A recirculation system ensures that everybody has hot water all the time. In a building without one, the person closest to the water heater has nearly instant hot water, but those living farthest from it have to wait for all the water that has cooled in the piping to flow out their faucets before heated water from the tank gets to them. With a recirculation line, or recirc as those in the trade say, water flows continually through the piping, drawn either by gravity or a pump back to the water heater from the farthest point in the plumbing.

The top photo shows a recirc line (red and yellow arrows) returning to a pair of commercial tanks. They show the path of the recirc line through the pump and up to a tee. This one has a check valve (white circle) and shutoff valves (blue handles) for each tank, so that the recirc can be used even if one tank breaks and has to be replaced. The bottom photos show typical recirc pumps. The lefthand one requires periodic lubrication with oil, while the righthand one is partially lubricated by water flowing through it.