Rinnai Continuum Series Service Manual page 31

When the hot water tap is turned off, the water flow sensor stops revolving, and the magnetic pulse ceases,
indicating to the PCB that there is no water flowing, in turn the PCB closes the gas valves. The combustion
fan continues to operate for 65 seconds. This is to provide quicker ignition when the tap is turned on and
off in rapid succession, as it removes the need for a pre-purge cycle and allows the burner to re-light
immediately when a hot water tap is opened again.
REU-2532FFU Series 31 Rinnai