A. You could even use 3/8" on the return line (after the last fixture). Best to go with 1/2" but there is no reason to use 3/4" pipe. When your system is entirely under pressure, while the Grundfos pump is "pushing" up, the water is also "dropping" down. Just make sure to get rid of ALL of air in your return line and design your system so that no air will be in your line.