Upsizing Dain Vent Pipe?


We want to Limit the number of Roof penetrations and want to combine some of the Vents.
What are the rules for taking say, 1.5, 1.5 and a 2" line and upsizing them to a 3" and venting the 3" out the Roof?
From what I've read you can downsize your Vent one size for your Drain. So a 1.5 Drain you can use a 1.25 Vent? So for most of ours we have vented the Same Size if that makes a difference when we upsize in the Attic.
Our Exit line to the Septic Tank is 3". We do not have a 3" from the main trunk line drain to a vent, they are all just the vents from sinks, showers and Toilets. 2" and Smaller.
Thanks, Scott<-
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Scott Townsend wrote:

There is a reason the plumbing license test takes four hours and has a 30 percent pass rate. Hint: there's much more to venting than can be explained here by me in response to an email like yours. I could print all the relevant code but you wouldn't know what it meant. You'd have to study plumbing system design, learn from a master how to put it in right, and finally learn how to understand and apply relevant code. By then you'd have invested considerable time and effort in learning the plumbing trade in order to properly complete one or two do-it-yourself projects.
Or skip all that and hire a real plumber to design and install your system in the first place.
MM
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Scott
My friends were a little critical because you spoke a little nonsense, but just between you and me, what you want to do is OK. Plumbers determine pipe sizes by developed length and fixture units.
Usually the codes require an undiminished stack from the sewer to the vent terminal. What this means is that somewhere in your house, there should be a 3" path all the way through the system to the roof. I liked the old codes that simply required the combined cross-sectional area of all the vents to equal or exceed the sewer size.
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Thank you for your reply. We were looking in our county code book for the reference to having a vent line from the waste line that is the same size. Though the only 'real' reference we could find was in some old photocopied 100 times diagram that seemed to imply that.
Our issue with the 3" vent at the end of the line is that there is no wall space for a 3" vent without adding it either outside the wall or furring out one to accommodate it.
I'm guessing the Design we chose for the Waste to Vent is fine as the county has already inspected that portion of the plumbing. Though now that the walls are in I need to get the vents out to the roof. So instead of having Several penetrations I want only 2 or 3.
Thanks again for your reply. Scott<-

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