Maintenance procedure · Written to ASD-STE100 Issue 9 · Formatted to ANSI Z535

Tankless Water Heater Descaling

Gas tankless (on-demand) water heaters with an isolation valve kit — the flush every maker publishes, and the numbers Rinnai, Navien, Noritz and Rheem do not agree on
ApplicabilityGas-fired tankless (on-demand) water heaters with an isolation valve kit already installed. Electric tankless water heaters are different; read your own documentation for one.
Task time1 hr 15 min to 1 hr 36 min editorial estimate, not a measurement
PersonnelOne
Revisionv2 · 2026-08-04
BEFORE YOU BEGIN

Before you begin. This procedure is general reference information, not professional advice. Equipment varies by manufacturer, model, and year. Always follow the documentation supplied with your equipment — where it differs from this page, follow the manufacturer. Verify that you have the tools, the working conditions, and the ability to do this task safely. If anything is unclear, if the equipment is damaged, or if the task exceeds your experience, stop and use a qualified professional. You are responsible for your own safety and for the outcome. This site accepts no liability for any loss, damage, or injury arising from the use of this information.

Specifications marked unverified could not be traced to a manufacturer source. Do not rely on them — obtain the value from your equipment documentation.

Summary

A tankless (on-demand) water heater has no tank for sediment to settle in, but hard water still leaves mineral scale inside the heat exchanger every time the burner fires. Left alone, scale narrows the waterways, cuts the flow of hot water, and can trigger an overheating error code. Clearing it out is called descaling, or flushing, and every major tankless maker publishes a version of the same basic job: close off the water heater, circulate an acid solution through the heat exchanger with a pump and a bucket, then rinse.

The steps are similar across brands. The numbers are not. We read Rinnai, Navien, Noritz and Rheem's own instructions, and no two of them agree on the descaling agent, the quantity, or how long to run the pump — Rinnai's own two documents even disagree with each other. Rheem states that a qualified service technician must do this work on the model we read. This page will not print a single figure that might be wrong for your water heater. It shows the shared shape of the job and sends you to your own documentation for every number.

Safety

WARNING

DO NOT CONTINUE THE FLUSH IF YOU SMELL GAS.
A gas leak can cause a fire or an explosion.

Necessary equipment

ItemNote
Circulation pumpThe pump must match the flow rate that your documentation gives.
Three flush hosesEach hose connects one port on the isolation valve kit to the pump or the bucket.
Bucket, 5 gallons or largerThe bucket holds the descaling agent and the pump.
Adjustable wrench optionalThe wrench opens and closes the isolation valve kit.
Descaling agent named in your documentation materialUse only the agent that your documentation names.
Clean water materialClean water rinses the agent from the water heater.

Find the correct method for your water heater

Estimated 6 min

NOTEThe manufacturers do not agree about the agent, the quantity, or the circulation time. Use the instruction for your own water heater.
Descaling agent chemical compatibility
Unverified — do not rely on this value
CONSULT THE MANUFACTURER DOCUMENTATION FOR YOUR WATER HEATER BEFORE YOU SELECT A DESCALING AGENT

No tier-1 source was found for this value. Take it from the documentation supplied with your equipment.

