Originally Posted by Wet_Boots
The closing force in a zone valve comes from the force differential on either side of the diaphragm assembly. Pressure loss through the valve is a part of that force. An oversized valve won't have the pressure loss of a standard valve, so that part of the force is absent or diminished. An oversized valve can even develop seat wear in certain low-flow situations, due to it opening incompletely (a phenomenon whose name I can't recollect at the moment)
It's the seldom seen Nottalottaopenup.
You described it well Shoes.
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