugged up.
3. Left sink is draining.

If 1 and 2 are similarly slow and 3 is fast, my guess is that it's the left sink's water's pull that speeds the right sink drain.
If 1 and 3 are fast and only 2 is slow, my guess is that it's a venting issue, and that the left sink is acting as a vent for the right sink.

Again, I have no training in this area and am just throwing out ideas since nobody else has suggested anything.
Good luck.

Thanks for the reply MattCarter. The answer is that 1 and 3 are fast and only 2 is slow. I believe you are correct that it's a venting issue. I'm a neophyte here, though. How should the sinks be vented? What do I need to do to add a vent?



Edited 1 times.

The purpose of a vent is to keep the air in the drain at atmospheric pressure to allow smooth draining without siphoning or glug-glug-glug'ing. If water siphons out of the drain, the trap can become emptied of water, thereby removing the protective seal that stands between the sewer gases and the clean air in your house.
The way a sink is supposed to be vented is that the trap arm (just downstream of the P-trap under the sink) should connect to a vent pipe that goes on an upward path through the roof. That vent should begin somewhere along the trap arm or or at the sanitary tee where the trap arm meets the drain.
Most likely, the sink's trap arm extends into the wall, so unless you open up the wall, it'll be hard to see a vent (or absence thereof).

In your case, my WAG is that there's a lack of venting, and the sink side that exhibits slow draining has a trap arm lower than that of the other sink side, so when that higher sink side is sealed, the slow side's drain has no vent and drains slowly with glug-glug-glug'ing. (The water going down the drain pushes the air at the top of the trap down the drain a little, then it bubbles back up due to gravity, and the process repeats over and over through the slow draining.) When the other sink drain is opened, a wet vent is created, allowing pressure to equalize at the top of the trap of the lower drain, thereby allowing smoother draining.

Again, this is pure guesswork, and I have no practical experience with drain venting.

If you do web searches on "drain venting" or "why do drains need vents" or "sink drain vent configurations", I'm sure you'll get lots of good info. If you post some pictures and more specifics, I think the experienced folks on this forum would chime in with their good advice.

Good luck!



Edited 1 times.