Recirculation lines keep hot water flowing to the farthest point of a plumbing system so the person farthest away from the water heater doesn't have to wait long periods for hot water. Otherwise, the water in the plumbing would cool off. At peak periods, however, it is possible for so much demand for hot water to develop that cold water will be drawn off the bottom of the tank and backward through the recirc line, causing people taking showers to get cold flashes.

Spring check valves are one option to prevent that -- and the one we prefer. They have a spring-loaded door inside that opens to permit recirc water to return, but shuts when water tries to flow the other way. Swing check valves are prone to stick open in areas with hard water. Then it's as if you had no valve at all...

Swing check valves have a nut on top, while spring check valves are cylindrical, as in the photos at right, showing side view and front and back views.