Shower drain acting as a vent for kitchen sink

[tjdonahoe]

My 31' 2013 classic when boondocking, after washing the dishes, you pull the drain the water drains from the sink quickly , venting into the shower, this happens even when the gray tank is empty. The vents are clean, I did check the one way air vents under the kitchen sink and under bathroom sink. If you plug the shower drain with a plug, the air will vent out of the bathroom sink. My 04 classic didn't act this way, I am thinking something might have a design problem ? Any ideas?

[switz]

Might need to run an eel down the drain line to clear any instructions in the drain pipe. Also remove the all the sink drain traps and clean them.

Could be a wasp nest at the top of the toilet vent pipe on the roof.

[tjdonahoe]

The drain lines are clean, the sink drains into the holding tank via a T into the vent and the shower and bath sink go into the tank elsewhere, there are no blockages. Any ideas?

[nrgtrakr]

My 75 AS has multiple vents. One for the kitchen sink, one for the shower and bath sink, and one for the toilet.

My guess is your kitchen sink vent is blocked.

[dznf0g]

Hmmm, if the kitchen sink drains into the vent and you Have a full column of water entering the section of the vent pipe between a y in the vent stack and the tank, there is nowhere for the tank to vent past the water column during sink draining. That pressure has to go somewhere like the shower. Sounds like a design flaw to me.

[Belegedhel]

If your sink is using the shower to vent, then it sounds like your vent pipe is obstructed, but it sounds like you have already ensured that it is clear.

I have seen this issue described on the Forums before, and in some cases it is a design issue where the venting "capacity" isn't equal to the amount of water that is rushing down the drain. As I recall the situation, the vent pipe comes off of the pipe that is draining the sink into the tank. If the pipe is full of water, then the air that needs to be displaced from the tank cannot go up the proper vent, and instead heads for the shower trap.

To confrim that it is design problem, you might try pouring water down the sink slowly and seeing if it vents from the shower still.

good luck!

[GeocamperAS]

It sounds like the vent stack in restricted between the T and outside. Can you look down the vent from the outside?

[dznf0g]

I'm pretty sure I have a dedicated stack for black and gray combined. The gray has 2 openings in addition to the vent. One for the shower and kitchen combined. And one for the bath sink. I have a 30 classic with split bath

[tjdonahoe]

Tomorrow I will pour a bucket of water slowly down the kitchen sink drain and see what it does, never noticed this before, but it doesn't seem to do it when the drain is open.

[nrgtrakr]

My gray tank is curb side rear of trailer. My kitchen sink vent is above the kitchen curb side center. My rear bath shower (curb side) and sink (center) is vented above the shower. The black tank is under the toilet street side rear and vented above the toilet.

I'd pour the water down the vents.

[dznf0g]

Originally Posted by tjdonahoe

Tomorrow I will pour a bucket of water slowly down the kitchen sink drain and see what it does, never noticed this before, but it doesn't seem to do it when the drain is open.

When the sink faucet is just running or a bucket is poured slowly, you don't have a full column in the pipe and air can escape past the draining water.

[HiJoeSilver]

I can tell you exactly what is going on. From your description in post 3, the drain from the sink T's into the vent stack, vent goes straight up, drain enters the t from the side. What is happening, is that when water is flowing quickly down that pipe and into the tank it is hitting the T and flowing down, this blocks the air from the tank from being able to flow back up the same pipe that is also the vent. This causes slight pressure to build up in the tank which works its way out by path of least resistance which in your case is the shower drain.
Seen this before in RV;s and houses. My bath sink is plumbed the same way in my 79 safari.
Several ways to fix this, the easiest being drain the sink slowly. This keeps the water from rushing and filling the pipe at the vent T and allowing air to escape.
The others would require some re-plumbing. Can you see the top of the tank? Are there any unused connection ports that you could route the drain or vent to? You could also try to put hepvo traps in the line for the shower drain, replacing the p-trap, but that would then likely try to push air back up another route, so you'd have to put them on all the other gray water drains as well. Does the T where the drain and vent merge have a 90 degree T or does it have some sweep/arch from the side drain inlet?

[SuperTrouper]

My 2008 27FB and others with the large round sink find that with the shower drain unplugged the dishwater actually comes up in the shower a bit if you drain the sink fast. It's a known flaw and if the sink water cuts off the vent on it's way to the gray tank just over the shower it explains why. I'm over it and rinse the shower if it happens. As we boondock some we usually use a collapsable washtub and dish rack for washing dishes and dump the dishwater in the toilet and leave the gray water tank to hold shower and hand washing water as gray always fills before black unless there's something terribly wrong. - Brad


- Brad Sent from my iPad using Airstream Forums

[tjdonahoe]

I filled the kitchen sink and let it drain out SLOWLY ,it did not vent into the shower. The kitchen sink that drains into the vent pipe is connected with a straight T. I am going to call the factory and tell them about this situation....

[ROBERT CROSS]

DW uses those hair ball collectors with the itty-bitty holes in all the drains.....I guess that's slow enough to not back up.

Bob

[KathleenBenk]

We had the same issue in our 2009 Classic. Called Airstream and they told me it was because of the water rushing so quickly as described by HiJoeSilver. I now place my hand over the drain to ensure it will drain more slowly, allowing smaller amounts of water to drain at a time. A pain, but it works.

[NOAZRK2690]

It sounds like your little problem has been nailed down so this information is just being passed along for whatever it might be worth. I've had good success with flushing my drain lines out with a little gadget I picked up at the Home Depot. It is a little bulb type thing that screws onto your water hose. I got a little inline ball valve so I could control the amount of water flow from inside the AS without running outside to a spigot. So you stick this thing in the drain hole and turn on the water. The bulb fills up and swells, completely closing off the drain. All water is forced down the drain at a high pressure which blows any clogs away. Works great in the shower drain too. Get the one inch size. It will swell to about two inches which is big enough for all AS drains.

[tjdonahoe]

There are no clogged pipes, all vents are clean, to keep the shower drain from being a vent is to drain the kitchen sink slowly, I guess I can live with this..