Tankless water heater descale interval by hardness

Shop links on this page are affiliate links: a purchase may earn us a commission at no extra cost to you. Disclosure

Short answer

There is no industry-wide descaling interval for a tankless water heater, but three of the four makers checked for this page print the same ceiling: Navien, Rinnai and Rheem all cap total hardness at 200 mg/L (about 11.7 grains per gallon, gpg) and exclude damage from water outside their limits from warranty. Only Navien bands a schedule: 0–4 gpg needs no flush; 4–9 gpg gets a softener or Navien PeakFlow recommended and, if untreated, a flush once a year; 9–12 gpg makes treatment highly recommended, with residential flushing once a year and commercial twice; above 12 gpg treatment is required. Rheem’s IKONIC manual says flush annually or as a professional recommends. Rinnai’s SENSEI RX manual says hard water must be treated, and its Service Soon reminder can be set to 6 months, 1 year or 2 years. Noritz prints no hardness number. Divide mg/L by 17.1 for gpg.

The picks

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
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

If you came for one “flush every six months” rule, leave now: none of the manuals opened for this page prints such a rule, and the Navien table your warranty hangs on has five different answers.

Your maker's descale rule at your hardness

Enter your numbers.

Conversion: gpg = mg/L / 17.1 (WQA). The rule text is the maker's own row, not an average across makers.

What is the formula?

Step one is a unit conversion, step two is a table lookup. Grains per gallon (gpg) and milligrams per litre as calcium carbonate (mg/L) both measure the same total hardness, and the Water Quality Association’s glossary converts between them at 17.1 mg/L per grain (17.118 unrounded). A utility report in ppm is the same number as mg/L.

Step two has no arithmetic. You find the row of your own maker’s hardness table that contains your gpg figure and read across to the treatment and flush columns. A maker that prints no table has no row, and the calculator says so instead of inventing one.

This page does not average the makers because the only table found is a warranty document: Navien’s page 15 states that damage caused by poor water quality is not covered under warranty. A blended “industry” number would bind no maker and protect no claim.

What does a worked example look like?

The numbers the calculator opens on: a Navien NPE-A2/S2, a utility report of 150 mg/L, no softener, a house.

Convert: 150 / 17.1 = 8.8 gpg.
Find the row: 8.8 sits in Navien’s Hard band, 7–9 gpg (121–160 mg/L).
Read across: a water softener or Navien PeakFlow is recommended.
Read the flush column: once a year, because the footnote says flushing is recommended if a treatment device is not installed.

Move the same house to 210 mg/L and the answer changes kind, not degree. 210 / 17.1 = 12.3 gpg, Navien’s Extremely Hard band, where a softener or PeakFlow is required rather than recommended, and 200 mg/L (12 gpg) is the manual’s maximum allowable total hardness. The hardness unit converter handles other units, and the hard water scale guide covers what the white deposit is.

What does each maker’s manual actually say?

Each row is from the document named, opened for this page. A blank cell means the document prints nothing on that point, not that the answer is zero.

Maker	Document read	Hardness threshold	Softener or inhibitor	Flush interval
Navien	NPE-A2/S2 Installation and Operation Manual, page 15	Five bands: 0–4, 4–7, 7–9, 9–12, over 12 gpg; maximum allowable 200 mg/L (12 gpg)	Recommended 4–9 gpg, highly recommended 9–12 gpg, required over 12 gpg; check local codes on softeners	None at 0–4 gpg; once a year at 4–9 gpg if untreated; 9–12 gpg residential once a year, commercial twice
Noritz	Scale deposits post and descaling how-to, noritz.com	None printed	None printed	Professional checkup after year one; descale at least annually only if significant scale is found
Rinnai	SENSEI RX Residential Installation and Operation Manual (100000839), Table 3	Total hardness up to 200 mg/L	Hard water “must be treated”; Rinnai names its ScaleCutter conditioning system	No fixed interval; Service Soon reminder settable to 0.5, 1 or 2 years; LC code means flush now
Rheem	IKONIC condensing tankless Use and Care Manual, Water Quality	Total hardness up to 200 mg/L	Treatment device or softener recommended where water exceeds the table	Annually or as recommended by a professional, based on water conditions and usage

The 200 mg/L ceiling is the makers’ number, not EPA’s. Navien and Rinnai both name the EPA secondary standards, 40 CFR 143.3, as the basis of their tables, but that rule lists iron at 0.3 mg/L and total dissolved solids at 500 mg/L and no hardness level at all.

