can I reduce a vent from 2" to 1.5" on the vertical run

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leighG

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My waste plumbing for a new bathroom group all runs horizontally in the crawl space however in order to run the vent for the toilet/tub and shower up through the wall to the roof I need to reduce to 1.5" vent pipe at the floor level in order to miss a rafter in the ceiling. Is it okay to reduce to 1.5" for the vertical vent run ?

Thanks
 

wwhitney

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Your zip code is in Missouri, which I believe uses the IPC. In which case it would be unlikely that the vent needs to be 2" in the first place. So the usual solution would be to reduce the vent takeoff to 1.5" and run a 1.5" vent for the whole run, possibly enlarging to 3" before the vent leaves the thermal envelope if you are in a cold climate.

But I'm having trouble envisioning a case where there's a ceiling joist in the way and you can't carefully adjust the vent location within the wall to avoid the joist.

Cheers, Wayne
 

leighG

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Your zip code is in Missouri, which I believe uses the IPC. In which case it would be unlikely that the vent needs to be 2" in the first place. So the usual solution would be to reduce the vent takeoff to 1.5" and run a 1.5" vent for the whole run, possibly enlarging to 3" before the vent leaves the thermal envelope if you are in a cold climate.

But I'm having trouble envisioning a case where there's a ceiling joist in the way and you can't carefully adjust the vent location within the wall to avoid the joist.

Cheers, Wayne
Hi Wayne
Yes I am in rural Missouri actually and there are no inspections permits or other encumbrances. I built the entire addition myself and it is totally self supporting, not attached to the single wide manufactured home. The room to the right of the screened porch is the new ensuite bathroom. So I want to bring the vent down the interior 2x6 wall which is right under one of the ceiling rafters and there is just enough room to go to the side of the rafter if I use 1.5" pipe in that wall. I try to do things right even though I don't technically have to so that's why I am asking the experts here.
 

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wwhitney

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Hi Wayne
So I want to bring the vent down the interior 2x6 wall which is right under one of the ceiling rafters and there is just enough room to go to the side of the rafter if I use 1.5" pipe in that wall.
Not quite following that in your picture--can you not use a 22.5 or 45 degree elbow at a strategic location to miss the rafter with a 2" pipe while still staying within the framing?

Anyway, like I said, if part of the vent is going to be 1.5", typically it would all be 1.5" (except possibly getting larger at the roof terminal to avoid frost closure). Not really sure if there's any prohibition on making part of 2" when only 1.5" is required.

Cheers, Wayne
 

leighG

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Hi Wayne, no the problem is where the top 2x6 is located against the ceiling. Over half of it's 5.5" width is under the rafter so to pass to the right side of the rafter and stay within the wall I only have 1.5" to play with so in reality I could reduce before exiting above the ceiling so that only the very top 12" or so would be 1.5" and the rest of the vent could be 2" going down.

Alternate way will be to run the vent on the shower side at the back of the wall in the corner I suppose. Funny how studying a picture of the problem reveals things you don't see when standing there and scratching your head :)
 
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