Four tier-1 manufacturer sources give four different answers. Rinnai (rinnai-tankless-flushing-procedure, accessed 2026-08-04) says: 'Use 4 gallons (15.1 liters) of undiluted white vinegar to clean your tankless water heater. Never use chemical solutions of any kind.' Noritz (noritz-how-to-descale-a-noritz-tankless-water-heater, accessed 2026-08-04) names a second acceptable agent: 'three to four gallons of distilled, food-grade white vinegar'. A Noritz knowledge-base page carries a wider wording naming a second product category, and this page does not quote it: that page was read with unrelated third-party spam links in its footer, which is a sign of possible compromise, and a document we will not register as a source is not one we will quote as guidance either. Navien (navien-how-to-descale-tankless-water-heaters, accessed 2026-08-04) names 'An approved tankless water heater descaling solution, or household white vinegar' without naming which other solutions are approved, while Navien's own owner's manual (navien-npe-users-information-manual, accessed 2026-08-04) does not name vinegar at all, only 'Cleaning solution diluted with water.' Rheem (rheem-tankless-users-information-manual, accessed 2026-08-04) names no home agent and states 'Annual Maintenance must only be done by a qualified service technician.' The manufacturer that permits an approved proprietary solution (Navien) and the manufacturer that says never use any chemical solution at all (Rinnai) sell the same class of appliance, and this page does not choose between them.
  1. Read the manufacturer instructions for your tankless water heater.
    • The instructions name the descaling agent for your water heater.
    • The instructions give the quantity, the circulation time, and the interval.
    3m
  2. Find the descaling agent that the manufacturer of your water heater names.
    • Rinnai names the white vinegar and forbids every other chemical solution.
    • Noritz names distilled, food-grade white vinegar.
    • Navien names an approved descaling solution or household white vinegar.
    • Rheem names a qualified service technician and gives no home method.
    • Use the agent for your own water heater only.
    2m
  3. Get the tools for the flush.
    • You need a circulation pump, three hoses, and a bucket.
    • Your water heater needs an installed isolation valve kit.
    2m

Prepare the water heater

Estimated 5 min

WARNING

DISCONNECT ELECTRICAL POWER TO THE WATER HEATER BEFORE YOU START THE FLUSH.
Contact with a live part can cause an electric shock.

  1. Disconnect the electrical power to the water heater.
    <1m
  2. If your water heater has a gas burner, close the gas supply valve.
    <1m
  3. Close the three water valves on the isolation valve kit.
    • The valves isolate the water heater from the water supply.
    1m
WARNING

CLOSE THE HOT WATER VALVE FULLY BEFORE YOU REMOVE A PURGE PORT CAP.
A cap removed too soon can release hot water and injure you.

  1. Remove the caps from the purge ports slowly.
    1m
  2. Connect a hose to each of the three purge ports.
    2m
  3. Open the purge ports on the isolation valve kit.
    <1m

Circulate the descaling agent

Estimated 49 min to 1 hr 4 min

Descaling solution quantity
Unverified — do not rely on this value
CONSULT THE MANUFACTURER DOCUMENTATION FOR THE QUANTITY

No tier-1 source was found for this value. Take it from the documentation supplied with your equipment.

The published quantities are not the same. Rinnai's two documents word it differently. Its general flushing page (rinnai-tankless-flushing-procedure, accessed 2026-08-04) says 'Use 4 gallons (15.1 liters) of undiluted white vinegar'. Its SENSEI technical bulletin (rinnai-tb188-sensei-flush-procedure, accessed 2026-08-04) is stricter about the vinegar itself: 'Pour four gallons of undiluted virgin, food grade, white vinegar into pail.' Noritz (noritz-how-to-descale-a-noritz-tankless-water-heater, accessed 2026-08-04) gives 'three to four gallons of distilled, food-grade white vinegar' — an overlapping but not identical range. Navien (navien-how-to-descale-tankless-water-heaters and navien-npe-users-information-manual, both accessed 2026-08-04) gives no quantity at all for its recommended solution, in either its blog page or its owner's manual. A quantity sized for one manufacturer's four gallons may be short or long for another model.
Descaling solution circulation time
Unverified — do not rely on this value
CONSULT THE MANUFACTURER DOCUMENTATION FOR THE CIRCULATION TIME

No tier-1 source was found for this value. Take it from the documentation supplied with your equipment.

