I have a T right above my return pump. One side is hooked the the pump. One side is plumbed back up to the main tank and the other line feeds back in to the return chamber. Both the both outputs have a valve on them. I leave the main line wide open and use the feed back line to adjust how much water is being sent up to the main. This design allows the pump to run full on with no real back pressure. If the system didn't have a feed back line and I closed off the main line to reduce the flow, I would be making the pump work extra hard to do less work. The extra wear and tear on the pump will shorten its life.