Jeff Wisnia wrote:
....

I don't see how a pool drain could suck with a force of more than one
atmosphere (14.7 psi) plus about another 1/2 psi for every foot of pool
water depth above the drain.
....
In large pools certainly, they're not atmospheric drains, they're inlets
to recirculating pumps so it's dependent on the suction head plus the
static head.

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