Trap Size Question

[MIKE@USA]

I am a DIY and have read some conflicting information regarding trap sizes. My question is I have a tub with standard 1 1/2 drain and I would like to step up to a larger trap size (2" or 3") to increase my maximum distance from fixture trap to vent. Is this an acceptable practice and are there any issues.

Thanks,

 

[Terry]

A 2" trap would be fine.

 

[Sponsor]

 

[Jadnashua]

You can always go bigger in the drainage path, but there are only a few exceptions for going the other way.

If you wanted a tub drain to be 2”, your choices are likely fewer., but you can go to 2” immediately after to the trap.

 

[wwhitney]

If I'm correctly reading UPC 1003.3 (not sure if the IPC is similar), a bath tub can have a 1-1/2" trap or a 2" trap, but not a 3" trap, and the trap arm diameter has to match the trap diameter.

1003.3 Size. The size (nominal diameter) of a trap for a given fixture shall be sufficient to drain the fixture rapidly, but in no case less than nor more than one pipe size larger than given in Table 702.1 . The trap shall be the same size as the trap arm to which it is connected

Cheers, Wayne

 

[Reach4]

1003.3 Size. The size (nominal diameter) of a trap for a given fixture shall be sufficient to drain the fixture rapidly, but in no case less than nor more than one pipe size larger than given in Table 702.1 . The trap shall be the same size as the trap arm to which it is connected

I wonder what the harm would be in increasing the trap arm size on the way to the vent.

 

[Plumber01]

I wonder what the harm would be in increasing the trap arm size on the way to the vent.

UPC 702.1 footnote 7-

Trap sizes shall not be increased to the point where the fixture discharge is capable of being inadequate to maintain their self-scouring properties

I assume it's the same for the trap arm as well. Preserving the self-scouring capabilities.

 

[MIKE@USA]

Based on the feedback, I will plan on going from 1 1/2" inch drain and using a 2" trap. According to the IRC code this will give me an extra 2' (6' to 8') to locate a vent.

Thanks for the help.

 

[Terry]

You have to go with 2" all the way if you expect to gain any length to the vent.
For example with UPC
Trap to vent
1.5" up to 42"
2.0" up to 60"

IPC
1.5" up to 72"
2.0" up to 96"

You can't install a 2" trap on a 1.5" trap arm.

 

[MIKE@USA]

Correct, I was going to come out of tub at 1 1/2", and then run at 1 1/2" for 24" before I transition to a 2" trap and then continue for 8' with 2" to the vent and then remain at 2" until I drop into the main waste line. The 24" run at 1 1/2" and the 8' run at 2" should provide me with 10' to hit the wall I want to use for my vent. Per the International Residential Code, I am allowed 30" measured from the centerline of the fixture to the centerline of the trap, which I think I can use to gain the additional 24" I mentioned above. I feel this sounds correct.