THE SHORT VERSION
- Know your number first. USGS calls water over 120 mg/L (about 7 grains per gallon) hard and over 180 mg/L (about 10.5 gpg) very hard. City water reports and well tests give the figure.
- Scale costs efficiency. In a lab study for the Water Quality Association, gas tanks on very hard water lost about 3 points of efficiency in two years, and tankless units fell from 80% to 72% before they needed deliming.
- Tankless heaters are the most sensitive. Rinnai, Rheem and Noritz put their limit at 200 mg/L (about 12 gpg), and all four big tankless makers exclude scale damage from the warranty.
- A softener stops scale but wears out a tank’s anode faster. A.O. Smith and Bradford White say to check the anode more often if you soften.
- Salt-free conditioners reduce scale without removing hardness. One independent university test found template-assisted crystallization (TAC) cut scale by more than 88%.
Hardness is calcium and magnesium, measured two ways
Water hardness is the amount of dissolved calcium and magnesium in your water, says the U.S. Geological Survey (USGS). It isn’t a health problem. It becomes a water heater problem because heating hard water makes those minerals come out as solid calcium carbonate, the white crust you see inside a kettle. USGS says this scale can shorten equipment life, raise water heating costs and clog pipes.
Labs and utilities report hardness in milligrams per liter (mg/L) as calcium carbonate, which is the same as parts per million. Softener sellers and some water heater manuals use grains per gallon (gpg). One grain per gallon is 17.1 mg/L, so divide mg/L by 17.1 to get gpg.
| Class | mg/L as calcium carbonate | Grains per gallon (approx.) |
|---|---|---|
| Soft | 0–60 | 0–3.5 |
| Moderately hard | 61–120 | 3.5–7 |
| Hard | 121–180 | 7–10.5 |
| Very hard | More than 180 | More than 10.5 |
Source: USGS Water Science School, “Hardness of Water.” Grain figures are Ember & Tank’s conversion (mg/L ÷ 17.1). Noritz’s manuals use a finer gpg scale with different break points, so a “hard” label in one document may not match another; compare numbers, not labels.
How to find your number
- City water: EPA requires community water systems to send customers an annual water quality report, the Consumer Confidence Report, by July 1. Hardness isn’t a regulated contaminant, so it isn’t always listed; if it’s missing, call the utility and ask for the average and the range.
- Private well: nobody tests it for you. EPA says private well owners are responsible for their own water quality. A certified lab test gives hardness along with iron, manganese and other items that matter to a water heater.
- Ask for mg/L, not a label. “Moderately hard” means different things in different documents. A number lets you compare it with your heater’s manual.
What scale does to tank and tankless heaters
Calcium carbonate is less soluble in hot water than in cold. A.O. Smith puts it plainly: “the more you heat, the more lime comes out.” So scale forms where water is hottest: the tank bottom over a gas burner, the surface of an electric element, and the narrow tubes of a tankless heat exchanger. A.O. Smith adds that higher hot water use, harder water and more heating surface all make a “limed-up” heater more likely. Popping and rumbling are a common symptom (see water heater noises).
The most detailed measurement we found is a 2010 lab study by Battelle, an independent research institute, done for the Water Quality Association (WQA), the trade group for the water treatment industry. Battelle ran 30 heaters, half on very hard water and half on softened water, in an accelerated test that compressed years of scaling into 90 days. Keep the funder in mind: WQA’s members sell softeners. The test water was far harder than most homes see.
| Heater tested | Scale per year, unsoftened | Scale per year, softened | Efficiency on unsoftened water |
|---|---|---|---|
| Gas tank, 40 gal, 38,000 BTU/h | 1.16 lb | 0.01 lb | Fell from 70.4% to 67.4% over 2 years’ equivalent use |
| Electric tank, 40 gal, 4,500 W | 2.00 lb | 0.03 lb | Held steady; researchers expect shorter element life |
| Gas tankless, 199,000 BTU/h | Not reported per year | Not reported | Fell from 80% to 72%; delimed at 1.6 years’ equivalent use, then recovered to about 77% |
Source: Contractor magazine’s report of the Battelle study for the Water Quality Association (2010). All heaters on unsoftened water eventually left the test because scale clogged the outlet piping. Softened units held their efficiency.
Two things stand out. First, tankless units suffer most: their heat exchanger passages are small, so scale cuts both heat transfer and flow. Second, sources disagree on electric tanks. USGS says scale lowers the efficiency of electric water heaters; Battelle found no change in electricity use, because the elements stay underwater and their heat has nowhere else to go. Both agree the element itself pays the price.
What that means in dollars, roughly
Say a gas tank’s efficiency slips from 70.4% to 67.4%, as in Battelle’s test. Fuel use rises by 70.4 ÷ 67.4 − 1, about 4.5%. On Ember & Tank’s estimate of $384 a year for a standard 50-gallon gas tank, that is about $17 a year. The bigger costs are the ones scale causes later: burned-out elements, noisy tanks, clogged tankless heat exchangers and shorter service life. For running costs by heater type, see water heater operating costs.
