From the Manufacturer The Laing Pump is installed under the sink or faucet farthest from the water heater - where hot water usually takes the longest amount of time to arrive. >A built-in temperature sensor automatically turns the pump on when the water temperature in the hot water supply line cools down to 77º F. This cool water in the hot water supply line is then pumped into the cold water line and back into the water heater. The Laing pump turns off automatically when the hot water supply line to the faucet reaches 91º F, ensuring the instant availability of shower warm water with maximum temperature hot water only seconds behind. When the pump is automatically turned off by the built-in thermostat, an auto closure device prevents hot or cold water from mixing in either supply line.