You should visit the heater at least once every three months.
Clean filter rinnai gas heater.
Never use chemical solutions of any kind.
The rinnai is typical.
Open the purge port valves by twisting them perpendicular to the position of the cold and hot valves.
Know that a tankless water is not a set it and forget it water heater.
Use 4 gallons 15 1 liters of undiluted white vinegar to clean your tankless water heater.