Softener, scale inhibitor or salt-free conditioner
There are three common ways to treat hard water before it reaches the heater, plus the option of treating nothing and descaling or flushing more often. A.O. Smith’s gas tank manual recommends “water softening or the threshold type of water treatment” where hard water exists; threshold treatment generally means feeding a small dose of a scale-inhibiting compound into the water.
Ion-exchange softener
Good
- Removes calcium and magnesium; the Battelle units on softened water stayed nearly scale-free
- Protects fixtures, dishwashers and shower heads too
- Named as a treatment option in Noritz’s and A.O. Smith’s manuals
Watch for
- Uses salt and discharges brine; some local water agencies regulate softeners
- Speeds up anode wear in tank heaters
- Needs regular salt and maintenance
Best for: Hard to very hard water, whole-house protection, tankless owners above about 200 mg/L.
Scale-inhibitor feeder
Good
- Small cartridge on the heater’s cold supply
- Sold by heater makers: Rinnai offers a ScaleCutter system that feeds scale-control compounds into the cold water
Watch for
- Cartridges must be replaced
- Treats only the heater it feeds, not the whole house
Best for: A single tankless heater on moderately hard water.
Template-assisted crystallization (TAC)
Good
- No salt; turns dissolved hardness into tiny crystals that stay suspended instead of sticking
- Cut scale by more than 88% in an independent university test
Watch for
- Doesn’t remove hardness, so spots on glass and fixtures can remain
- Media needs replacing; Navien’s PeakFlow S lists every 2 years
- Iron, manganese, hydrogen sulfide and polyphosphate must be removed first, per Navien’s limits
Best for: Homes that want scale control without salt or brine, where the water meets the device’s limits.
The independent test is a 2014 study by Arizona State University for the WateReuse Research Foundation, funded partly by Western water agencies that want fewer salt-discharging softeners. It ran heated test rigs on three Arizona waters. TAC cut scale formation by more than 88%; electromagnetic and “electrically induced precipitation” devices cut it by about 50%; ion exchange and capacitive deionization removed scale-forming minerals as expected. The researchers also noted there is no easy way for an owner to tell whether a salt-free device is working, other than watching for scale.
| Technology | Average equipment cost | Average yearly upkeep | 10-year cost |
|---|---|---|---|
| Ion-exchange softener | $2,048 | $168 | $3,478 |
| Template-assisted crystallization | $1,098 | $27 | $1,326 |
| Magnetic treatment | $760 | $11 | $855 |
Source: WateReuse Research Foundation, “Evaluation of Alternatives to Domestic Ion Exchange Water Softeners” (2014), Table 4.5, averaging three to five vendors’ quotes per technology. 2014 prices, equipment only; installation and today’s prices will differ. Magnetic devices cut scale by only about 50% in the same study.
Softened water wears out a tank’s anode faster
Tank water heaters rely on an anode rod that corrodes so the steel tank doesn’t. A.O. Smith’s Bulletin 44 explains why a softener speeds that up. Softening swaps calcium and magnesium for sodium, which lowers hardness but not conductivity, and conductive water makes the anode work harder. Leftover salt from a poorly rinsed softener raises conductivity further.
There’s a second effect. A thin film of scale actually protects bare steel at welds and pipe connections. A.O. Smith reports a 33% drop in steel corrosion rate once a film under 1/16 inch formed in 17-grain water. Softened water never builds that film, so the anode carries the whole load.
- A.O. Smith: customers using softeners “should expect more rapid anode consumption,” and should inspect anodes more often.
- Bradford White: inspect every 2 years normally, more often with softened or phosphate-treated water.
- Rheem: says softened water makes anode current demand “drastically higher,” and lists rotten-egg odor as more common in softened water. See rotten-egg smell and discolored water.
What warranties say about hard water
Tankless warranties are explicit about water quality. Tank warranties usually say less, but scale and sediment damage is still excluded. Read the certificate for your exact model; these are the documents we checked.
| Maker and document | Hardness limit | What’s excluded |
|---|---|---|
| Rheem gas tankless warranty (RTGH) | Up to 200 mg/L | Failures “caused by lime, mineral build-up, or scale”; coverage limited where water is outside the listed ranges |
| Rinnai condensing tankless manual | Up to 200 mg/L | “Damage caused by lime/scale build-up is not covered”; hard water “must be treated” |
| Noritz condensing tankless manual | 12 gpg (200 mg/L) | Damage from water above 12 gpg “is not covered” |
| Navien NPE tankless manual (2016) | Not stated in the owner’s manual | “Hard water deposits, lime or mineral build-up” |
| A.O. Smith tank warranty (Lowe’s, 2018) | Not stated | Failure of the heater or parts “due to sediment build-up” |
Sources: the manufacturer documents listed under Sources. Rheem and Rinnai cite EPA’s secondary drinking water guidelines for their other water limits (iron, chlorides, pH and so on). Warranty terms change; check the certificate that came with your heater.
In practice this means a tankless heater on untreated water above about 200 mg/L is running without heat exchanger coverage for scale, which is the most likely way it will fail. Below that limit, scale damage is still excluded, so treatment or regular descaling is the owner’s job. More on coverage in water heater warranties.
