Would a non-overflow drain flow less?

vegan_vicky

Online advice says that an overflow lavatory sink will drain more slowly if a non-overflow drain is installed. The explanations are unconvincing. What am I missing? How much slower would it be? Would commonly available non-overflow drains fit? Except for the overflow feature are the two types interchangeable?

Originally asked by stretch on Stack Exchange (diy) (CC BY-SA 4.0).

uphilltech

Great question — answering as the UphillTech team:

If the sink has an overflow port, it acts as an additional vent and the water flowing down the drain does so faster. I have witnessed this first hand and panicked when I installed a new sink in a rental property and filled the sink to check for leaks. Upon opening the drain the water ran slowly making me think there was a partial clog in the drain...but NO! the chrome ring cover for the drain was still wrapped in the plastic covering. I removed it and the water drained just fine. If you install a drain without the access for the overflow you lose this additional venting...as well as losing the overflow protection. Can they be interchanged? That depends on the sink design. Sometimes the bowl...

Full answer by RMDman: https://diy.stackexchange.com/a/270092 (CC BY-SA 4.0).

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