Grey water drain vent problem?

[bob5560g]

Something strange is going on with my gray water drains. Details are below.

FYI, kitchen sink is farthest from gray tank. It's diagonal drain pipe (see photo) is visible under the bathroom sink cupboard, but is separate from the bathroom sink drain (at least before they both separately disappear beneath the floor).

Also, as described below, during troubleshooting I removed the grey water vent device (vacuum break?) from the vertical pipe under the kitchen sink (see other photo).


2015 Forester MBS 2401WS motorhome

1. Added a bit of bleach down bathroom sink and tried to fill gray water tank via the bathroom sink water faucet. After a few minutes of continuous water into bathroom sink, it started to not drain. Checked kitchen sink and shower and they all drained slowly if at all. Eventually heard lots of bubbling from gray tank.

Pumped water from gray tank into 6 gallons jugs. Only got about 12 gallons from the 32 gallon gray tank so it was less than half full.

2. Gray tank now empty. To speed things up for bleach soak, used garden hose to fill kitchen sink (to the top!). When I removed the sink’s drain plug, it quickly drained a lot but then water started backing up out of the bathroom sink. (Shower drain stayed dry). Took a while to drain kitchen sink as water kept pushing out of bathroom sink drain. Bubbling sounds periodically from gray water drain pipes or tank.
3. Removed drain vent device (vacuum break?) from top of vertical vent pipe under kitchen sink.
4. Ran test again with same results. Filled kitchen sink to the top then pulled the plug. Sink drains 80-90%, then stops draining and water surges up from bathroom sink drain. This goes on for a minute or two to get that last 10% to drain from the kitchen sink. THIS IS WITH THE VENT DEVICE REMOVED FROM UNDER THE KITCHEN SINK COUNTER. CURRENTLY JUST AN OPEN VENT PIPE FOR TROUBLESHOOTING PURPOSES.
5. RV was slightly nose down for this test so I turned it around and placed it back in the same spot but facing the other way, so slightly nose up. Got the same exact results from test.
6. Doesn’t make sense to me. Why would the full kitchen sink drain 90% then suddenly begin to choke and spit water out the bathroom sink drain, especially since they appear to be separate drain pipes?

[bob5560g]

Forgot to attach photos. Here they are.

Photo with big diagonal drain pipe between blue and red PEX lines is under bathroom sink (which is over the gray tank). That diagonal pipe is the kitchen sink drain.

The other photo is under the kitchen sink.

[bob5560g]

Oops, wrong photo! Here's the correct ones.

[nomad297]

It sounds like you have a partial stoppage in the portion of the drain after where the lavatory drain connects to the kitchen drain beneath the floor and before the tank. It’s probably just grease buildup combined with hair and other stuff that gets into your lavatory drain. Snaking either drain should take care of the problem. Just pick the easiest drain to snake and go at it.

EDIT: bleach will not effectively take care of this, but if you don’t want to snake and you want to use a chemical, sulphuric acid is what you need.

Bruce

[bob5560g]

Thanks Bruce. I recently drained gray water into a water can (now dedicated for gray water) while boondocking. This was a first for me. It stunk so I thought I'd give the gray tank an overnight bleach soak.

I'll try snaking both sinks.

[nomad297]

Originally Posted by bob5560g

Thanks Bruce. I recently drained gray water into a water can (now dedicated for gray water) while boondocking. This was a first for me. It stunk so I thought I'd give the gray tank an overnight bleach soak.

I'll try snaking both sinks.

You don’t need to snake both — just one of them — pick the easiest one to snake and do it from there because both drains will lead to the area of the partial stoppage.

Bruce

[bob5560g]

I snaked all three gray water drains (both sinks plus shower). Found no evidence of blockage. Ran test again, this time the shower backed up instead of the kitchen sink. Maybe by draining a completely FULL kitchen sink all at once it simply overloads what the plumbing can handle. A full sink is about 9 gallons draining fast and non-stop.

It's going in the shop in three days to get the issue checked.

[eye95]

Originally Posted by nomad297

It sounds like you have a partial stoppage in the portion of the drain after where the lavatory drain connects to the kitchen drain beneath the floor and before the tank. It’s probably just grease buildup combined with hair and other stuff that gets into your lavatory drain. Snaking either drain should take care of the problem. Just pick the easiest drain to snake and go at it.

EDIT: bleach will not effectively take care of this, but if you don’t want to snake and you want to use a chemical, sulphuric acid is what you need.

Bruce

Why sulphuric acid? Is it least likely to eat at the material in the pipes? Are there different chemical recommendations for different pipe materials?

Thanks in advance.