OwnerSpec

How to flush a tankless water heater: the Rinnai, Rheem, Noritz, Navien and Takagi procedures as each manual prints them

Rinnai: 4 gallons of white vinegar for 1 hour. Rheem: no more than 2 gallons, 45 minutes. Noritz: 3-4 gallons, 1 hour. Navien: 45 minutes. Takagi prints none.

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A white wall-mounted tankless water heater with red and blue isolation valves below it, two braided hoses running down into a white bucket holding a small submersible pump

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Short answer

Flushing a tankless water heater means closing its isolation valves, pumping a cleaning fluid from a bucket through the service ports and the heat exchanger, rinsing, and cleaning the cold inlet filter. Every maker that prints the procedure prints that shape; the numbers differ. Rinnai’s SENSEI RX manual says four gallons of undiluted food-grade white vinegar circulated at least 1 hour at 4 gallons per minute. Rheem’s IKONIC manual says no more than 2 gallons of white vinegar for 45 minutes in its Flush Mode. Noritz says 3 to 4 gallons of distilled white vinegar for at least one hour, swapped at 30 minutes. Navien’s NPE-2 manual says a diluted cleaning solution for at least 45 minutes and prints no quantity. Takagi’s manuals print a 7 grains per gallon hardness limit and a filter-cleaning step but no flush procedure at all. All four procedures end with a rinse of 5 minutes or more.

Do not buy the vinegar yet: if your maker’s hardness row says no flush at your gpg, or your water is over the 200 mg/L every one of these manuals caps at, the right purchase is a softener, not a pump.

What does each manual actually say?

One row per maker, every cell from the document named.

Maker and documentFluid the manual namesQuantityCirculation timeRinseScale alert printed
Rinnai SENSEI RX, manual 100000839, section 6.3Undiluted virgin food-grade white vinegar4 gallonsAt least 1 hour at 4 gpm (15.1 L/min), dES mode counts down 60 minutes5 minutesLC code: beginning to lime or scale up, must be flushed; SS reminder at 0.5, 1 or 2 years
Rheem IKONIC, Use and Care ManualVirgin food-grade white vinegarNo more than 2 gallons45 minutes, in Flush Mode (auto-returns to normal after 2 hours)5 minutesA01L: buildup of lime deposits, contact a dealer or technician
Noritz, descaling how-to (noritz.com)Distilled food-grade white vinegar3 to 4 gallonsAt least 1 hour, vinegar replaced after 30 minutesAt least 5 minutes or 20 gallonsError code 11 appears during the flush because the gas is off; power cycle to clear
Navien NPE-2, User’s Information Manual 4.3White vinegar or a solution formulated for tankless heaters (Installation manual, page 15), diluted with waterNot printedAt least 45 minutes5 minutesDescaling reminder setting: No Alarm (default), 6, 12 or 24 months
Takagi, manuals 62S051 and 68A161None printedNone printedNone printedNone printedFault codes 651 and 661 (flow adjustment and bypass valve): scale buildup locking the motor drive is one listed cause

Rinnai’s Table 3 bands flush frequency by hardness: none at 0–60 mg/L, once a year residential and twice commercial from 61 to 160 mg/L with a softener or ScaleCutter recommended, and strongly recommended from 161 to 200 mg/L. Navien’s page 15 table runs the same way in grains: none at 0–4 gpg, once a year at 4–12 gpg (twice a year commercial at 9–12), a softener or PeakFlow required over 12 gpg. The descale-interval calculator looks your number up, and the hardness unit converter turns mg/L into gpg (17.1 mg/L per grain).

What do you need before you start?

Isolation valves with flush ports on the hot and cold lines: every procedure assumes them, Rinnai strongly recommends them, Navien calls its kit Plumb Easy, Rheem lists flush kit RTG20124. A unit on plain shut-offs needs valves cut in first, which is plumbing work.

Then the kit the Noritz page lists: a five-gallon bucket, two 3/4 inch hoses, a submersible pump, and the vinegar.

Still choosing? The gas tankless review and the electric tankless review cover these makers, and the sizing calculator covers the flow number.

How is the flush done, step by step?

The shape all four procedures share, with each maker’s detail where it differs.

  1. Shut down as the manual says. Rinnai: gas off, controller off. Rheem: SERVICE menu, FLUSH HEATER? YES. Noritz: unit powered on, gas valve off. Navien: power off and disconnected.
  2. Close both isolation valves (Rinnai’s V3 and V4, Navien’s C and D, Noritz’s red and blue isolators) and drain through the service ports; Noritz condensing units also open the drain cock beneath the heater.
  3. Pump outlet hose to the cold service port, return hose from the hot service port into the bucket, pump in the bucket.
  4. Add the fluid: Rinnai 4 gallons, Noritz 3 to 4 gallons, Rheem no more than 2 gallons, Navien a diluted solution with no quantity printed.
  5. Open both service ports and start the pump for the time in the table. On a Rinnai, press B on the PC board for 1 second until the display shows dES, then A; it counts down from 60 minutes.
  6. Rinse: return hose to a drain, close the cold service port, open the cold isolation valve, run 5 minutes (Noritz: 5 minutes or 20 gallons). Then close the hot service port and open the hot isolation valve.
  7. Clean the inlet filter (next section) and return to normal: Rinnai’s display shows End, Rheem’s SERVICE menu gets FLUSH HEATER? NO, Noritz power-cycles to clear code 11.

