Linear drain to 1.5" pipe

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Mike Albanese

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Looking for for advice as I've had some conflicting info from different plumbers who have come to the house. I'm doing a master bath remodel and switched from a tub/shower combo to a curbless shower.

The shower will have a Schluter 68" linear drain body. My drain line is only 1.5" and it's on a second level, so switching it all out to 2" would be pretty extensive work.

My understanding on the reason 1.5" was ok in a tub, was the ability to hold a little water that wasn't draining as fast as a 2". Does the Schluter drain body give this same idea? Would a 2" to 1.5" bushing on the Schluter drain be ok or should I change it all out to 2" at a pretty big price.

The shower will have two shower heads, both 2.5 gpm.
 

Mike Albanese

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FWIW, your plumbing code (National Standard Plumbing Code) says that 1-1/2" is OK for a shower with one showerhead, while 2" is required for a shower with 2 or more showerheads. Go to page 96 here:

http://epubs.iapmo.org/NSPC/2021/

Cheers, Wayne


THanks, for some reason I thought I had saw it was by GPM not fixtures, but clearly that says 2" for two heads. From a practical standpoint, do you think it would be an issue running the 1.5"?

My thought just was if I had a regular drain I'd be more concerned, but given the linear drain would be able to hold a decent amount of water before it flowed back up into the shower, it seemed like it wouldn't present an issue of standing water in the shower if it didn't drain as fast as a 2".
 

wwhitney

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THanks, for some reason I thought I had saw it was by GPM not fixtures, but clearly that says 2" for two heads.
That would be the IPC, which many eastern states use, just not NJ. The IPC cutoff for a 1.5" drain on a shower is 5.7 gpm.

You need to have 2 showerheads simultaneously? As your easiest code compliance solution is to have at most one showerhead at a time.

Cheers, Wayne
 

Tuttles Revenge

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From a practical standpoint it comes down to gallons per minute. Input must not exceed output. The code was written LONG before water conservation was at the point that it is now and like most codes is sloooow to catch up with reality..

Washington state recently adopted a code requirement that only ONE shower head of 1.8gpm may operate in a shower at a time. So when we have multiple heads in a shower or are converting a tub to a shower on a 1.5" drain, we install a transfer valve that limits the use of the shower heads to only One.
 

Mike Albanese

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From a practical standpoint it comes down to gallons per minute. Input must not exceed output. The code was written LONG before water conservation was at the point that it is now and like most codes is sloooow to catch up with reality..

Washington state recently adopted a code requirement that only ONE shower head of 1.8gpm may operate in a shower at a time. So when we have multiple heads in a shower or are converting a tub to a shower on a 1.5" drain, we install a transfer valve that limits the use of the shower heads to only One.


So let's say in theory both shower heads are going at the same time, both are 2.5. So at most I'm looking at 5 gpm. Would that be a problem for a 1.5" drain given that I'm using a channel body drain? Is it worth the extra cost to replace the pipe all the way to my crawl from the second floor, or will I be ok using a bushing on the schluter drain before it hits the trap?
 

Tuttles Revenge

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I'm sure i've installed something like that without knowing it. Or say you were to follow the letter of the code til you passed an inspection and just changed out the shower heads and removed the limiting switch in the transfer valve and had 5gpm outta both heads.. logic would tell us the user would turn off the flow if it overwelmed the drain.. and then installed the correct lower flow heads to not overwhelm the system n the future. The code just preemts that process and gives us a

Lifted this from another article discussing your same scenario.
"...For more than 20 years, many plumbing codes have listed the minimum shower drain as 1 1/2 in. This was based on the lowering of the showerhead flow rate to 2.5 gal. per min. As the plumbing engineers pointed out, a 1 1/2 in. shower drain pitched 1/4 in. per ft., flowing half full, will accommodate 4.9 gpm with the roughest piping material, which happens to be cast iron. If you use PVC, the flow rate at half full flow increases to 8.3 gpm..."

Another section of the code has a couple tables that indicate a 1.5" trap can flow up to 3 DFU drainage fixture units.. 1DFU = 7.5gpm. \

I think you're safe sticking to 1.5".... BUT if that 1.5" is galvanized steel pipe.. and any of it is horizontal... change it.. its garbage and the actual diameter is less than 1.5 due to build up of gunk.
 

Jadnashua

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There's a bit more to think about with a shower drain than the gpm capability...probably not with a linear drain, but for a single drain, you might stand on the thing, and if you're not paying attention, start to get water accumulating, or say you drop a washcloth and water starts to back up. Once you realize that, you want that excess to be able to drain out rapidly so it doesn't spill out. That's especially important if you're planning a curbless shower where the actual volume of water it can store is very minimal.
 
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