Ideally in new construction I would add a wall box but water hook-ups are already in position. Currently, our washing machine drains into the utility tub-doing some remodeling I’d like to add a standpipe drain and utilize all the existing washing machine water connections that are currently in place. I plan to T off of the sink P trap and add a new P trap with a 30” standpipe for the washer drain. Do I need to add an additional air lock vent or will the current sink vent handle this?

I would think if the top of the standpipe is open and you're using 2" PVC, that should be enough. Make sure you're draining through only one trap.

The plumbing code in Wisconsin requires new installations of standpipe and p-trap for wash machine discharge to be 2" in diameter. If your sink drain is 2" in diameter you may be able to make it work. Discharging the wash machine waste to a laundry tub is code compliant as the drain on the tub can still be 1 1/2" in diameter.

Chris

Just make sure that if your line freezes, then you thaw it out, that you hook the drain back up. Stupid me, I forgot to do that in our recent cold snap. Now have an insurance adjuster coming to look at all of the floor I ruined. ugh!

Menards sells an 1-1/2 standpipe kit but if 2" is code- I might have to drop through the floor and T into the existing PVC soil run- I assume running a 2" standpipe & T into the 1-1/2" drain is no no?

You would need to tie the standpipe/p-trap into a 2" line. As a home owner you could still do it all in 1 1/2" pipe as that diameter has worked for decades. If you sell your home, a home inspection may ding you but I doubt it as at one time 1 1/2" was the standard.

Chris

Thanks Chris- it appears that the WI Code changed when front load washers (which we have) became popular, they discharge at a faster rate, albeit utilizing less water. I would probably be fine with 1-1/2' as this is still code in other states, but I'll run 2" to be safe and to code.

All new washing machines discharge faster than the old ones. Our code in NC requires a 2" pipe with a 34" stand pipe. The 2" drain must be increased to 3" as soon as another fixture is tied into it such as your kitchen sink.