This is sort of a follow up from a previous question. I'm converting a walk-in shower to a whirlpool bathtub / shower. My existing drain configuration is all 2". I'm bringing it down to 1 1/2" for the new whirlpool drain. Should I install a 2" trap then reduce the raiser down to 1 1/2" or can I reduce it right where I am cutting the old trap off at? The latter part is what my plan is right now. The new trap will be about 15" from the old one.

Hi Mike.. If it were me, I would install 2" up to and including the trap IF SPACING AND VENTING ALLOW IT!! Then, I would reduce using a 2"x 1.5" reducing bushing to an 1.5" desanco fitting (trap adapter) to connect to the tub waste and overflow assembly of the whirlpoool unit. It's not that you cannot reduce earlier...YOU CAN...no problem... BUT...oh yes....BUT... an 1.5" drain clogs faster than a 2" drain... so why not use the increase in size... what really matters is that there is a vent pipe within, say, 5 feet or so of the trap... the vent is VERY IMPORTANT (allows turbulent/self-scouring/healthy drain system VS. laminar/clogging/un-healthy system). SO... up to you to decide... and always remember if you are not sure... DO NOT HESITATE to contact your local plumbing inspector and ask what they think (or surrounding towns... *wink, wink*).

I'm with the MassPlumber on this. A 2" trap and raiser with a 2 X 1/1/2" compression fitting on the raiser to pick up the 1 1/2" tub waste.
Since you're converting a shower I assume the vnt's already in place.
Good luck. Tom

Xx

I will go with the advice then. I appreciate it.

what really matters is that there is a vent pipe within, say, 5 feet or so of the trap.

But certainly no more than 5 feet from the vent for a 2" trap arm, right?

Agrees with I am growler.. *smile*