Systems that use a thermostat or timer to automatically turn on the pump whenever water temperature drops below a set-point, or when the timer reaches a setting. Hot water recirculation systems can be activated by the push of a button or by a thermostat, timer or motion sensor. Such systems ensure that hot water is always available without any waiting time. Hot water recirculation systems generally consist of a pump, an integrated electronic controller, and a zone valve. When the activation button is pushed, or when another type of control turns the system on, the pump starts re-circulating cooled water that has been sitting in the hot water line and sends it back to the water heater through the cold water line. When the water reaches a desired temperature, a control closes the zone valve and turns off the pump. It is much like turning on the hot water faucet and letting the water run until it gets hot, but instead of the water going down the drain, it is simply returned back to the water heater.