Omission of relief vent for waste stack?

[John_187]

This question is about waste stacks and and relief vent connections at the base of the stack.

The situation is a toilet only waste stack and vent stack that is 5 stories tall. There is a dedicated 2" vent stack and 4" waste stack for these toilets. First section 907 of the 2015 IPC mentions relief vent connections for horizontal offsets for waste stacks 5 stories or more.

However, 711.2.1 and 711.3 over ride the requirement for a vent connection at the stack base though. As long as the waste stack piping is sized at 1 size greater than requirements for a building drain (Which it is at 4"), no relief vent at the base of the stack is required. The concept is that over sized stacks minimizes any positive pressure developed by slugs going down the stack.

Another aspect of this, is these toilet stacks terminate in a parking garage space above ground. They turn horizontal and are connected to nearby bathroom lav and shower waste stack vents. Some of these lav and shower stacks might be 3-4 feet away from the toilet stacks. All these collect and route down columns in the garage then go below grade. This is the only thing I'm not sure about with this - I don't know if these lav stacks or shower stacks qualify as "horizontal branch connections" downstream of the lowest toilet stack offset. It does't define this aspect of it. But still, the toilet waste stack is 4", so the thought is pressure developed would be minimized because of that over sized waste stack.

See attached document with code sections and diagrams. Do you believe that no relief vent connection is required for this situation?

Thanks for any input. Venting is most obscure, hard to interpret part of plumbing

 

[PEDARRIN2]

Venting is a challenge. At first glance, it seems you may have issues.

Few things.

Make sure you are not confusing branch intervals with stories. If you only have 5 stories of toilets, you likely only have 4 branch intervals. So you may not have an issue. What may throw this into more confusion is if there are more horizontal connections to the lower vertical portion of the toilet stack. This could be construed as more branch intervals and require the venting or oversized horizontal offset.

Check your fixture count. If the fixture units for a building drain requires a 3" (710.1(1) which is 36 or less than upsizing would be 4". If you have more than 36 fixture units, the building drain size would be 4" and your oversize would be 6".

Also be careful the developed length your 2" vent stack is not more than what is allowed in 906.1 which will depend on the drainage fixture units and the developed length of the vent stack. I would estimate you need to be 3"

IPC commentary states "All horizontal drains above the elevation of the building drain are horizontal branches." So, I would assume the horizontal pipe from the other stacks would fit and would need to connect 10 pipe diameters from the base of the toilet stack.

 

[John_187]

Thanks for the response.

Regarding the other connections to the sanitary piping below the lowest toilet - that occurs once the piping has turned completely horizontal in the garage space. Again these are 5 story lavatory and shower stacks for example. They are about 3 feet and could probably connect in a little farther away from the toilet stack. So right around above 10 pipe diameters.

For the toilet stack, 5 toilets at 3 DFU each = 15 DFU, so the building drain size required is 3". The waste stack is sized at 4".

In terms of protecting these stacks that are close to the base of the toilet stack - section 711.3 states that for the lowest offset, sanitary stack piping does not need to be up sized 1 size larger than the building drain, because there are no horizontal branches to protect downstream. To me that implies that up sizing the toilet stack sanitary pipe one size more would protect any horizontal branches downstream. (The toilet sanitary stack is 4" and thus is one size larger than required by the building drain).

So I would hope this situation would fall more under section 711.2.1, since the sanitary stack is in fact sized 1 size greater than building drain requirements. That section doesn't mention any risk to any additional downstream horizontal branches. Overall this is somewhat ill defined by the code, just stating how I might interpret it.

Also, I'm sorry I can't access the commentary right now, where is the 10 pipe diameters away from base of the stack listed?

Thanks

 

[PEDARRIN2]

The reference to the 10 pipe diameters is 704.3

711.3 references vertical offsets which are offsets greater than 45 degrees. I was assuming you had a horizontal offset which is an offset less than 45 degrees. The comment about the additional connections downstream of the offset had more to do with the question of whether your toilet stack met the branch interval of 5 or more category. Five floor levels does not equate to five branch intervals.