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Returning flow back to the sump actually makes a centrifugal pump run hotter not cooler. When you restrict the discharge side you reduce the amount of work the pump is required to do and reduce the wattage consumed meaning less heat produced.
When you return part of the flow to the sump you are requiring the pump to work harder to pump the total volume of water even though you are only using part of the water. More work equals more power consumed whcih equals more heat produced.
All pumps create heat as they are not 100% efficient. Some are better than others though.
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