Water from your water heater should have no more than a 20 degree drop in temperature when water is sent out and then circulated back to the water heater. This is called the temperature differential for the water system. To accomplish this you will need to use the proper size water circulating pump. The size of the water pump is determined by calculating the recirculation flow for a building.
Determine the total length of water piping for the building. Measure the distance of the water pipe with the tape measure. Multiply the total length by two to account for the return pipe.
Multiply the total distance by 30, which is the maximum amount of water that travels per linear foot of pipe. This is the amount of BTUH for the building.
Divide the BTUH amount by 10,000. This is the recirculation flow for the water system.
Measure any branches that extend out from the main line. Multiply these branches by 30 and divide the result by 10,000. Repeat this step for the remaining branches, if any.
Add the recirculation flow for the main line and any branches. This is the total recirculation flow that is required for the building.
Find a circulation pump that matches the recirculation flow for the building. Obtain a pump that meets the minimum recirculation flow requirements.
The acronym BTUH stands for British Thermal Units per Hour. The acronym GPM stands for gallons per minute.