The scale is the same calcium carbonate as the white crust on your faucets; vinegar dissolves it, nothing more.

What about the inlet filter screen?

Rinnai and Navien build the filter into the flush. Rinnai says remove the in-line filter, clean out any residue and replace it. Navien’s section 4.4 adds a recirculation inlet filter on A2 models: rinse under clean cold running water, scrub with a brush if needed, reinsert and tighten.

Rheem’s manual says clean the water filter monthly, unscrewed from the base of the cold inlet line, soft brush for severe sediment. Takagi’s 68A161 prints only this step (page 54): gas off, power off, inlet valve closed, drain, brush the filter, hand-tighten the plug. Stiebel Eltron’s manual says never run the unit without the filter screen.

Is an electric tankless flushed differently?

Same bucket, pump and service valves, shorter clock, power off instead of gas off. Stiebel Eltron’s tankless FAQ says scale may need flushing every six months to a year depending on hardness: a bucket about half full of water with its Flow-aide kit (part 540000), or white vinegar or CLR, circulated 30 to 45 minutes, then flushed 3 to 5 minutes or until clear. Its Use and Care manual says descaling should be performed as often as necessary.

That “as often as necessary” is why the electric tankless review lists hardness as a buying spec.

Before you flush a second time

A flush due twice a year is a hardness problem wearing a maintenance costume. Rinnai, Rheem and Navien all cap total hardness at 200 mg/L and exclude scale damage from warranty, and Rinnai’s table moves to “softener strongly recommended” at 161 mg/L. Get a hardness number; if it lands above the row your manual flushes at, size the softener with the water softener size calculator before buying more vinegar.

What should you do first?

  1. Find your maker’s row, then read your own manual’s flush section; another series may print different minutes.
  2. Check for isolation valves with flush ports; without them the job starts with a plumber.
  3. Buy the fluid your manual names in the quantity it prints; a 5 gallon bucket covers every figure above.
  4. Flush, rinse 5 minutes, clean the inlet filter, clear the service mode or code.
  5. Note the date against the reminder setting (Rinnai SS 0.5, 1 or 2 years; Navien 6, 12 or 24 months), and test hardness if the unit scaled before the reminder fired.

What owners buy for this

The top picks from Tankless water heater descale interval by water hardness: what Navien, Noritz, Rinnai and Rheem actually publish, with a calculator, compared on published specs and certifications.

  1. Your unit already has isolation service valves and your manual's row says flush

    6699 tankless water heater flushing kit: 1/6 HP pump, two hoses, 3 gallon pail

    Listing states a 1/6 HP submersible pump, two hoses and a 3 gallon pail: the pump, hoses and bucket the Navien appendix and the Noritz procedure both describe

    Check price on Amazon Why this one

  2. Your unit was installed with plain shut-offs and no flush ports

    HYDRO MASTER tankless service valve kit, 3/4 inch IPS, with pressure relief valve

    Listing states 3/4 inch IPS isolator valves with a pressure relief valve; the listing title says clean brass and names no lead-free standard, so confirm that on the maker's sheet before it goes on a potable line

    Check price on Amazon Why this one

See the full comparison and who should skip each one

Diagnose the problem, size the unit, match the part, then compare the shortlist. These pages continue the chain.

Frequently asked questions

How long do you circulate vinegar through a tankless water heater?

As long as your manual says, and the manuals disagree. Rinnai's SENSEI RX manual says at least 1 hour at 4 gallons per minute. Noritz says at least one hour, replacing the vinegar after 30 minutes for best results. Rheem's IKONIC manual says 45 minutes, and Navien's NPE-2 user's manual says at least 45 minutes. Stiebel Eltron's electric tankless FAQ says 30 to 45 minutes.

How much vinegar does a tankless water heater flush take?

Rinnai's SENSEI RX manual says four gallons of undiluted food-grade white vinegar. Noritz says 3 to 4 gallons of distilled food-grade white vinegar. Rheem's IKONIC manual says no more than 2 gallons. Navien's user's manual says a cleaning solution diluted with water in a 5 gallon or larger bucket but prints no quantity. A 5 gallon bucket covers every figure printed.

Do you turn the gas off to flush a tankless water heater?

Rinnai's SENSEI RX procedure starts by shutting off the gas supply and turning the controller off. Noritz says to keep the unit powered on but turn the gas valve off, and to expect error code 11 because the gas is off. Navien's procedure turns the power off and disconnects it.

