That's your valve--usually they have a recirc position.
But I've found if I add the Poly to the return stream it's STILL very effective.
I like to get all my chems pretty much balanced, then add 1 or 2 quarts of Poly (last couple of seasons I just used one for a 20k pool). I then wait 48 hours for the FC to drop like a stone (an effect of lots of Poly) then boost the FC back to the correct shock level, and close the pool.
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