Rinnai V-series non-condensing tankless (V53DeN, V65, V75, V94): quick sheet

When a hot tap opens, cold water passes the inlet filter and a turbine-type water flow sensor. Once flow reaches the activation threshold (Rinnai lists about 0.4 gpm for the V65e/V75i) the board starts the combustion fan, proves airflow, ignites the burner and proves flame with the flame rod. Below the threshold the unit simply does not fire, which is the most common 'no hot water at the bathroom sink' complaint.

The board modulates gas input (roughly 10,300 to 150,000-199,000 BTU/h depending on model) to hold the outlet temperature. It reads an inlet thermistor and an outlet thermistor and uses an electronic water control device plus a fixed bypass to blend and limit flow when incoming water is very cold, so the set temperature can still be reached.

Out of the box the unit is set to deliver 120°F to reduce scald risk; higher settings (typically up to 140°F on residential controls) are available through the controller or parameter settings, and commercial controllers are needed for anything hotter on models that support it. Check the model's manual before raising it.

Built-in safeties include flame failure detection, boiling (overheat) protection with a thermal fuse/bi-metal on the heat exchanger, a combustion fan RPM check and automatic frost protection. Frost protection uses small electric heaters and, when needed, the burner, so it only works while the unit has 120 V power and gas. Rinnai rates indoor models to about -22°F (-30°C) and outdoor models to about -4°F (-20°C) when shielded from wind; exposed external piping is not protected.