Vent Size Question

[Nick_A]

Hello,

We are putting on an addition (2 full baths/laundry) and had a new septic system installed. We moved it to a different place in our yard, so we still have our current plumbing (kitchen/1 full bath) on our old septic. We will keep it this way until we finish the addition. When we had the new system installed, we had them make two separate stub outs into the basement, each is 4-inch. We are in the process of plumbing the addition to one of the new stub outs now. Once we have another working bathroom... we plan to redo the plumbing on the old part of the house and hook up to the new septic at the other 4-inch stub out.

We have been reading what we can and also trying to understand the venting functions - but we have a couple of questions. We have seen some topics on this site that mention needing the aggregate opening in the roof at least equal to the outlet to the sewer. First question is, does that mean we would need to have 25sqin of roof opening total since we have the two 4-inch stub outs? Does each stub out need its own vent system to work properly, or can they be tied together eventually? And can that total area be equal or greater than just one 4inch outlet (12.5sqin)?

Also, it seems in the code maybe that the roof openings need to be at least as large as the minimum required outlet, which we though was 3inch minimum? However, we are more interested in having a working system than meeting any minimum code requirements, but just looking for help on how to size this overall.

Thanks in advance, and sorry if the question is not clear, let us know if we can provide any more info.

Nick

 

[wwhitney]

Looks like Iowa uses the UPC? Then you are subject to the requirement on aggregate vent area through the roof, that's a UPC requirement. It is based on the minimum building drain that would be allowed for all the fixtures in the house. Generally that's 3" for up to (3) toilets, and 4" above that. You are of course welcome to have more vents through the roof, but generally the UPC requirement is sufficient and is significantly more than the IPC.

Also, if you are in a cold climate, then there is a provision to increase the size of any vent just before it leaves the thermal envelope of the building, to reduce the change of the vent frosting closed. I think 3" is generally the larger size used, not sure. And I would think for the aggregate area rule that you wouldn't get credit for 3" based on that upsizing, rather it would be based on the smaller continuous size of the vent.

Cheers, Wayne

 

[Sponsor]

 

[Nick_A]

Thanks Wayne,
We were planning to run any individual vents back into a 3" main vent directly on top of our 4" main stack. Tie everything into that 3" and then put a short section of 4" back on to go out the roof for the frost issue. Glad to hear it sounds like we can keep to just this single opening in the roof if the distances and such allow.

Nick