Rinnai and Rheem answer the interval question with a reminder rather than a table. Rheem’s baseline is once a year; Rinnai leaves the choice to its 6-month, 1-year or 2-year setting. The tankless sizing calculator covers the flow side.

How is the flush done?

By the maker’s procedure, which all four describe the same way in outline: close the isolation service valves, circulate an acid through the heat exchanger with a pump, then rinse.

Step	Navien NPE-A2/S2 (Appendix 8.2)	Noritz (descaling how-to)	Rinnai SENSEI RX (6.3)	Rheem IKONIC
Isolation	Navien Plumb Easy Valve Kit on the hot and cold lines	Red isolator valve on hot, blue on cold	Service valves; Rinnai strongly recommends isolation valves	Service valves; Rheem lists isolation valve kit RTG20220AB
Pump and hoses	Circulation pump in a flushing kit	Submersible pump, two 3/4 inch hoses	Pump and hoses, “dES” descaling mode on the PC board	Flush kit RTG20124; the heater has a Flush Mode of up to 2 hours
Fluid	White vinegar or a solution formulated for tankless water heaters	Three to four gallons of distilled, food-grade white vinegar	Four gallons of undiluted food-grade white vinegar	No more than 2 gallons of food-grade white vinegar
Duration	Manual refers to the User’s Information Manual, section 4.3	At least one hour, vinegar changed after 30 minutes	At least 1 hour at 4 gallons per minute	At least 45 minutes

Navien’s appendix is a diagram with a pointer to section 4.3 of its User’s Information Manual, so its duration cell stays empty. The gas connection stays untouched throughout; Navien’s manual says only a licensed professional connects the gas supply.

Read your row before you buy a pump

A flush kit is the wrong first purchase at 3 gpg on a Navien, where the Soft row lists no flush, and the wrong only purchase above 12 gpg, where the manual requires a softener or PeakFlow.

If your row says flush and the unit already has service valves, the 6699 flushing kit is the pump, two hoses and 3 gallon pail both procedures describe. If the installer fitted plain shut-offs, the HYDRO MASTER 3/4 inch service valve kit adds the flush ports and a relief valve, with one admitted flaw: its listing says clean brass and names no lead-free standard, so confirm that on the maker’s sheet before it goes on a potable line. Cutting into the supply to fit valves is plumbing work; where a permit applies, it is the licensed trade’s.

Frequently asked questions

How often should a tankless water heater be flushed for hard water?

As often as your maker's manual says. Navien's NPE-A2/S2 manual bands it: no flush at 0-4 gpg, once a year at 4-9 gpg if untreated, and once a year residential or twice commercial at 9-12 gpg. Rheem's IKONIC manual says annually or as a professional recommends. Rinnai's SENSEI RX reminder can be set to 6 months, 1 year or 2 years.

Does a tankless water heater need a water softener?

Navien's NPE-A2/S2 manual makes a water softener or Navien PeakFlow required above 12 grains per gallon (200 mg/L), highly recommended at 9-12 gpg and recommended at 4-9 gpg, and says to check local codes on softeners. Rinnai's SENSEI RX manual sets a 200 mg/L guideline and says water in a hard-water area must be treated, and Rheem recommends a softener or treatment device where water exceeds its 200 mg/L level.

How do I convert water hardness from mg/L to grains per gallon?

Divide milligrams per litre as calcium carbonate by 17.1, the Water Quality Association's rounded conversion (17.118 unrounded). A utility report of 150 mg/L is 8.8 gpg, which lands in Navien's Hard band. Going the other way, 12 gpg is 205 mg/L, which is why Navien's table prints the top of its Very Hard band as 200 mg/L and 12 grains per gallon together.

Does hard water scale void a tankless water heater warranty?

Navien's NPE-A2/S2 manual states that damage caused by poor water quality is not covered under warranty, and it sets the maximum allowable total hardness at 200 mg/L (12 grains per gallon) based on the EPA secondary drinking water regulations, 40 CFR Part 143.3. Rinnai and Rheem print the same 200 mg/L ceiling and the same warranty exclusion, though that EPA rule itself lists no hardness level.

