Bathtub trap question


Old 07-08-19, 09:52 AM
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Bathtub trap question

I have a bathtub with a 1 1/2 drain. I was wondering if I can transition to using a larger trap (2" or 3") so that I can increase the distance before a vent is required. For example it appears that I can increase the distance from the trap to the vent by 2 feet if I use a 2" trap instead of a 1 1/2" trap. I am not sure if this is acceptable or something that is regularly done.
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Old 07-08-19, 01:43 PM
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It is not just the trap, the entire drain must be increased to 2 inch to meet code. 2 inch is standard for a shower and using it on a tub may cause the inspector to raise his eyebrows a bit although I don't know that it is specifically prohibited.
Old 07-08-19, 05:00 PM
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It looks like if I transition to a 2" trap, it will provide me an additional 2' before a vent is required according to IRC code (6' to 8'). A 3" trap might be excessive and not code.
Old 07-08-19, 05:55 PM
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It is not just size of the trap, but the size of pipe in horizontal run.
The distance specified is so that the horizontal pipe is not full of water is starts siphoning water out of the trap.
If you have too much slope between trap and vent, siphoning will happen even if the distance was within the code.

Easiest way out I can think of is adding an air admittance valve. There is one that can be installed inside of the wall with a recessed box and louvered cover. You can hang a picture or mirror over it to hide.
Old 07-09-19, 04:06 AM
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It's my understanding that a drain must go from small to large as it it travels down stream. The reasoning is that anything that can pass the small drain size will pass the larger without a problem. But not in reverse.

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