You can reduce the flow using a ball valve on the outlet side of your pump. This is good for limiting the flow up to about 20% without adding too much strain on your pump. If you need to reduce it more use a Tee fitting and attach a second ball valve to the return just before the first valve to return water to your sump and prevent over pressurization of the pump. Please excuse the crudeness of the diagram.
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You can reduce the flow using a ball valve on the outlet side of your pump. This is good for limiting the flow up to about 20% without adding too much strain on your pump. If you need to reduce it more use a Tee fitting and attach a second ball valve to the return just before the first valve to return water to your sump and prevent over pressurization of the pump. Please excuse the crudeness of the diagram.