Combination waste and vent system

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North Jersey

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At some point, a washing machine appears to have been removed from the basement of my house; I see a 12-inch concrete platform right next to a wye with a test plug in the 2-inch branch inlet. As I replace that 4-inch cast iron building drain, I would like to reinstall a basement washing machine and a laundry tray. I'm going to try to run a vent through a nearby utility chase, but I'm considering incorporating a combination waste and vent system if I'm unsuccessful.

My understanding of NSPC section 12.17 is that I can treat the building drain as a combination waste and vent system where the proposed standpipe and laundry tray connect to the building drain. All of the fixtures draining into the building drain upstream are conventionally vented. I have three upstream bathroom groups (9 DFUs), a washing machine standpipe (3 DFUs), and a kitchen sink (3 DFUs). Would I omit of those otherwise vented upstream fixtures when calculating the load on the combination system?

If I must include the DFUs from the conventionally vented upstream fixtures, then I'll be right at the limit (20 DFUs) for a 4" combination system with 2% pitch. A soil stack from the second floor joins the building drain in the basement immediately before the house trap. This stack has a 3-inch vent. I assume the building drain ceases to be a combination system at this point.
 
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For the combination waste and vent, can I use a section of the 4-inch building drain as described above without factoring in the DFUs of the conventionally vented upstream fixtures?
 

wwhitney

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From a physics point of view, seems like the total DFU flowing through a horizontal pipe would affect its ability to function as a combined waste and vent.

So my instinct says "no," but I haven't parsed the NSPC.

Cheers, Wayne
 

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For what it’s worth, the NSPC says that combination systems cease being combination systems when they connect to conventionally vented drains.
 

wwhitney

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For what it’s worth, the NSPC says that combination systems cease being combination systems when they connect to conventionally vented drains.
I take that to mean downstream of where you need the drain to also function as a vent. I would think that for the portion of the drain that you also need to function as a vent, the total DFU through that portion is what matters.

Cheers, Wayne
 

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In that case, I'll just size up the trap arms to make them serve as the combination system until they hit the building drain. I want to leave a little headroom to add fixtures.
 

John Gayewski

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I don't think a washing machine can be incorporated with a laundry washing machine. Mainly becuse the trap needs to be above the floor. But also (and I'd have to read the specific standard your trying to comply with, but combination waste and vent is for certain fixtures and certain circumstances and not for common systems.
 

wwhitney

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I don't think a washing machine can be incorporated with a laundry washing machine. Mainly becuse the trap needs to be above the floor.
The NSPC section on combination waste and vent lists the allowable fixture types, and it definitely includes standpipes, and IIRC includes sinks.

I believe that combination waste and vent typically has an allowance for a short vertical section to handle these fixtures. That means you end up with something that looks like an S-trap, but the vertical pipe is oversized like the rest of the combination waste and vent, so it doesn't siphon.

However, my familiarity with combination waste and vent is limited, so take that with a grain of salt.

Cheers, Wayne
 

John Gayewski

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The NSPC section on combination waste and vent lists the allowable fixture types, and it definitely includes standpipes, and IIRC includes sinks.

I believe that combination waste and vent typically has an allowance for a short vertical section to handle these fixtures. That means you end up with something looks like an S-trap, but the vertical pipe is oversized like the rest of the combination waste and vent, so it doesn't siphon.

However, my familiarity with combination waste and vent is limited, so take that with a grain of salt.

Cheers, Wayne
I've seen diagrams of what your describing for ipc compliant systems. Not sure about nspc
 

Jeff H Young

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12.17.1 says installation is only permited on certain conditions basicaly my opinion is it rquires AHJ to agree to allow it agreeing its a hardship or impossible to build it right . therefore Id ask befor building it .
also your system needs to connect to the conventional piping 22.5 to 45 degrees above the horizontal so the main needs to be deep enough. Only time I seen the combi system is on floor sinks in supermarket type jobs. But read through it real good draw it up and visit building department is my recomendation
 

North Jersey

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Before I start replacing the cast iron building drain, I’m going to use an endoscope to see if I can squeeze a vent in a space that is mostly obstructed by a 3-inch drain, gas piping, and domestic water.

Does your reading of 12.17.8 indicate that a combination drain and vent from the standpipe could be connected to a 4x4x3 wye in the conventionally vented building drain with the branch inlet coming in the top? If it’s rotated out slightly from the wall to account for the slope of the building drain, it would connect at under 45 degrees measured horizontally along the longitudinal axis.
 

Jeff H Young

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I briefly looked at your provided code , but a lot of details im not sure of , I was thinking for a combination waste and vent system the traps will be under floor laundry standpipe might actualy have its trap underfloor.
I think the Combination waste and vent system is seperate for a time and then enters the conventional system through a wye that must be horrizontal and rolled up between 22.5 and 45 degrees . Not sure what you are getting at ? do you want a combination waste and vent system with only the laundry stand pipe? but I see no provision for a "combination waste and vent and conventional waste and vent system " Its 2 systems so to speak not an all inclusive single system
 
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