Drain Vent ing- How Wrong Is This?

randy427July 2, 2010

A friend is having a two-story addition put on her house, The venting for the bathrooms is obviously messed up, but I'd like for someone to tell me how many ways it's wrong.

Lower bath with toilet, tub and one basin are all vented up with a single 1 1/2" sched 40 pipe, which then has a 6' horizontal section in the attic to join with the 1-1/2" vent from the upstairs bathroom (1 tub, 1 shower, 1 toilet and a 2-basin lav) which includes another 6' horizontal (NO slope)section. The vents join to a single 1-1/2' sched 40 vertical section that, as of now, vents into the attic. (I'm hoping that the plumber at least plans to go through the roof)

I didn't have the time to note all of the details, but this gives a general picture of what I saw.

My friend is getting another plumber to evaluate it, but I'd like to give her some specifics so she can see if the second plumber is any better.

TIA

R3

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lazypup

If you are under the IRC it is fine as it is with the exception that the horizontal pipe should have a 1/4" per foot pitch.

    Bookmark   July 2, 2010 at 12:25PM
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randy427

Thanks.
They'er in rural Virginia and I don't know which code they ascribe to.
It just seemed to me that 2 toilets, two tubs, one shower and three basins would be a bit under-served by a single 1-1/2" vent. I was also under the impression that a single toilet would require at least a 2" vent and that the 6' horizontal runs were too long.

    Bookmark   July 2, 2010 at 10:54PM
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lazypup

The Virginia Plumbing Code is based primarily upon the IRC.

The UPC and the IRC have some dramatically differing rules on venting.

Under the IRC all structures are required to have one "Main Vent". which must run undiminished in size from the building "Main Drain" through the roof.

Once the main vent is achieved whenever it is necessary to install an auxiliary vent the auxiliary vent may be reduced to 1/2 the diameter of the line it serves but not less than 1-1/4".

The IRC does not limit the length of a horizontal offset but if the total developed length of the vent exceeds 40' the entire vent line must be increased by one nominal trade size.

In regards to the vents in your original question, the vents are a result of an addition to the existing structure, therefore we can assume that the original structure has the required "Main Vent" therefore the new vents are auxiliary vents. The water closets require a 3" drain line and we may reduce the size of the vent to 1/2 the diameter of the drain line they serve, ergo your new vents are 1-1/2"

To confirm the 3" drain we must compute the combined DFU load.

W.C. =/tub/shwr............... 2dfu
lav.................... 1dfu

You have two W.C. (6dfu), two tub/shwr (4dfu) and three lavatories (3dfu) for a total combined load of 13dfu's and a 3" horizontal drain line with a 1/4" per ft pitch is rated for a maximum load of 20dfu's so here again, the 3" drain is well within maximum limits and the vent may be 1/2 the diameter of the drain line.

Under the UPC all vents must terminate through the roof, but under the IRC the Main Vent must terminate through the roof. After that is achieved auxiliary vents may terminate through the roof, in the attic space (never a good idea) and subject to a long list of limitations they may terminate through a side wall. In addition, while the UPC will only allow a maximum of one AAV per structure once the main drain is achieved the IRC permits an unlimited use of AAV's (here again, not a good idea).

Now in regards to the material to construct the vents. If they are using PVC it MUST BE schedule 40 because schedule 40 is the only PVC that is listed as approved for DWV applications.

The horizontal sections must be pitched at 1/4" per foot with the slope dropping back towards the vertical drop down to the drain.

From your description the only thing that i would insist on changing is to make sure they terminate that vent through the roof. Even though the IRC does permit terminating in the attic space that is never a good idea because it will result in excessive moisture in the attic space which is ultimately absorbed into the insulation, which then degrades the value and efficiency of the insulation as well as potentially could lead to a mold problem in the attic space.

    Bookmark   July 3, 2010 at 12:59AM
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randy427

Many thanks for your time and expertise.
R3

    Bookmark   July 3, 2010 at 2:34PM
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manhattan42

Lazypup's advice is generally good, but he is confusing code sections of the UPC under which he works with non-existent code requirements of the IRC under which he has little experience.

For example, lazypup said:

"Under the IRC all structures are required to have one "Main Vent". which must run undiminished in size from the building "Main Drain" through the roof."

Not true. This "requirement" does not exist under the IRC.

All that is required is that "The vent system serving each building drain shall have at least one vent pipe that extends to the outdoors." (P3102.1)

There is nothing in the IRC that requires a building drain to have a "main vent" that goes directly to and through the roof, or that this "main vent" run undiminished in size.

NO 'main vent' is even required under the IRC.

Lazypup also wrongly claims:

"Even though the IRC does permit terminating in the attic space that is never a good idea..."

Again, lazypup's lack of expertise with the IRC has caused him to make more false assumptions.

The IRC does not allow ANY vent to terminate in an attic space.

All vents under the IRC must terminate to outside air OR terminate in an Air Admittance Valve.

An Air Admittance Valve is NOT a "vent terminal", however.

It is an air admittance terminal through which vent air can only enter and through which no vent gases can ever escape. There is therefore no ability for "excessive moisture in the attic space which is ultimately absorbed into the insulation, which then degrades the value and efficiency of the insulation as well as potentially could lead to a mold problem in the attic space" under the IRC provisions as lazypup wrongly claims.

-------------

lazypup did get right the part about making sure that Randy427's vent which now terminates in the attic needs to terminate to the exterior, but this vent can also terminate into an air admittance valve in the attic provided that there is at least 1 vent in the building's vent system that terminates through the roof.

Bottom line: lazypup should stick with advice regarding UPC only... because he's having a very hard time understanding and applying the plumbing codes for the IRC.

    Bookmark   July 4, 2010 at 5:32AM
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