What to do at your hardness level
| Your water | Tank heater | Tankless heater |
|---|---|---|
| Soft or moderately hard (up to 120 mg/L, about 7 gpg) | Flush yearly per your manual; check the anode on the manual’s schedule | Descale at least yearly (Rinnai, Navien, Rheem); clean the inlet filter |
| Hard (121–180 mg/L, about 7–10.5 gpg) | Flush every 6 months (Rheem’s advice for hard water); consider a softener | Descale every 6–12 months, or add an inhibitor or softener; Noritz calls for treatment or yearly flushing |
| Very hard, up to 200 mg/L (about 12 gpg) | Softener or threshold treatment (A.O. Smith); check anode more often if softened | Treat the water; descale on schedule |
| Above 200 mg/L | Treatment strongly advised | Treatment is effectively required: above the Rheem, Rinnai and Noritz limits |
Sources: USGS classes; flushing and descaling intervals from Rheem, A.O. Smith, Rinnai, Navien and Noritz documents listed below. Ember & Tank’s summary; your manual has the final word.
If you’re buying a new heater in a hard-water area
- Get your hardness number (mg/L) before you get quotes
- For tankless, ask the installer to fit isolation valves with service ports so descaling is quick (see the tank vs tankless guide)
- Ask whether the quote includes water treatment, and which kind
- If you already have a softener, ask which anode the new tank has and when to check it first
- Don’t set the temperature higher than you need: Rinnai says high settings speed scale build-up (see water heater temperature)
Common questions
Is hard water bad for a water heater?
It shortens its life and cuts efficiency. Scale forms on the hottest surfaces, and in a lab study for the Water Quality Association, gas tanks on very hard water lost about 3 points of efficiency in two years while tankless units dropped from 80% to 72%. Softened units held steady.
What water hardness is too hard for a tankless water heater?
Rheem, Rinnai and Noritz set their limit at 200 mg/L, about 12 grains per gallon. Above that, scale damage is clearly outside the warranty, and all of them exclude scale damage even below it, so treatment or regular descaling is expected.
Do I need a water softener for a tankless water heater?
Not always. In soft or moderately hard water, yearly descaling is usually enough. In hard water, Rinnai says the water must be treated, and a softener or a scale-inhibitor feeder are the usual choices. Above about 12 grains per gallon, treatment is effectively required to keep warranty coverage.
Does a water softener damage a water heater?
Not the heater itself, but it uses up the tank’s anode rod faster. A.O. Smith and Bradford White both say to inspect the anode more often with softened water. Tankless heaters have no anode, so this doesn’t apply to them.
Do salt-free water conditioners work?
Some do. In a 2014 Arizona State University study for the WateReuse Research Foundation, template-assisted crystallization (TAC) cut scale by more than 88%, while magnetic and electronic devices cut it by about half. They don’t remove hardness, so water spots remain.
How do I convert grains per gallon to mg/L?
Multiply grains per gallon by 17.1. So 10 gpg is about 171 mg/L, and 12 gpg is about 205 mg/L (manufacturers round it to 200).
Does hard water make the water heater noisy?
Often. A.O. Smith says limed-up tanks pop as water trapped under deposits boils, and electric elements sizzle. Flushing helps with loose sediment; hardened scale needs deliming by a plumber.
Sources
Sources checked October 5, 2026. Prices, codes and rebates change. Check current details with your installer and local permit office.
- Hardness of water ↗ · U.S. Geological Survey, Water Science School
- Consumer Confidence Report information for consumers ↗ · U.S. EPA
- Private drinking water wells ↗ · U.S. EPA
- Battelle finds water softeners aid energy efficiency, appliance life (2010; study funded by the Water Quality Association) ↗ · Contractor magazine
- Evaluation of alternatives to domestic ion exchange water softeners (WRRF-08-06, 2014) ↗ · WateReuse Research Foundation and Arizona State University, hosted by California State Water Resources Control Board
- Technical Bulletin 13: Mineral build-up ↗ · A.O. Smith
- Technical Bulletin 44: Water softeners ↗ · A.O. Smith
- Signature 100 gas water heater installation instructions and use & care guide (100341468, 2021) ↗ · A.O. Smith
- Residential limited warranty (100286376 Rev D, 2018) ↗ · A.O. Smith
- Installation and operation manual, gas-fired water heater (238-51007-00R) ↗ · Bradford White
- Technical Service Bulletin 1201: Anode rods, cathodic protection and the porcelain (glass) lining ↗ · Rheem
- Technical Service Bulletin 1206: Smelly water and the rotten egg odor ↗ · Rheem
- Residential tankless gas water heater limited warranty (RTGH) ↗ · Rheem
- How to keep your tank and tankless water heaters running at peak performance (Dec. 22, 2025) ↗ · Rheem
- CX/CXP condensing tankless installation and operation manual ↗ · Rinnai, hosted by Lowe’s
- NRC111 condensing tankless installation manual (water treatment tables) ↗ · Noritz, hosted by SupplyHouse
- NPE condensing water heaters operation manual (version 5.1, 2016) ↗ · Navien
- PeakFlow S scale prevention system specifications ↗ · Navien