Small bathroom plumbing using air admittance device

[k-n]

I have a 600 sq foot cabin in progress and it's time for the bathroom. I have it roughed in 60" wide and 8' long. I plan to place a 30" sterling ensemble direct to stud shower/tub, 10" rough-round toilet, and a 24" shallow vanity. This will be the only bathroom. For the wall that will house the plumbing lines, I framed it using 2x6 instead of 2x4 so I have plenty of room to work. I have city water but a septic system on my property.

I don't want to poke holes in the roof if I can avoid it and plan to use an air admittance device on a 3" vent stack. My plan is to build something almost exactly like the picture below. My only issue might be beams in the way. My joists run parallel to the plumbing (long 2x4 in the pic below) below and I'm between them so they won't be an issue. I also have 6" wide beams running perpendicular below the joists. I believe the floor joists are 2x6 (maybe 2x8) so I have 5.5" minimum to go above the beams without having to punch through joists. Are there any issues with this plan that I haven't considered? I know some don't like AADs but I'm set on using it with an access panel to check it/replace it if needed. Not sure if I'll use Oatey or Studor but they both make an air admittance valve that will work for up to 4" pipe (hence why I framed that wall with 2x6)

 

[Kreemoweet]

AAV's are intended to supplement the atmospheric vents, not to replace them. As I recall, the UPC requires actual
vents-thru-roof that's at least equal in cross-section to the building drain. Your own local code likely has a similar provision.
Proper plumbing does not always equal the cheapest, most convenient plumbing.

 

[Sponsor]

 

[MACPLUMB]

You must a have a vent "Through" the ROOF !
Before you can use a Studor vent !

 

[k-n]

Why does Studor advertise otherwise?

https://www.studor.net/maxi-vent

A snippet from the product and notation that these are used all over Europe without any "roof" penetration per legal code.

"The Maxi-Vent eliminates roof penetrations and can be used with the Mini-Vent to vent an entire building without the need for passive vents pipes. It is resistant to extreme temperatures. Subject to being used with the insulating cap, Studor guarantees the performance of the Maxi-Vent from -40°C to +60°C, which far exceeds the requirement of EN12380 for the AI rating (which is “only” from -20°C). See our tech sheet here for more detailed information."

Further, in reading the IPC code, there is this:

"917.3 Where permitted.
Individual, branch and circuit vents
shall be permitted to terminate with a connection to an individ-
ual or branch-type air admittance valve. Stack vents and vent
stacks shall be permitted to terminate to stack-type air admit-
tance valves. Individual and branch-type air admittance valves
shall vent only fixtures that are on the same floor level and con-
nect to a horizontal branch drain. The horizontal branch drain
having individual and branch-type air admittance valves shall
conform to Section 917.3.1 or Section 917.3.2."

I believe this describes my situation. I am in a single story cabin. I have one bathroom and a small kitchen sink. The above drawing would connect to a horizontal branch that would pick up the sink on the way to the septic. My total length of horizontal branch will be about 18 feet where it will turn down into the 4" line heading for the septic.

Bypassing the AAV discussion for the moment, is there anything wrong with that diagram? I could always take the stack straight up and punch through the roof.

 

[hj]

Most AAVs DO specify that you need at least one roof vent. IF you were on a city sewer, you could probably get away with it, but NOT with septic system since that creates a "closed system" that will NOT drain properly, if at all.

 

[Terry]

You have to allow for somewhere for all that fermenting gas to escape from the septic tank.
All plumbing codes in the US require a vent through the roof for that.

 

[k-n]

Thank you for the clarification. As I said above, I can always go straight up and go through the roof which is what I certainly don't want to do but may have to do... So assuming that, is the above diagram a reasonable way to plumb up the bathroom? The stack in that picture would literally go straight up and through the roof. The bottom would go straight down and make a 90 degree turn and shallow slope for about 18' where it would take another 90 degree turn down into the septic line. I believe when going down from the main vent to the horizontal, that is supposed to be a 90 and not 2 45's correct? Same at the septic entry? (I'm entering the septic at the top of the run)

 

[Terry]

That's a pretty standard layout.
With UPC, I use a long turn when going from vertical to horizontal.
That is the same as two 45's. You want to ease down the line as smoothly as possible.

 

[k-n]

thanks, Terry! Last, but not least, my main horizontal will be 3" and I'm joining a 4" septic line that is already underground. I have a couple of options: I can join the septic underground at the very end of it (it was accidentally cut when electric went in -- conveniently right where I have to join in) or I can Wye into it and repair the septic line. It goes back under the cabin for about 15' where there is a cleanout. My biggest question is going from 3" to 4". I understand the long turn 90 going from the main stack to the above ground horizontal and then again a long 90 going from the above ground horizontal to the below ground septic 4" but I'm not sure the best way to go from 3-4" so I don't create an odd transition that causes solids to hang.

 

[k-n]

Thanks for your help, everyone... Back to the top... I'm getting ready to start and I'm gathering materials and I want to make sure all is correctly planned before I start.

-Main vertical vent stack will be 3".
-2" piping from sink and tub into the stack (see above drawing)
-Main vent stack will size down to 2" before it goes through the roof. From the tables it seems that's more than enough for my total load.
-Vent stack will use a 90 degree long sweep as it transitions to horizontal
-horizontal will run about 18' and will drop 1/4" per foot or 4.5" of drop total
-horizontal will have a 3" combo wye at about 7' down slope from the bath for a small kitchen sink to join the horizontal and another one about 14' down slope for a possible washing machine drain line to join the horizontal.
-horizontal will turn down to the septic at 18' and go down about 30" from the horizontal
-long sweep 90 from end of horizontal to the drop and again from the drop back to horizontal in the septic line (which is 4")

I have just one more thing to work out.... I have to go 3" to 4" somewhere because horizontal will be 3" and the septic is already 4". I assume the best place to do this would be on the drop at the end of the horizontal run so there's no uneven surface where solids might hang up on the way. Would that be the ideal location or is there another way to do that easily. (should I make my main vent stack 4" all the way? I have 2x6 walls so it's possible but I don't see 2" to 4" type fittings)