When the burner in a tankless water heater fires up to heat water, there's a small amount of scale that can happen. Scale occurs when heat separates the dissolved minerals in water. The harder the water in your area, the more this happens, and these minerals will build-up on the unit's heat exchangers. Left unchecked, this scale can lower the water heater's efficiency and even reduce the life-span of the unit.
There is a simple maintenance routine, which I explain in the video below, that should be done periodically - once a year if the unit is used frequently. It's a straight-forward procedure to clean your tankless water heater and remove scale, and it works regardless of the unit’s manufacturer. Tankless units from Rinnai, Takagi, Rheem, Noritz, Bosch, and others can all be cleaned using this method, provided the unit was installed with flush valves to begin with. If it wasn't, call the plumber back and get those installed, then watch this video.