Vent Stack reduce from 4 in to 3 in ?

If it is a one story house it is okay.

The main waste vent stack must remain undiminished in size from the drain to the roof. That means that if you have a 3″ drain, your main vent stack must be at least 3″ up to the roof. Ditto for a 4″ main drain.

NtP

Why is that ? -Pete

---

In reply to message posted by Gonz:
Why is that ? -Pete

---

Because the plumbing code says so: see IRC 3109.3

NtP

Is there a way to access the IRC via the internet ? Only thing I found are sites that will sell you the code books / CD.

On the Vent Stack question, I was wondering what the physics were behind the code. Why should it matter that you reduce the diameter of the vent pipe ? There’s not a massive volume of air that has to rush through the pipe.

-Pete

---

In reply to message posted by Gonz:
Is there a way to access the IRC via the internet ? Only thing I found are sites that will sell you the code books / CD.

I don’t know about accessing the Codes on the Net. I guess there must be a way to do so. I have all the Code Books in my shop library and on CD ROM.

—On the Vent Stack question, I was wondering what the physics were behind the code. Why should it matter that you reduce the diameter of the vent pipe ? There’s not a massive volume of air that has to rush through the pipe.

---

—I don’t know why the code says that the vent stack must be undiminished as it goes up. I’m not an engineer, but I trust that the engineers who wrote the codes had a good reason for their recommendation. To be honest, I’ve had the same question about the volume or air and I can’t imagine why reducing a vent stack from 4″ to 3″ should make any practical difference in a single family dwelling. It might make a difference in multistory buildings, however.

NtP

I understand this question fully.

This homeowner probably has maybe 1 or 2 baths in the entire home, and back in the day when it was built, 4″ was used alot. Now in my area,,,,on the 4th toilet…….you must increase to 4″.

Sizing is usually determined by the amount of DFU’S the stack is serving,,,,with the minimum being 3″.

But in the time that house was probably built, codes required 4″, and in today’s codes that would not apply.

So, if that is the only stack in the house, and there are less than 4 toilets in the home, you can reduce to 3″ and continue through the roof at 3″ and use a reducing bell in the drain system either above or below the basement floor. I would go with underground reducing bell.

I have done large scale gut outs on systems in my area where 4″ was used on a one bath home, and it was just overkill due the the amount of drain fixture units that 4″ can handle within code limitations.

UPC code says you run the same size pipe to test tee and then reduce .

Gonz, I might be able to help with the why. For the plumbing system to be properly vented you need to have the same cross sectional area of vent going thru the roof as you have building drain going out of the house. Therefore a 4″ drain going out must have 12.56 sq in of vent area going thru the roof. You can achieve that by having four 2″ vents or one 4″ vent and any other combination that would add up to that. You will notice many new homes being built with no vents larger than 2″.
My understanding of why is that if the drainage system was completely plugged up and full of water and then cleared. You need enough air to enter the system to prevent the traps on fixtures from being syphoned dry.

---

In reply to message posted by Gonz:
On the Vent Stack question, I was wondering what the physics were behind the code. Why should it matter that you reduce the diameter of the vent pipe ? There’s not a massive volume of air that has to rush through the pipe.

-Pete

---

I think its 6litres of air to every one litre of water

All advice is given with-out seeing the job, and hence all advice MUST be taken as advice with limited knowledge on the exact situation. NO responsibility can or will be taken. And yes, I am a licensed Plumber and Drainer with my own business in Brisbane Australia.

According to the Kentucky State Plumbing Code book, AT LEAST one main vent coming from a water closet shall extend 3 inches, UNDEMINISHED through the roof and terminating at least one foot above the roof unless the roof is used for purposes other than conveying rain water, THEN the vent shall terminate no less than 5 feet from the roof.

» This message has been edited by ZeekThePlumber on 04 May 2004

I am strictley novice when it comes to plumbing but I know that there have been problems in Winter with ice forming and closing off small vent lines hence minimum vent sizes by code