Venting basement bathroom via stack vent

[Radio Flyer]

I'm installing a basement bathroom with a Saniflo macerating toilet. My main drainage line is 3" PVC running horizontally in the basement. The line is stack vented, continuing to the roof on the upstream side.

I plan to connect the 3/4" pumped waste outlet on the toilet into the 3" horizontal drainage stack with 3/4" pressure PVC and a wye. This will be several feet upstream from where the upper floor bathroom drainage enters the stack. No other fixtures enter the drainage stack upstream from this point.

My question is about where to connect the combined vents for the toilet pump, sink, and shower. Since the main stack is vented and no other fixtures drain upstream of the new bathroom, can I connect the vent into that main 3" line upstream of where I connect the drainage? If so, how far upstream should the vent be from where the drainage enters? And the vent should connect at the top of the 3" line, right?

In other words, can I connect the drainage near the left side of this photo and the vent somewhere on the right side?

Thanks for any help and advice!

 

[wwhitney]

Are you sure that nothing drains through the 3" horizontal PVC shown in your picture? To my understanding, it would be at least unusual for that to be the case. I'm not aware of a venting method that can use that 3" horizontal PVC and its upstream extension(s) as a vent for any downstream fixtures, when there are no upstream fixtures.

Cheers, Wayne

 

[Sponsor]

 

[Radio Flyer]

Are you sure that nothing drains through the 3" horizontal PVC shown in your picture? To my understanding, it would be at least unusual for that to be the case. I'm not aware of a venting method that can use that 3" horizontal PVC and its upstream extension(s) as a vent for any downstream fixtures, when there are no upstream fixtures.

Cheers, Wayne

Yes, strange as it seems, it appears nothing drains through that line right now. To the right of the photos, the line turns vertical and goes to the attic. The home is a modular home, so maybe that had something to do with it?

For venting, do I need to connect into that stack vent at the point it turns vertical? I could instead run a separate vent in the exterior wall, but it would end up just joining the same vent stack in the attic. (I would basically be running a vent right next to this vent, so I'm assuming it would just be easier to connect into it down in the basement, right? Maybe I'm missing something.)

 

[wwhitney]

So say you cut the pipe in your picture and capped the end. On the upstream side we'd be left with a horizontal pipe that turns vertical and goes through the roof, with no side connections. Are all the fixtures that connect to the horizontal drain downstream of the cut still properly vented? Or is there some weird venting that is being attempted via the horizontal 3" pipe that will be disrupted? In particular, is there another stack that goes through the roof for an atmospheric vent?

If you could cut the pipe, and use one side for the macerating discharge, and the other side for the vent, then you could certainly connect to the vent side in the basement. But if the pipe stays connected, and if the horizonal drain line gets clogged and backs up, then there's the possibility of the backup reaching your vent and fouling it, if the vent connection is too low.

That's why vents usually can only connect or turn horizontal at 6" above the flood rim level of the associated fixture(s). I'm not sure how that rule should apply to a pumped discharge with check valve. If the 3" line clogged downstream of the pumped discharge but before any fixture, wouldn't the discharge back up to the maximum head of the pump? So for that scenario, you'd want to connect your vent above the maximum pump head, since your vent is also connected to your drain.

Even if that's not plausible because of the macerating action, certainly at the point the next branch drain connects to the horizontal 3" line, you could get a clog just downstream of that. So whatever fixture has the lowest flood rim level on that branch, I'd suggest connecting the vent at least 6" above that level.

There's some chance this question has exceeded what I'm knowledgeable about and my answers aren't sufficient.

Cheers, Wayne

 

[Radio Flyer]

Ah, that's a very good point. I could just cut the 3" line and use one side as vent and the other side as drain. (I could also install a cleanout cap on the end, I suppose.)

I had thought that perhaps the horizontal drainage line needed to be vented for some reason, but I already have other vents protecting the traps in the other two bathrooms above. The master bath has separate vents below the vanity and shower traps, plus a vent downstream of the toilet just before the line drops to the basement. The other bathroom has a vent below the vanity and shower traps, which I assume wet vents the toilet. The kitchen sink is Studor vented.

So I guess my only question is why the builders bothered installing a 3" stack vent extending up from the main drainage line if every trap is separately vented. Odd.

 

[Reach4]

Note that in snow country, you want a bigger pipe through the roof to avoid freezing shut.

Are you saying that you have several roof penetrations for vents?

 

[Radio Flyer]

Note that in snow country, you want a bigger pipe through the roof to avoid freezing shut.

Are you saying that you have several roof penetrations for vents?

Actually, at the moment NONE of the vents I mentioned penetrate the roof, amazingly. It seems the job wasn't finished when the home was assembled on site 12 years ago. All the vents are just stubbed into the attic with little foil caps and orange flags.

Once spring comes, I'm planning to join the vents together into one or two 3" vents continuing through the roof. A half-bath on the side of the house does have a 3" vent though a lower-level roof for the toilet, but that's the only roof penetration. Again, very odd.

 

[Radio Flyer]

This is the half-bath toilet vent I mentioned. No other vents currently go through the roof.

Again, though, it seems that no waste goes through the 3" stack vent, which is on the other end of the house.

 

[Radio Flyer]

So just to confirm before I cut this horizontal 3" PVC line, there is no reason that the main 3" horizontal drainage line in the basement ceiling would need to be vented on the upstream side, since all other traps appear to have their own vents, right?

Thank you again for the helpful advice.