What is an overflow valve?

Answer by  William7526 (69)

It is a secondary drain near the lip of a tub or sink, designed to prevent an overflow due to a surface obstruction. Since the overflow value eventually meets back up with the drain pipe, it will not stop overflows caused by a clog in the pipe itself.

Answer by  y3636363 (1380)

Yeah I'm guessing that an over flow valve is basically what that is. It prevents the water from overflowing and that is how it acts as a valve as well.

Answer by  Atta (17)

An overflow valve is a device used in plumbing and other applications to maintain pressure on the line above the valve and it prevents all overflow.