In order to determine the hot water return flow, need to calculate the heat-loss rates of the hot water supply piping in each section and then calculate the flow rate to cover the total calculated heat loss in each section of supply pipe. After determining a flow rate in each section, determine the pipe size for return in the same friction loss that have been used for the sizing the supply pipe. .
As a rule of thumb method and for preliminaries of sizing circulation piping and circulation pumps follow the following rules:
1- Consider 1 gpm for every 20 fixtures (not fixture unit) in the hot water system.
2- Consider 0.5 gpm for each ¾” and 1” supply riser.
3- Consider 1 gpm for each 1 ¼” and 1 ½” supply riser.
4- Consider 2 gpm for each 2” and 2 ½” supply riser
5- Consider 3 gpm for each 3” and larger supply riser.
