Help with drain/vent plan for bathroom

[Dr_1400]

In the picture, I've drawn a crude schematic of my off-grid cabin’s plumbing. It’s a simple bathroom – toilet, shower, sink – draining into a septic tank. Nothing else goes into the septic. I’m planning to improve (if only slightly) the waste drains/venting this summer. We have noticed a periodic sewer smell since purchasing the place.

All waste lines are exposed underneath the cabin, which sits on piers. The shower trap is exposed underneath, as well (we don't use that in the winter).

Related (?): We often get a very noisy “whooshing” sound upon flushing the toilet. I assume this may be the adjacent shower trap siphoning off (toilet and shower drains are only about 3 feet apart).

In the drawing, I've shaded the vent pipe which vents all 3 fixtures. From it's origin just before the sink empties into the main drain pipe, this vent runs about 8 feet west, then turns and runs 8 feet south. After going under the rim joist, it turns vertical and runs up the outside of the cabin. It then terminates under the roofline (presumably made this way to avoid puncturing the metal roof and to avoid snow issues ?!)

So, I can only assume this is really inadequate and potentially contributing to the sewer smell issue.

What would be a better venting plan? AAV for sink help at all?

Hope the drawing makes some type of sense. Any help appreciated!

 

[Reach4]

Vent line you drew is horizontal is in the crawl space until it goes up outside?
Does the vent cross under the shower drain or above the shower drain? I presume above, with a connection of some sort.

AAV for sink help at all?

Yes. However you can work around that by low flow of the lavatory water before shutting off. That will refill the S-trap.

I think the venting for the shower is more worrisome. What kind of trap is that?

 

[Sponsor]

 

[Dr_1400]

Vent line you drew is horizontal is in the crawl space until it goes up outside?
Does the vent cross under the shower drain or above the shower drain? I presume above, with a connection of some sort.


Yes. However you can work around that by low flow of the lavatory water before shutting off. That will refill the S-trap.

I think the venting for the shower is more worrisome. What kind of trap is that?

Thank you for the reply, much appreciated.

Yes that's correct, vent line is generally horizontal (with some upward vertical slope) until it goes up the outside.

The vent crosses under the shower drain. I've attached another picture here which hopefully illustrates it better. (Heat tape was wrapped on shower trap after a winter freeze up). This was part of a panoramic picture which got distorted, so the PVC of the vent line appears to have some waves.

 

[Dr_1400]

Thank you for the reply, much appreciated.

Yes that's correct, vent line is generally horizontal (with some upward vertical slope) until it goes up the outside.

The vent crosses under the shower drain. I've attached another picture here which hopefully illustrates it better. (Heat tape was wrapped on shower trap after a winter freeze up). This was part of a panoramic picture which got distorted, so the PVC of the vent line appears to have some waves.

View attachment 74424

Reach4, I've been thinking about how to vent that shower. I'm now wondering if after the shower trap, I can run a vent back up through the shower wall (shown in black below). I have a large access panel and there's no insulation in the wall. I could put a studor vent in that panel about 6 feet above drain level.

This seems to be the only way to go vertical without going under the rim joist first, which requires originating the vent way downstream.

I wonder if that, in combination with an AAV at the sink, would make sewer smell less likely? (I forgot to note that the sink is currently two S traps in series, one right above the other...)

Any thoughts?