The traditional hot water recirculation system uses the existing cold water line as return line from the point of use located farthest from the hot water tank back to the hot water tank. The first of two system types has a pump mounted at the hot water heater while a "normally open" thermostatic control valve gets installed at the farthest fixture from the water heater and closes once hot water contacts the valve to control crossover flow between the hot and cold lines. A second type of system uses a thermostatically controlled pump which gets installed at the farthest fixture from the water heater. These thermostatically controlled pumps often have a built-in "normally closed" check-valve which prevents water in the cold water line from entering into the hot water line. Compared to a dedicated return line, using the cold water line as a return has the disadvantage of heating the cold water pipe (and the contained water). Accurate temperature monitoring and active flow control can minimize loss of cold water within the cold water line.