tempering valve on return line

Dan_61

I am going to be putting in a new heater for my radiant floor system and have read that you can run the return line into the cold side of a mixing valve. Supply goes into the hot, mixed goes into the floor. So if you have hot water returning it can just go back into the floor. Does this work? Is it much more efficient? Do I need any extra pumps, controls, sensors,etc?