Even Rinnai's own two documents disagree with each other. Rinnai's general flushing page (rinnai-tankless-flushing-procedure, accessed 2026-08-04) states the whole flush 'may take up to 45 minutes', while Rinnai's Technical Bulletin 188 for SENSEI models (rinnai-tb188-sensei-flush-procedure, accessed 2026-08-04) requires the pump to run for 'at least 1 hour at a rate of 3-4 gallons per minute'. Navien (navien-how-to-descale-tankless-water-heaters and navien-npe-users-information-manual, both accessed 2026-08-04) gives 'at least 45 minutes' in both its blog page and its owner's manual. Noritz (noritz-how-to-descale-a-noritz-tankless-water-heater, accessed 2026-08-04) gives 'at least an hour', adding 'For maximum effectiveness, change the vinegar solution after 30 minutes and repeat.' A circulation time correct for one water heater can stop too early or run needlessly long on another.
WARNING

USE ONLY THE DESCALING AGENT THAT YOUR DOCUMENTATION NAMES.
A different agent in your drinking water can harm your health.

  1. Put the descaling agent that your documentation names into the bucket.
    • Rinnai gives four gallons of undiluted white vinegar.
    • Noritz gives three to four gallons of white vinegar.
    • Navien gives no fixed quantity for its approved solution.
    2m
  2. Put the pump and the free hose ends into the bucket.
    <1m
  3. Start the circulation pump.
    <1m
  4. Circulate the agent through the water heater for the time that your documentation gives.
    • Rinnai's general instruction gives up to 45 minutes.
    • Rinnai's instruction for SENSEI models gives at least 1 hour.
    • Navien gives at least 45 minutes.
    • Noritz gives at least 1 hour, with a solution change at 30 minutes.
    45m
  5. Stop the circulation pump.
    <1m
NOTICE

DO NOT REUSE A DESCALING AGENT FOR A LATER FLUSH.
A used agent has less strength and gives a weaker result.

  1. Discard the used descaling agent.
    1m

Rinse the water heater

Estimated 7 min to 12 min

Post-descale fresh water rinse time
Unverified — do not rely on this value
CONSULT THE MANUFACTURER DOCUMENTATION FOR THE RINSE TIME

No tier-1 source was found for this value. Take it from the documentation supplied with your equipment.

Rinnai's SENSEI technical bulletin (rinnai-tb188-sensei-flush-procedure, accessed 2026-08-04) and Noritz (noritz-how-to-descale-a-noritz-tankless-water-heater, accessed 2026-08-04) each give five minutes. Rinnai's general flushing page states no rinse time at all - it gives 45 minutes for the whole flush and then moves straight to closing the purge valves, though Noritz adds a volume test: 'Flush for at least five minutes or until 20 gallons of water have passed through the heater.' Navien doubles that figure in its own blog page (navien-how-to-descale-tankless-water-heaters, accessed 2026-08-04): 'always flush with fresh water for a minimum of 10 minutes', but Navien's own owner's manual (navien-npe-users-information-manual, accessed 2026-08-04) gives 'Allow water to flow through the water heater for 5 minutes' at the equivalent step — so even Navien's two documents disagree with each other by a factor of two.
  1. Close the purge port that carried the agent.
    <1m
  2. Open the water valve on the same side.
    <1m
  3. Let the clean water flow through the water heater for the time that your documentation gives.
    • Rinnai and Noritz give at least 5 minutes.
    • Navien gives at least 10 minutes.
    5m
  4. Stop the water flow.
    <1m
  5. Disconnect the hoses after the water stops.
    <1m

Restart the water heater

Estimated 6 min to 8 min

  1. If your water heater has a filter on the cold inlet, clean it.
    • Rinse the filter under clean water.
    • A soft brush removes stuck deposits.
    2m
NOTICE

TIGHTEN A PURGE PORT CAP BY HAND ONLY.
Extra force can break the rubber seal inside the cap.

