Tankless water heater manufacturers recommend flushing every 6-12 months. This is especially important in areas with hard water, as minerals can buildup inside the heat exchanger and cause performance problems. I’ll run down the basics and I’ve collected the descaling instructions from the three main manufacturers of tankless water heaters: Takagi (sometimes labeled A.O. Smith), Noritz, and Rinnai.
Before flushing, you need to ensure that your tankless unit is equipped with isolation valves. Without isolation valves, you can’t flush the unit without disconnecting the attached piping. If you don’t have them, call a plumber. The valves cost around $100 for the set, and I should take about an hour or two of plumbers time, so estimate $200-$300 for this work.