I've seen diagrams where by code a laundry sink can be installed without a trap if it is drained into a 2" washing machine standpipe that has a trap, provided the sink is not more than 30" away. Why is the limit 30"? A 1 1/2-inch sink drain with a trap can be installed 42" from a main stack. And the only reason for that 42" dimension is so there is not a possibility of the drain line flooding and pulling the trap dry. I understand the reasoning for not allowing a trapped drain line to flow into another trapped drain line. But if a laundry sink without a trap is drained into a washing machine standpipe that has a trap, there is no risk of sewer gases entering the house even if the laundry sink is more than 30" away, or more than 42" away, or even more than 60" away. The 2" washing machine trap cannot become dry with this laundry sink drainage arrangement. I know codes are in place to provide safe installations, but the 30" limit on a laundry sink draining into a washing machine standpipe doesn't seem reasonable. What am I missing?
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