What do I flush a tankless water heater with?

What the manual names. Navien's manual says white vinegar or a solution specifically formulated for tankless water heaters, circulated with a pump through its Plumb Easy isolation valve kit. Rinnai's SENSEI RX manual says four gallons of undiluted food-grade white vinegar for at least an hour, and Rheem's IKONIC manual no more than 2 gallons for at least 45 minutes.

Fact sheet and how to cite this page

Answer: There is no universal flush interval, but Navien, Rinnai and Rheem all cap total hardness at 200 mg/L. Navien alone bands a schedule: no flush at 0-4 gpg, once a year at 4-9 gpg if untreated, treatment required over 12 gpg. Rheem says flush annually or as a professional recommends, Rinnai says hard water must be treated, and Noritz prints no number (1 gpg is 17.1 mg/L).
Formula: Hardness (gpg) = hardness (mg/L as CaCO3) / 17.1; then look the gpg value up in the maker's own hardness table (Navien: 0-4, 4-7, 7-9, 9-12, over 12 gpg) and read the treatment and flush columns for that row
Inputs: Maker: Navien NPE-A2/S2
Hardness: 150 mg/L as CaCO3
Treatment device installed: no
Use: residential
Output: 150 / 17.1 = 8.8 gpg, Navien’s Hard band (7-9 gpg, 121-160 mg/L): a water softener or Navien PeakFlow is recommended, and with no treatment device the manual’s flush frequency is once a year.
Assumptions: The Navien table is the one printed on page 15 of the NPE-A2/S2 Installation and Operation Manual; other Navien series may print a different table, so read your own manual’s water quality page
1 gpg = 17.1 mg/L as calcium carbonate, the Water Quality Association’s rounded conversion; the unrounded figure is 17.118
Noritz publishes no gpg or mg/L threshold: its guidance is a professional checkup after the first year and descaling at least annually only if significant scale is found
Rinnai’s row is from the SENSEI RX Residential Installation and Operation Manual (document 100000839) and Rheem’s from the IKONIC condensing tankless Use and Care Manual; other series from either maker may print different text
40 CFR 143.3, the EPA secondary standards Navien and Rinnai name as the basis of their tables, lists no hardness level, so the 200 mg/L figure is the makers’ own
Method: The calculator converts the reader’s hardness to grains per gallon, picks the maker, and returns the treatment and flush text from that maker’s own published row. It does not blend makers into an average rule, and it says so when a maker publishes no rule.
Primary source: Navien, NPE-A2/S2 Installation and Operation Manual, page 15 About Water Quality and Appendix 8.2 Flushing the Heat Exchanger (PDF) (manufacturer or vendor document)
Facts verified: 27 September 2026
Last updated: 27 September 2026
Canonical URL: https://ownerspec.com/plumbing/calculators/tankless-water-heater-descale-interval-by-hardness/
Cite as: OwnerSpec, "Tankless water heater descale interval by water hardness: what Navien, Noritz, Rinnai and Rheem actually publish, with a calculator", https://ownerspec.com/plumbing/calculators/tankless-water-heater-descale-interval-by-hardness/, facts verified 27 September 2026.

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

Sources

Navien, NPE-A2/S2 Installation and Operation Manual, page 15 About Water Quality and Appendix 8.2 Flushing the Heat Exchanger (PDF) (manufacturer or vendor document)
Navien, NPE-A2 series downloads page (manufacturer or vendor document)
Noritz, Understanding Scale Deposits in Your Tankless Water Heater (manufacturer or vendor document)
Noritz, How to Descale a Noritz Tankless Water Heater (manufacturer or vendor document)
Rinnai, SENSEI RX199iN product page, Documentation: 100000839 SENSEI RX Residential Installation and Operation Manual (Table 3 water quality, sections 4.13 and 6.3) (manufacturer or vendor document)
Rheem, IKONIC condensing tankless water heater Use and Care Manual (199,900, 180,000 and 157,000 Btu/hr models), Water Quality and Flushing of the Heat Exchanger (PDF) (manufacturer or vendor document)
40 CFR 143.3, Secondary maximum contaminant levels (Legal Information Institute) (university or extension service)
Water Quality Association, Glossary of Terms: Calcium Carbonate Equivalent (standards or certification body)