Hello,

I have a very old house and I'm trying to upgrade portions of my DWV plumbing.

I am trying to figure out what size my main vent should be. I previously replaced the old 1.5-inch galvanized main vent because it failed, but I only replaced it with 1.5-inch PVC. I believe that it would be better to upgrade this to 2-inch PVC. In the space that I have access to, I doubt if I could fit in a 3-inch PVC main vent without major work and demolition.

My main stack is a 4-inch cast iron that appears to be in good / very good condition.

From the basement, my main vent (is this called a stacked vent?) ties in a washer and utility sink... on the 1st floor is my kitchen sink with garbage disposal and dishwasher, and a bathroom sink (toilet is plumbed straight into main stack)... the second floor has a bathroom group with the tub and toilet connecting into the vent on the second floor and the sink making it's connection in the attic space, where the vent system connects into the 4-inch cast iron and exits through the roof.

I doubt if the current 1.5-inch PVC vent meets today's standards, but given my scenario, would a 2-inch main vent be adequate? What else would you recommend?

The only other fixtures in the house that require venting is a bathroom group in the basement (sink, shower, toilet), which I plan to vent to an un-used 1.5-inch galvanized vent that exits the house on the basement stairway landing and runs adjacent to the brick, through the soffit, and vents above the roof line. Is this adequate for my basement bathroom group?

Thanks for your help,

-Chad

HI Chad,

'My main stack is a 4-inch cast iron that appears to be in good / very good condition."
I would retain the 4" stack vent as my main soil stack. This isn't the main vent is it?

"I doubt if the current 1.5-inch PVC vent meets today's standards, but given my scenario, would a 2-inch main vent be adequate? What else would you recommend?"
While 1 1/2" vents are acceptable in some codes we use 2" vents in today's plumbing.

"From the basement, my main vent (is this called a stacked vent?) ties in a washer and utility sink...on the 1st floor is my kitchen sink with garbage disposal and dishwasher, and a bathroom sink (toilet is plumbed straight into main stack)...the second floor has a bathroom group with the tub and toilet connecting into the vent on the second floor and the sink making it's connection in the attic space, where the vent system connects into the 4-inch cast iron and exits through the roof."
It's not called a "stacked vent" it's called a vent stack. By "tie in" do you mean it vents or drains? Is ANYTHING draining into the vent stack or does every fixture drain into the stack vent and the ventstack's a dry vent? If you have fixtures draining into the vent stack where are they located in relationship to elevation? (What's over what?)

"The only other fixtures in the house that require venting is a bathroom group in the basement (sink, shower, toilet), which I plan to vent to an un-used 1.5-inch galvanized vent that exits the house on the basement stairway landing and runs adjacent to the brick, through the soffit, and vents above the roof line. Is this adequate for my basement bathroom group?"
If it's already there and it's a dry vent the I'd use it. Good luck, Tom

Hello Tom,

Thanks for your answers and I'll try to answer your questions the best I can...

'My main stack is a 4-inch cast iron that appears to be in good / very good condition."

I would retain the 4" stack vent as my main soil stack. This isn't the main vent is it?

No, it's not the main vent, and I plan on retaining the 4-inch cast iron stack vent as my main soil stack. Is the soil stack also called the stack vent? And, is the main vent stack called the vent stack?

"I doubt if the current 1.5-inch PVC vent meets today's standards, but given my scenario, would a 2-inch main vent be adequate? What else would you recommend?"

While 1 ½" vents are acceptable in some codes we use 2" vents in today’s plumbing.

I'm upgrading the main vent stack from 1.5-inches to 2-inches. I believe that still falls short given the fixture demand, because it's venting a basement laundry group (washer and utility sink), a 1st floor kitchen group (sink, dishwasher, and garbage disposal), a first floor bathroom sink, and a second floor bathroom group (sink, tub, and toilet). I believe the fixture demand adds up to 11 or 12, depending on how you count it, and the 2-inch main vent stack is good for 10. Is this calculation correct? Any other ideas? This main vent stack is dry (no drains - only vents) from the basement to the attic.

"From the basement, my main vent (is this called a stacked vent?) ties in a washer and utility sink...on the 1st floor is my kitchen sink with garbage disposal and dishwasher, and a bathroom sink (toilet is plumbed straight into main stack)...the second floor has a bathroom group with the tub and toilet connecting into the vent on the second floor and the sink making it's connection in the attic space, where the vent system connects into the 4-inch cast iron and exits through the roof."

It's not called a "stacked vent" it's called a vent stack. By "tie in" do you mean it vents or drains? Is ANYTHING draining into the vent stack or does every fixture drain into the stack vent and the vent stack's a dry vent? If you have fixtures draining into the vent stack where are they located in relationship to elevation? (What's over what?)

Nothing is draining into the vent stack, so it only vents and is dry from the basement to the attic.

"The only other fixtures in the house that require venting is a bathroom group in the basement (sink, shower, toilet), which I plan to vent to an un-used 1.5-inch galvanized vent that exits the house on the basement stairway landing and runs adjacent to the brick, through the soffit, and vents above the roof line. Is this adequate for my basement bathroom group?"

If it's already there and it's a dry vent the I'd use it. Good luck, Tom

Yes, it's a dry vent. Is 1.5-inches adequate for a bathroom group (sink, shower, and toilet)?

Thanks,

-Chad

"Is the soil stack also called the stack vent? And, is the main vent stack called the vent stack?"
Yes, A stack vent is a stack the fixtures discharge into. Above the highest fixture that discharges into it the stack vent turns into a dry vent stack. A vent stack is dry all the way up unless it wet vents a fixture. Al;l very confusing at first but makes sense after you're familiar with it.

"Nothing is draining into the vent stack, so it only vents and is dry from the basement to the attic."
Great! You have a perfect set up.

"Yes, it's a dry vent. Is 1.5-inches adequate for a bathroom group (sink, shower, and toilet)?"
I'm going to go out on a limb and say yes. There will be some younger plumbing experts that may disagree but I'm a old timer. I've seen 1 1/2" vents that have worked for decades. If you're going to pull a permit check your local codes. Good luck, Tom