Can you flush a Takagi tankless water heater with vinegar?

Takagi's installation manuals 62S051 (models 110, 310, 510) and 68A161 print hardness limits of 7 grains per gallon for single-family use, an inlet filter cleaning procedure and fault codes that list scale among their causes, but no heat exchanger flush procedure, fluid or time. Takagi's FAQ says the heat exchanger may require periodic flushing by a plumber and recommends a softener. Ask Takagi or that plumber.

What does the LC code on a Rinnai or A01L on a Rheem mean?

Scale. Rinnai's SENSEI RX manual says an LC diagnostic code indicates the unit is beginning to lime or scale up and must be flushed, and that scale damage is not covered by the warranty. Rheem's IKONIC manual lists A01L as a buildup of lime deposits and says to contact a dealer or qualified technician. Takagi's 651 and 661 are valve faults that list scale among their causes.

Fact sheet and how to cite this page

Answer
Every gas tankless maker that prints a flush procedure prints the same shape (close the isolation valves, pump a cleaning fluid through the service ports, rinse), but the numbers differ: Rinnai 4 gallons of white vinegar for at least 1 hour at 4 gpm, Rheem no more than 2 gallons for 45 minutes, Noritz 3 to 4 gallons for 1 hour, Navien at least 45 minutes. Takagi prints no procedure.
Basis
The flush sections of the Rinnai SENSEI RX Installation and Operation Manual (100000839, section 6.3), the Rheem IKONIC Use and Care Manual, the Noritz descaling how-to on noritz.com, the Navien NPE-2 User’s Information Manual (section 4.3) and the NPE-A2/S2 Installation and Operation Manual (page 15), the Takagi 62S051 and 68A161 manuals, and Stiebel Eltron’s tankless FAQ and Use and Care manual for electric units.
Assumptions
  • A residential gas tankless heater fitted with isolation (service) valves, which every procedure assumes; a unit on plain shut-offs needs a valve kit first, and cutting into the supply is plumbing work.
  • The cleaning fluid is what each manual names, food-grade white vinegar in every case that prints one; Navien also allows a solution formulated for tankless heaters and Stiebel Eltron also names its Flow-aide kit or CLR.
  • The gas connection is not touched: Rinnai says shut the gas off, Noritz says keep the unit powered but turn the gas valve off, Navien says power off and disconnected.
  • Procedures are from the series named; another series from the same maker may print different minutes or quantities, so the reader’s own manual overrides this page.
Method
Procedure, fluid, quantity, circulation time, rinse time, filter step and scale code read from each maker’s own manual or support page, opened on 11 October 2026, and set side by side without averaging. Where a maker prints no step, the table says so rather than borrowing another maker’s figure. No flush was performed for this page.
Primary source
Rheem, IKONIC condensing tankless water heater Use and Care Manual, Flushing of the Heat Exchanger (PDF) (manufacturer or vendor document)
Facts verified
Last updated
Canonical URL
https://ownerspec.com/plumbing/guides/how-to-flush-a-tankless-water-heater/
Cite as
OwnerSpec, "How to flush a tankless water heater: the Rinnai, Rheem, Noritz, Navien and Takagi procedures as each manual prints them", https://ownerspec.com/plumbing/guides/how-to-flush-a-tankless-water-heater/, facts verified 11 October 2026.

Every figure above also appears, with its source, in the article. All sources · How OwnerSpec verifies numbers

Sources

  1. Rinnai, SENSEI RX199iN product page, Documentation: 100000839 SENSEI RX Residential Installation and Operation Manual (Table 3 water quality, section 6.3 Flushing the Heat Exchanger, Quick-Flush) (manufacturer or vendor document)
  2. Rheem, IKONIC condensing tankless water heater Use and Care Manual, Flushing of the Heat Exchanger (PDF) (manufacturer or vendor document)
  3. Noritz, How to Descale a Noritz Tankless Water Heater (manufacturer or vendor document)
  4. Navien, NPE-A2/S2 Installation and Operation Manual, page 15 About Water Quality and Appendix 8.2 (PDF) (manufacturer or vendor document)
  5. Navien, NPE-2 User's Information Manual, sections 4.3 Flushing the Heat Exchanger and 4.4 Cleaning the Inlet Water Filter (PDF) (manufacturer or vendor document)
  6. Takagi, Installation Manual and Owner's Guide 62S051, models 110, 310 and 510 (PDF) (manufacturer or vendor document)
  7. Takagi, Installation Manual and Owner's Guide 68A161, Inlet Water Filter Cleaning and fault codes (PDF) (manufacturer or vendor document)
  8. Takagi, Frequently Asked Questions (manufacturer or vendor document)
  9. Stiebel Eltron USA, Tankless Electric Water Heater FAQs (manufacturer or vendor document)
  10. Stiebel Eltron USA, Tankless Use and Care Manual (PDF) (manufacturer or vendor document)