  1. Replace the caps on the purge ports.
    1m
  2. Open the three water valves on the isolation valve kit.
    1m
  3. If your water heater has a gas burner, open the gas supply valve.
    <1m
  4. Connect the electrical power to the water heater.
    <1m
  5. Open a hot water tap.
    <1m
  6. Let the water run until it flows without air.
    • Trapped air can cause a brief sputtering sound.
    • This is normal.
    2m

Complete the work

Estimated 1 min

Descaling interval
Unverified — do not rely on this value
CONSULT THE MANUFACTURER DOCUMENTATION FOR THE DESCALING INTERVAL

No tier-1 source was found for this value. Take it from the documentation supplied with your equipment.

The manufacturers publish different kinds of answer, not one number to compare. Rinnai (rinnai-tankless-flushing-procedure, accessed 2026-08-04) gives a floor: 'it's important to flush those mineral deposits at least once a year.' Navien (navien-how-to-descale-tankless-water-heaters, accessed 2026-08-04) gives the same floor, 'at least once a year', and says the true interval 'can vary' with mineral content, set temperature and hot water use, without stating a harder number for any of those cases. Noritz's descaling post (noritz-how-to-descale-a-noritz-tankless-water-heater, accessed 2026-08-04) gives the procedure and no interval at all, so Noritz is absent from this comparison rather than agreeing with it. Rheem's own site (rheem-how-often-to-flush-water-heater, accessed 2026-08-04) gives numeric intervals only for storage tank water heaters and separates tankless units out with 'periodic descaling' and no interval number at all. A floor, a variable, a deferred chart and an explicit non-answer are four different shapes of instruction, and averaging them would invent a precision none of the four sources offers.
  1. Record the date of the descale.
    • A record of the date shows when the next descale is due.
    1m

Terminology

This page uses one term for each item and holds it throughout, as ASD-STE100 requires. Equipment sellers and other guides use the names below for the same parts.

tankless water heater
Also called: on-demand water heater, on demand water heater, instant hot water heater, instantaneous water heater, continuous flow water heater, the box on the wall that makes hot water, gas hot water heater with no tank
descaling
Also called: flushing, flush, tankless flush, vinegar flush, clean my tankless water heater, remove the scale, de-scale, decalcify
scale
Also called: limescale, lime scale, mineral buildup, calcium buildup, hard water deposits, white flakes inside my water heater, the chalky stuff
heat exchanger
Also called: coil, burner coil, the part inside that gets hot, tankless coil
isolation valve kit
Also called: flush kit, service valve kit, hose bib kit, hot and cold shutoff valves, purge valves, flush valves
circulation pump
Also called: submersible pump, flush pump, descaling pump, sump pump

When to use a professional

Read your own manufacturer instructions before you start the flush. Rheem requires a qualified service technician for annual maintenance on the model in this record. If your water heater is a Rheem unit, call a technician for the flush.

Stop the flush if you smell gas. Leave the building. Do not touch a light switch or use a phone inside the building. Call your gas supplier from outside, or call the fire department.

Stop if your water heater has no isolation valve kit installed. Call a licensed plumber to fit one. Stop if you find a water leak, a gas smell, or damage to the water heater. Call a licensed professional.

Improper maintenance can affect your warranty. Navien and Noritz both state this in their own documentation. Read your own warranty terms before you start.

Common questions

What is the white flaky stuff inside my tankless water heater?

It's scale, mostly calcium and magnesium left behind when hard water heats up inside the unit. All four manufacturers we read describe the same problem: scale narrows the waterways inside the heat exchanger, so the burner works harder and the flow of hot water drops. Navien's own manual lists overheating error codes (E016 and E030) that name flushing the heat exchanger as the fix. Scale isn't dangerous to touch, but it does shorten the life of the water heater.

How much vinegar do I need to descale a tankless water heater?

There's no single number, and this page won't invent one. Rinnai gives four gallons of undiluted white vinegar, in both its general instructions and its technical bulletin for SENSEI models. Noritz gives three to four gallons, close to Rinnai's figure but not identical. Navien names 'an approved descaling solution or household white vinegar' and gives no quantity at all, in either its blog page or its own owner's manual. Use the amount your own documentation gives, not a number copied from a page like this one.

