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.

Isolation Valves
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.

Isolation Valves

The Basics
The basic descaling kit is a submersible pump, a bucket, two clothes water connection hoses, and descaling solution.  You hook up your hoses to the isolation valves, and run the solution through the heat exchanger to remove scale.  For a descaling solution, most manufacturers  recommend using white vinegar, but it’s interesting that Takagi notes “Please note that vinegar is known to be ineffective at removing scale.”  Descaling kits can be purchased for about $150, and Flow-aide, the only dedicated descaling solution that I know about, will run you about $60.  This is why people probably like using vinegar!

The Procedure
While the basics of flushing are the same, each manufacturer has a slightly different procedure.  Find the manufacturer of your tankless water heater and click on the appropriate link below.

Descaling Procedure_Takagi.pdf

Descaling Procedure_Noritz.pdf

Descaling Procedure_Rinnai.pdf

Plumber vs DIY
While this is a pretty simple procedure when you have the descaling kit and solution, some homeowners may not feel comfortable with tackling such a job.  It should be noted that Noritz warns in their descaling procedure that “Any unauthorized use of this manual may result in voiding the warranty.”  If you have any lingering doubt, call a licensed plumbing contractor.