How to Flush a Tankless Water Heater: What Each Brand Says

The Short Answer: Flushing a tankless heater means pumping white vinegar or a tankless descaler through the heat exchanger for at least 45 minutes to an hour, then rinsing and cleaning the inlet filter. It needs isolation valves with service ports. How often depends on water hardness. Navien’s and Rheem IKONIC’s owner manuals include the steps; Rinnai, most Noritz models, Takagi and Rheem’s other lines send it to a professional.

Who each brand says should do it

The U.S. Department of Energy’s tankless guideline (opens in a new tab) sums it up: flushing frequency “can vary from twice per year to once every three years, depending on the manufacturer and individual use patterns.” Rheem’s maintenance article (opens in a new tab) lists flushing the heat exchanger among tasks best left to a licensed professional, and suggests descaling about every six months in hard-water areas.

How hardness sets the schedule

Navien’s installation manual (opens in a new tab) has the clearest table. Its footnote says flushing is recommended if a treatment device isn’t installed:

Your water utility’s annual water quality report usually lists hardness, or a test kit will tell you. Rinnai, Navien and Noritz all exclude scale damage from the warranty, and Noritz’s NRC111 owner’s guide (opens in a new tab) excludes water over 12 grains per gallon (200 mg/L) unless it’s treated.

What a technician does

For the brands that assign descaling to a professional, the visit looks broadly the same, though each brand’s sequence differs:

• The technician turns the heater off and shuts the gas. Rheem’s technician manual (opens in a new tab) says the gas must stay off throughout. Most brands also disconnect power, but Noritz models with a blue FLUSH connector, such as the NRC111 (opens in a new tab), stay powered in a flushing mode. Older Noritz procedures unplug the heater.
• They close the isolation valves and connect hoses from a small submersible pump and a bucket to the valves’ service ports.
• They circulate the solution through the heat exchanger. Rinnai’s manual (opens in a new tab) calls for four gallons of undiluted food-grade white vinegar for at least an hour; Rheem’s technician procedure uses about two gallons for 45 to 60 minutes; Noritz’s flushing mode runs for an hour.
• They drain the solution, rinse fresh water through the heater, and clean the inlet filter.
• They purge the air by running a hot tap before restoring gas and power, then check the heater fires normally.

Rinnai’s bulletin 188 (opens in a new tab) adds a power cycle before flushing Sensei models, which positions internal valves, and says to discard used vinegar rather than reuse it. Rheem’s IKONIC models have a Flush Mode in the service menu that stops normal operation and keeps the inlet water valve open and the bypass valve closed, for up to two hours.

Doing it yourself on a Navien

Navien’s owner’s manual (opens in a new tab) gives a 15-step procedure for NPE-2 models, and the NPN manual (opens in a new tab) gives the same one. Both carry this caution: “Flushing the Heat Exchanger is a somewhat complicated procedure… If you are uncertain about any of the steps in the procedure, contact an authorized technician or licensed professional. Keep in mind that improper maintenance can void your warranty.”

Before you start, check that you have what the procedure assumes: the Navien valve kit, with A and B service ports and C and D shutoff valves, a pump, three hoses and a bucket of at least five gallons. Then follow the manual’s own sequence. In outline, it has you turn the heater off and disconnect its power, close the C and D valves, connect the hoses and pump to the A and B ports, circulate white vinegar or a tankless descaler for at least 45 minutes, and rinse for five minutes, reopening the valves as the rinse ends. You then disconnect the hoses and clean the inlet filter (Navien’s filter section has you shut the water supply and drain the heater first). Before reconnecting power, run a hot tap until water flows steadily with no air, as Navien’s refill steps describe, and check for leaks.

Stop and call a plumber if a valve or service port won’t open, close or reseal, if anything leaks, or if your heater has plain shutoff valves with no service ports. It can’t be flushed this way until a plumber adds them. If you smell gas afterward, don’t restart the heater; leave and call the gas utility or 911 from outside.

Cleaning the inlet filter

The cold-water inlet filter is owner maintenance on Rheem and Navien heaters. Rheem’s manual (opens in a new tab) says to clean it monthly, and Navien says periodically; its installation manual tells installers to instruct owners that the filter must be cleaned periodically to maintain proper water flow. Rheem’s steps start by turning the heater off and disconnecting its power; Navien’s turn it off at the Power button and close the gas. You then close the water valves, drain the heater, and unscrew the filter at the bottom. Rinnai’s general maintenance rule assigns all maintenance to a professional, so check your manual.

Codes that mean it’s time

• Navien E760: “Flushing/service alarm.” Navien’s manual says to contact the installer for routine service. The installation manual’s descaling reminder is off unless the installer sets an interval. See Navien tankless error codes.
• Rheem and Richmond 1L or 88: lime buildup, or the flush reminder. Rheem’s technician manual says 88 is a warning and the unit will eventually shut down to protect itself. See Richmond tankless error codes.
• Rheem IKONIC A01L: lime buildup. T111 is the service reminder.
• Rinnai SS: Service Soon, a timer set at installation. Older Rinnai models show LC for scale buildup; Rinnai’s bulletin 75 (opens in a new tab) notes it’s the only code that lets the heater keep running.
• Noritz C codes: the heater has detected scale buildup. The NRC111 owner’s guide says the unit may lock after repeated scale codes until the heat exchanger is flushed.

Clearing a reminder without flushing only hides it. For how resets work by brand, see how to reset a tankless water heater.

Frequently Asked Questions

Q: Can I use a descaling product instead of vinegar? A: It depends on the brand. Navien allows “white vinegar or solutions that are specifically formulated for use on tankless water heaters.” Noritz’s installation manuals recommend a calcium, lime and rust remover, and its help center (opens in a new tab) also allows pure vinegar. Rheem’s and Rinnai’s procedures specify food-grade white vinegar.

Q: My tankless has no service valves. What now? A: Every procedure depends on isolation valves with service ports. Rinnai includes them with Sensei heaters and strongly recommends them; Rheem calls for a cold-side service valve and offers a kit. A plumber can add them, which also makes every future flush faster.

Q: Is a softener better than flushing? A: They work together. Navien, Noritz, Rinnai and Takagi all recommend water treatment on hard water, and Navien’s table treats flushing as needed when no treatment device is installed. Treated water mainly reduces how often a flush is needed. For tank heaters, see how to flush a water heater.