Can I use CLR instead of white vinegar?

It depends entirely on which brand you own, and here the manufacturers actually disagree. Noritz names distilled, food-grade white vinegar. Navien allows an approved proprietary descaling solution as an alternative to vinegar. Rinnai's own instructions say the opposite: 'Never use chemical solutions of any kind', and add that the wrong chemical could be harmful because your drinking water flows through the same unit. So a product one manufacturer names by category is a product another manufacturer's own words rule out. Read the instructions for your own water heater before you buy anything.

How long do I need to run the pump when I flush a tankless water heater?

It varies by manufacturer, and even by model within one manufacturer. Rinnai's general page says the whole job 'may take up to 45 minutes', but Rinnai's own technical bulletin for SENSEI models requires at least a full hour at 3 to 4 gallons a minute. Navien gives at least 45 minutes. Noritz gives at least an hour, and suggests changing the vinegar after the first 30 minutes for a better result. Follow the time your own documentation gives, not a single figure repeated online.

How often should I descale a tankless (on-demand) water heater?

Rinnai and Navien both give the same floor: at least once a year, more often if your water is hard, your water heater is set hotter, or your household uses a lot of hot water. Neither one gives a specific number of months for hard water, and Noritz sends you to a chart inside your own owner's manual instead of one figure here. Watch for a drop in flow or a rise in error codes as a practical sign it's due, alongside the calendar.

Do I need a plumber, or can I descale my tankless water heater myself?

It depends on your water heater and your own comfort with the job. Rinnai, Navien and Noritz all publish steps a homeowner can follow, though Navien adds that damage from an incorrect descale could void your warranty and recommends hiring a professional for that reason. Noritz says a homeowner can do it but a plumbing contractor should be brought in if you have any hesitation. Rheem is the outlier: its own manual for the model we read states that annual maintenance, including heat exchanger work, must be done by a qualified service technician. If you own a Rheem unit, or you aren't confident with hoses, pumps and shutoff valves, call a professional.

What's the difference between flushing a water heater and descaling one?

Rheem's own site draws this line for its products: a tank water heater is flushed to remove sediment that settles at the bottom, while a tankless water heater is descaled to dissolve mineral scale built up inside the heat exchanger. Rheem states plainly that tankless water heaters 'require periodic descaling instead of flushing'. The words get used interchangeably online, including by some of the manufacturers we read, but the underlying job — circulating an acid solution rather than draining sediment — is what a tankless unit actually needs.

Will descaling the wrong way void my warranty?

Possibly, and two of the four manufacturers say so directly. Navien's owner's manual states that improper maintenance 'can void your warranty', and its blog page repeats that damage from an incorrect descale could void coverage. Noritz's limited warranty separately lists 'improper, dangerous, or destructive maintenance procedures' as a condition that voids the warranty. None of the four documents we read name a specific chemical as a warranty condition by itself; the risk named is doing the job incorrectly, not the brand of vinegar.

My water heater shows an overheating error code. Will flushing it fix that?

Sometimes, according to Navien's own manual. Its error code table lists flushing the heat exchanger as one remedy for error code E016 (overheating of the heat exchanger) and error code E030 (exhaust overheating), alongside cleaning the cold water inlet filter and restarting the unit. That's Navien's own guidance for Navien units with those specific codes. If your water heater shows a different code, or the error returns after a flush, follow your own manufacturer's instructions and consider a professional.

Sources

Every specification on this page is traced to one of these sources. Tier 1 is a manufacturer document, a published standard, or a government safety authority. See Standards for the full tiering rule.

