Washer machine in the garage is 24" lower than the ff from the bathroom, I'm installing a p-trap 6" from the bathroom finish floor. What is the height of the washer box that needs to be? The bathroom is located on the other side of the wall of the garage, Need to know the height of the washer box from the garage finish floor? Can I drain with a 1 1/2" pipe?
It will depend on the local code as well as manufacturers' requirements. Typically, the vertical standpipe (the portion between the p-trap and the laundry box) must be no shorter than 18" and no more than 42", and this pipe must be 2" in diameter.
The issue is that you want capacity in the standpipe to account for any back-pressure caused by the volume of water discharged by the pump in the washing machine when completing a drain cycle. This water is transitioning from being forced to gravity, and if there is insufficient capacity within the standpipe, you could have water overflow out of the top of the pipe. The height of the standpipe is limited by the size of the pump in the washing machine. The pump will be designed to pump only up to a set maximum height, and if the standpipe connection is higher than that height, the washing machine will not drain as the pump cannot overcome the head pressure.
