To add some perspective to hot water circulation - it is strictly convenience item and make no technical sense.
I build new house and in California I had to pay for Title 24 energy efficiency design.
Having tankless water heater on exterior wall on east side, the west side guest bathroom has about 80 feet of piping to it.
I measured and it took over 3 gallons of dumping cold water to get hot there.
Yet Title 24 will not allow me installation of 2nd heater on the bathroom wall as that would not be energy-efficient.
Water here cost less than 1 cent a gallon, while energy is expensive and circulating the hot water on 80' of piping would loose lot of energy.
