Wet Vented Bathroom - why won't this work?

[tropostudio]

I'm trying to get through plumbing rough-ins in a renovation of my old house, and am trying to understand why my inspector won't allow this horizontal wet vented bathroom group.

Specifics: Have to follow 2015 MN Plumbing Code (UPC). No 2" trap arm that joins the wet vent portion of the drain in my current layout exceeds 60" developed length. All horizontal drain fittings are flat wyes - there are no mis-used sanitary tees or rolled wyes. Water closet is less than 72" developed length from the main stack. I come up with 7 DFU at the dry vent and 4 DFU discharge into the wet vent, so used 2" pipe until I had to bring in the WC. Drainage slope is a consistent 1/4" per foot.

My inspector wants the WC separated from the group, replacing the 3" wye with a 3" 45 elbow and dropping into the stack. He wants the shower trap arm dry vented to the 2" tub dry vent above the tub rim, and the drain line for those 2 fixtures routed to drop into the 2 inch washing machine drain using a 2" combo.

What's wrong with my layout? Thanks much...

 

[Stuff]

With your layout whenever the toilet is flushed there will be a vacuum on the shower trap. This will eventually suck it dry and you will get gas/smells.

 

[Sponsor]

 

[tropostudio]

That occurred to me, but isn't the dry vent immediately upstream of the shower trap arm what prevents that from happening? I asked the inspector if installing a dry vent in the shower trap arm that tied vertically into the tub dry vent above the tub rim would cut it, without separating the tub and shower drain line from the toilet and he said 'No.'

I'm thinking my current layout is really no different than the Private Bathroom set-up in the right hand schematic below. These are sample diagrams for horizontal wet venting to clarify MN Plumbing Code, by the State.

I'll re-work if I have to, but I want to understand how this actually violates UPC code. Even with floor trusses, this install is rough. A concealed girder truss has opposing diagonals with double chords and big metal plates. There is really very little room to work.

 

[hj]

Pragmatically, it makes no difference what we think or say. He is the inspector and unless you satisfy him you will NOT get a "green tag" on the installation. The saying goes, "The inspector may not always be right, but he will ALWAYS be the inspector".

 

[tropostudio]

HJ-

Believe me, I hear you. But I'm still asking for 2 reasons:

1) My inspector is tough, but by the book. He was going to fail a similar wet vented arrangement in another bathroom, but I walked him through it and he agreed it met code and would work.
2) I build science museum exhibits for a living, including water exhibits with automated control and treatment systems. I like to understand how stuff works and why it wouldn't work. Code is 'supposed' to be based on sound principles, right?

I'll do whatever I need to do to get done, but if there isn't a code violation here, I'd prefer to not re-build. Stack configuration you can't see under sub-floor is really tight - there is no room for repair couplers for a quick re-arrangement. It's careful removal of short pipe stubs from inside tees and wyes or cutting out a significant chunk of work to replace

 

[Stuff]

You are correct that the vertical dry vent should prevent shower from siphoning. Sorry, I was seeing a different scenario in my head.

And yes, similar to the 3rd pic. Some codes want wet vent up-sized - with 2" traps go to 3" up to dry vent. But I see from pic there is note that with 4 DFU 2" is required so should be good as is.

Instead of double san-tee should be double fixture fitting for lavs.
Remote possibility: Some want a wye connecting drains to stack. Instead of san-tee you might need to use combination wye-1/8 as stack not used for vent on those connections.

Hard to read their mind so why not ask inspector to give you the code references that you are violating?

 

[tropostudio]

Stuff - thanks for the reply. This 2nd floor bath is not as critical as the basement and first floor, as I have sheetrock scheduled for next week for lower levels. Inspector suggested I could pressure test as is and if it works, I proceed and deal with any changes to second level plumbing later. He also pointed out onus is on me no matter what. I think I will politely submit my drawings to him and ask for him to cite to specific code violation references.

I am the homeowner. Not a plumber, but I have read the code pretty carefully. MN Plumbing Code allows double sanitary tees provided the opposing inlets are 2 pipe sizes smaller than the vertical portion. I'm running 1-1/2" lav drains into the 3-3-2-2 double tee through bushings, and he is fine with that. Sanitary tees at the stack instead of combo wyes are not an issue either. I thought about the transition from 2"-3" in the drain, and he didn't mention that. As you noted, it meets code per the sample drawings. I would have used 3" in entire wet vent, but the slope of opposing truss diagonals wouldn't let me get sufficient drainage slope with a 3" drain for the entire wet vent.

Thanks again for the review.