1.5 vs 2" P-trap for tub

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Old 10-24-16, 01:41 PM
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1.5 vs 2" P-trap for tub

I'm curious what thoughts anyone has on making a standard bathtub p-trap 2" instead of 1.5". I've used 2" p-traps on tubs where the vent distance is near maximum, to make sure I kept 2" pipe all the way to the tub.

The tub drain assembly itself is pretty much always going to be 1.5" pipe and fittings so you'd have to reduce with a bushing as you connect to the 2" trap.

Assuming I have the space for a 2" trap is that a good idea to prevent clogs?

Or, is the larger trap just reducing the drain velocity thereby making it more likely that debris builds up in the trap?

I think there's little value to arguing that a 2" trap would let someone in the future replace the tub with a shower since likely the trap would have to be moved anyways.
 
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Old 10-25-16, 09:03 PM
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Well, I didn't get any replies.

On my latest project a 2" p-trap gave the perfect offset whereas a 1.5" p-trap required some extra fittings to get it located just right for the tub drain. I went with the 2".

Would still be interested in hearing thoughts on if it matters.
 
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Old 10-25-16, 09:55 PM
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I doubt you'd see much difference in water velocity...the tub drain assembly being the limiting factor in every standard tub I've seen....

2" is a little easier to snake should that become necessary.
 
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Old 10-25-16, 10:13 PM
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I used to have a large (about three feet by five feet) stall shower in my home. I removed that and installed a whirlpool tub. Since the existing drain was 2 inch I installed a 2 inch P-trap with the reducing fitting to the 1-1/2 inch tub drain. I am right at the limit of trap arm length without having a re-vent at the trap.

This was about ten years ago and I have had NO problems. The drain DOES gurgle when draining but I asked on a professional plumber's forum and the consensus was it was merely scouring the trap, a good sign. I have often gone several months without using the tub and have never "lost the seal" in the trap.

So, in my opinion, which will get you a cheap cup of coffee if you also slip the waitress a buck, you are good to go. Your LOCAL plumbing inspector may have a different opinion.
 
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Old 10-26-16, 11:04 AM
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I don't see any benefit (or detriment) to using a 2" trap. It's rare that a bath or shower trap gets clogged. It's usually the stopper or grate that gets all gunked up over time.

Installing a 2" pipe like you did is often a good idea depending on the length and also enables future changeover to a shower.
 
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