What Size Should My Overflow Plumbing Be?

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My advice is to go larger than you think you need to. (2x1.5") There is no harm in making the drain too large, but problems if you make it too small.

Further, by making two of such drains you can run the "silent siphon", or "Herbie" method. Use a gate valve to cut back the flow on one of the overflows until that overflow is max'd. At this point, it will run like a siphon and be completely silent.

The second drain has a vertical standpipe and durso. This drain is only used if the first drain becomes plugged. It's an emergency backup only.

For this to work, both drains must be in the same overflow box.

The exact flow potential of a drain is a complex thing to estimate. Every situation is different.