The rule of thumb for calculating the circulating pump for domestic hot water return is to count the number of plumbing fixtures (not fixture units) which using hot water such as lavatories (bathroom sinks), kitchen sinks and showers in each riser (or group of the fixtures) then consider a one gpm for each 20 fixtures. After estimating the total gpm in each riser (or group) the pipe size can be find from the pipe sizing charts for 3 psi/100 ft pressure loss, the velocity of water in pipe shall not exceed by 5 fps. By this procedure can size the entire return pipe and estimate the total gpm in hot water return system.
The capacity of the circulating pump would be equal to the total estimated gpm in the hot water return system.
For calculating the head of the circulating pump, the pressure loss in the longest pipe run (circulating loop) should be calculated:
Pressure loss = (2 L x 1.25 ) 3/100 + Pₒ
L = longest pipe run in hot water return pipe (multiply by 2 for supply and return pipe)
1.25 = adding 25% of L for elbows & fittings
Pₒ = Pressure loss in water heater (from the manufacturer catalogue)
3/100 = pressure loss in each 100 ft of pipe