Recirculation pump

Hot water travels through the pipes at the speed of the water, so a distant tap waits while the cold water in the pipe clears. A recirculation pump moves hot water through a dedicated return line, or through a crossover valve at the far fixture, so the pipes stay warm and the wait drops from tens of seconds to a few. It does not make hot water instant at the moment of the draw, and it adds gas or electricity use while it runs.

Rinnai's SENSEI units support recirculation through Circ-Logic with an external pump or crossover mode; the RXP models carry the pump built in, and Navien's A2 platform offers its ComfortFlow configuration. Timers and controls limit the loop's running time.

A recirculation loop that cannot move enough water reports low-flow codes: Rinnai's code 63 lists the bypass plug and filter configuration, fouled filters, parameter settings, pump supply and air in the return line as the checks. A stuck-open crossover valve is also the classic hidden cause of a rising gas bill, because it lets the burner cycle against pipe losses around the clock.