Washer backs up into kitchen sink, but sink seems to drain fine.

[tomsteve]

take a drinking straw and stick it in a cup of water. put your finger over the end and lift it out. it wont drain because there air pressure isnt equal on both sides.
house drains need the vent to provide air- to equalize the pressure on both sides of the water draining and allow it to flow.
also to vent gasses out.

[rwe2156]

I would check to see if a vent for the washer even exists.

I'll bet you'll find there isn't one.

[Snipe Hunter]

I would check to see if a vent for the washer even exists.

I'll bet you'll find there isn't one.

It's the vent stack for the sink or just down stream from the sink drain. There probably isn't a washer vent stack although there should be. Possibly even somewhere else. It's using the sink drain opening as it's vent because the waste vent is clogged or partially clogged. The vents are used to displace the air so water can flow freely without compressing the air. If the air doesn't escape, the water has to back up somewhere and in this case, it's in the sink. The air can't escape the drain pipe fast enough to allow the water to flow freely.

You also need the vents to stay clear behind the draining water (in this case it would be at the washer drain) so when the water passes another drain (like the sink for instance), it doesn't suck the P-Trap dry at the washer. It could also be a partially clogged drain but I think they already ruled that out. If you hear gurgling in the drain pipes, that's a pretty good indicator that a vent stack is clogged. What you are hearing is the P-Traps being sucked dry (gasping for air) trying to get make-up air behind draining water.

If the vent stack is clogged downstream from the drain, it will need to be "burped" or even burp itslef.

[Snipe Hunter]

I would check to see if a vent for the washer even exists.

I'll bet you'll find there isn't one.

You're probably right. The gap where the drain hose from the washer enters the standpipe is often times the only vent there is at that end. If there's no air gap there, it can contribute to the problem. I've seen rags stuffed in there to prevent splashing out and in essence, plugging the vent.

[bennybmn]

There's an air gap at the washer drain, that I'm sure of. Not sure of a washer vent, I'm considering opening up the wall. Adding one of those air admittance valves will only let air into the drain, not out, so I don't think that's what's causing push-back into the sink. A clog in the main stack vent could I guess. Time for snakey snakey.

[EricU]

for the longest time, Studor valves were the only vent valves allowed in most places.  Now there are cheap ones, but they don't work as well in my experience.  I would put a vent valve in the washer stack

[rwe2156]

Benny we had the same problem in our house - no vent for washer drain (my error when I built the addition).  The only vent was upstream for the kitchen sink.  We had constant problems with gurgling noise, etc.  When the washer was running, sink drainage was slow.

Fast Fwd we bought a new washer.  Its a front loader and uses much less water than the old one - gurgling noises disappeared.

Obviously I 'm not a plumber but I think the options are 1) install a vent to the washer drain.  Difficult to do without demo :-(  Could be installed on the outside wall in conjunction with a Studor valve to minimize obtrussiveness.   2) if there is a clean out beyond the kitchen sink, you could try opening it and see what happens.  If the problem goes away, this would prove the vent for the kitchen sink is between the trap and the drain.  It that works, then you could add the vent pipe.

Last but not least: 3) Call a plumber.

[TDKPE]

How far did the plumber snake the line?  All the way until it gets larger than the 1-1/2" of the sink and washer drain, after they combine?  If it's not able to flow fast enough at the rate it's flowing, it will back up, and if the sink is teed into that line, it can back up into the sink.  It only flows because it's pushed, and it's gravity and the water depth that's pushing it, so it sounds (from way up here) that it needs more depth to push hard enough to flow at the rate it's coming out of the washer (equilibrium). 

It may seem like it's flowing fast through the sink drain, but does it flow as fast on its own as the washer is draining?  Can't know, at least not from just observation and discussion.  But it sounds like the equilibrium state is where the depth is equivalent to an inch in the sink, and whatever depth that translates to where the pipe turns horizontal in the slab.

[bennybmn]

That's an interesting theory too. I always assumed the depth in the sink was a function of the amount of water coming out of the washer. I'll have to play around. I don't mind opening up the wall behind the washer, my wife doesn't want to put more projects on my plate and see me get frustrated...

[Roly]

I have also heard where excessive suds can cause drain problems.   Could you try just water (no soap) in the washer and see if it backs up ?   Not sure what to do about it other than a different soap if that works.   Roly