  1. Tier 1How do I flush clean my Rinnai tankless water heater? (13-Step Tankless Water Heater Flushing Procedure)Rinnai America Corporation · accessed 2026-08-04 · re-verification due 2027-02-01
  2. Tier 1Technical Bulletin 188: Optimized Flushing Procedure for SENSEI Tankless Water HeatersRinnai America Corporation · accessed 2026-08-04 · re-verification due 2027-02-01
  3. Tier 1How to Descale Your Tankless Water HeaterNavien, Inc. · accessed 2026-08-04 · re-verification due 2027-02-01
  4. Tier 1User's Information Manual — NPE Condensing Water Heaters (NPE-180A, NPE-210A, NPE-240A, NPE-150S, NPE-180S, NPE-210S, NPE-240S)Navien, Inc. · accessed 2026-08-04 · re-verification due 2027-02-01
  5. Tier 1How to Descale A Noritz Tankless Water HeaterNoritz America Corporation · accessed 2026-08-04 · re-verification due 2027-02-01
  6. Tier 1Noritz America Corporation Limited Warranty — Tankless Hot Water Heaters (SBA8743)Noritz America Corporation · accessed 2026-08-04 · re-verification due 2027-02-01
  7. Tier 1User's Information Manual — High Efficiency Condensing Tankless Water Heater, 199,000 Btu/hr Model (Natural Gas or Propane)Rheem Manufacturing Company · accessed 2026-08-04 · re-verification due 2027-02-01
  8. Tier 1How Often To Flush Your Water Heater (And How To Do It)Rheem Manufacturing Company · accessed 2026-08-04 · re-verification due 2027-02-01

Revision history

Current version 2 · updated 2026-08-04 · next review due 2027-08-04

  1. v2 · 2026-08-04Removed content taken from the Noritz knowledge-base page that version 1 had already decided not to register as a source, because its footer carried unrelated third-party spam links - a sign of possible compromise. The decision was recorded and then the page was quoted anyway, in a step, in the FAQ and in a conflict note: 'vinegar or Calcium, Lime, and Rust product' and 'based on the guidelines in the manual' appear nowhere on the clean noritz.com post this record cites. A document not trusted enough to register is not trusted enough to quote. Also corrected two Rinnai attributions: 'four gallons of undiluted virgin, food grade, white vinegar' is the wording of Technical Bulletin 188 alone - the general flushing page says '4 gallons (15.1 liters) of undiluted white vinegar' - and the general page states no rinse time at all, so the five-minute figure belongs to TB188 and Noritz only. No specification value changed: all five already routed the reader to their own manufacturer documentation, so no wrong number was ever published. What was wrong was the evidence shown for the disagreement.
  2. v1 · 2026-08-04First draft. Sourced to Rinnai (general flushing page and Technical Bulletin 188 for SENSEI models), Navien (blog page and NPE owner's manual), Noritz (blog page and limited warranty PDF), and Rheem (199,000 Btu/hr tankless owner's manual and 'How Often to Flush' article), all read directly on 2026-08-04. No specification publishes as verified: the agent, the quantity, the circulation time, the rinse time and the interval all route to the reader's own manufacturer documentation, and four of the five carry a documented disagreement between named manufacturers, or between two documents from the same manufacturer, rather than a house figure. A Noritz knowledge-base page (help.noritz.com) with the same technical content was not registered as a source because its footer carried unrelated third-party spam links at the time of reading, a sign of possible compromise; the clean noritz.com blog page was used instead. Third-party blogs describing a Rinnai 'chemically treated water' warranty exclusion as if it covered descaling chemicals were not used; the actual warranty PDF was read and that clause is about pool and spa heater applications.

Corrections to this page are listed in the public corrections log. Reports that are credible and not yet verified are listed in the public errata register, and an open one renders above the step it concerns.

Comments and corrections

Found an error? A correction with a manufacturer source is applied and logged publicly. You can also write to corrections@fixchecklist.com.

Every step above carries its own report control, which tags the report with that step so a correction reaches the right place. For anything about the procedure as a whole — the order of the sections, a step that is missing, something that did not match your equipment — use this: report something about this whole procedure.

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