You need much more than just putting a pump o the hot water line, you need all the plumbing that goes with it. If you stick a pump on the hot water line by itself, you will blow the line apart overtime time.
Also you don’t use the cold water line as the return line. If this was the case you would never have cold water.
A hot water recirculating system is 1 big loop of the hot water line with drops for each kitchen and bath. The water from the heater flows in a circle keeping the water at a temperature you program at the pump, not at the heater. If you set the water heater at 120 and the recirculating pump is set at 122, your water will run constantly because it’s trying to heat the water to something higher than your heater. Don’t do this. I always set my pumps to a temp of 3-5 degrees colder so the pump would shut off for periods and when we needed water, it was only a couple degrees off the max setting ofthe heater and we had instant hot water at every location
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