OakTendGet started free

How to descale a tankless water heater

Updated . By the OakTend team.

Descaling (also called flushing) means circulating white vinegar through a tankless heater's heat exchanger, through the service valves under the unit, to dissolve mineral scale.

How often
At least once a year, according to Rinnai and Noritz, and more often with hard water or heavy use.
What you need
Service valve kit on the heater (most installs have one), Hoses for the service valves, Bucket, 4 gallons of undiluted white vinegar, A small submersible pump, if your manual calls for one

Why it matters

Noritz says hard water builds scale inside the heat exchanger, which cuts heat transfer and restricts flow. Orange County water is hard, so this is not a job to skip.

Steps

  1. Turn off the power to the heater, and close its gas supply.
  2. Close the cold, hot and main water valves at the unit.
  3. With the hot valve fully closed, slowly unscrew the purge port caps to let the pressure out.
  4. Connect the hoses to the service valves and run the outlet hose into the bucket.
  5. Open the purge ports and circulate the vinegar as your manual directs. Rinnai says this can take up to 45 minutes.
  6. Close the purge ports, take off the hoses and screw the caps back on snug, not overtightened.
  7. Open the water valves, restore gas and power, and run a hot tap until the water flows steadily without air.

When to call a pro

Call the installer or a plumber if your heater has no service valves (they can add them), if anything leaks, or if error codes come back after a flush.

Safety

Rinnai says to use only white vinegar and never chemical solutions. Release pressure slowly, and only after the hot valve is shut.

Go deeper

General information, not professional advice. Your equipment's manual comes first when it says something different.

Sources

More water heater and plumbing