Maintenance
Water Hardness and Descaling Interval Chart
Researched from the DOE test procedures, NFPA 54 and NEC code tables, and manufacturer specifications. Updated .
Quick answer
Water hardness is usually reported in one of two units: grains per gallon, or gpg, common on water softener literature and test kits, and parts per million, or ppm, common on municipal water utility consumer reports. The conversion between them is exact: 1 grain per gallon equals 17.1 milligrams per liter, which is the same as 17.1 parts per million. Knowing which band a water supply falls into is the first step to estimating how quickly mineral scale will build up inside a tankless heat exchanger.
Whether to treat that water, through softening or filtration, is a decision that belongs to homewatercalc.com. What belongs here is the water heater side of the question: how hardness translates into a practical descaling schedule so a tankless unit's heat exchanger keeps performing at its rated efficiency. The descaling interval calculator runs this same lookup for a specific hardness figure and setpoint.
What is the exact conversion between grains per gallon and parts per million?
One grain per gallon of water hardness equals exactly 17.1 parts per million, or 17.1 milligrams per liter. This is a fixed unit conversion, not an estimate, so a reading in either unit converts cleanly to the other by multiplying or dividing by 17.1. A municipal water utility's annual consumer confidence report almost always states hardness in ppm or mg/L, while a home test kit or a softener's control valve almost always reads in gpg, so converting between the two is a routine first step before using either the classification bands or the descaling table below.
What are the standard water hardness classification bands?
| Classification | Grains per gallon | Parts per million |
|---|---|---|
| Soft | Under 3 gpg | Under 51 ppm |
| Moderately hard | 3 to 7 gpg | 51 to 120 ppm |
| Hard | 7 to 10 gpg | 120 to 171 ppm |
| Very hard | 10 to 15 gpg | 171 to 257 ppm |
| Extremely hard | 15 to 20 gpg | 257 to 342 ppm |
| Severe | Over 20 gpg | Over 342 ppm |
Published standard Source: Ppm figures calculated at the exact 17.1 ppm per gpg conversion factor. Band names follow a commonly used water hardness classification; the conversion itself is exact, the band edges are a widely used grouping of it.. Softened water, typically reduced to well under 1 gpg by a properly maintained softener, falls into the soft row and inherits its descaling interval below.
How often should a tankless water heater be descaled at each hardness band?
No test procedure or code publishes a descaling interval. That absence is exactly why a table like this exists: it converts a hardness reading into a practical planning schedule rather than leaving an owner with no starting point at all. Every figure below is a convention, and a manufacturer's own service manual can specify a different interval for a specific model.
| Hardness band | Interval at 120 F setpoint | Interval at 140 F setpoint |
|---|---|---|
| Soft | 24 months | 18 months |
| Moderately hard | 18 months | 14 months |
| Hard | 12 months | 9 months |
| Very hard | 9 months | 7 months |
| Extremely hard | 6 months | 5 months |
| Severe | 4 months | 3 months |
Convention Source: Planning intervals only; no test procedure publishes a descaling interval. The 140 F column is the 120 F interval multiplied by 0.75, reflecting faster mineral precipitation at a higher storage temperature, with a 3 month practical floor.. A household with unusually heavy tankless usage, well above typical daily draws, will accumulate scale faster than these intervals assume even at the same hardness band.
My water utility reports ppm, not grains. How do I find my band?
| Utility report reading | Converted hardness | Band |
|---|---|---|
| 100 ppm | 5.85 gpg | Moderately hard |
| 150 ppm | 8.77 gpg | Hard |
| 200 ppm | 11.70 gpg | Very hard |
| 250 ppm | 14.62 gpg | Very hard |
| 300 ppm | 17.54 gpg | Extremely hard |
| 350 ppm | 20.47 gpg | Severe |
Published standard Source: Calculated as ppm divided by 17.1 to convert to grains per gallon, then matched against the classification bands above.. A reading between these rows converts the same way; divide the exact ppm figure from a utility report by 17.1 rather than rounding to the nearest row shown here.
Why does a higher setpoint shorten the descaling interval?
The minerals that form scale, mainly calcium and magnesium carbonate, become less soluble in water as temperature rises, so they precipitate out of solution faster on a hot heat exchanger surface than on a cooler one carrying the same hardness. A tankless unit held at 140 F is asking the same hard water to give up its dissolved minerals faster than the identical water would at 120 F, which is the mechanism behind the shorter interval in the 140 F column above rather than an arbitrary adjustment.
Does softening my water eliminate the need to descale?
Softened water, typically well under 1 gpg after a properly functioning softener, falls into the soft band above and inherits that band's longer interval rather than needing no descaling at all. A tankless unit still benefits from periodic flushing on that schedule, since even soft water carries some residual mineral content and a flush also clears sediment unrelated to hardness. Whether softening, filtration or another treatment approach is the right fix for a specific hardness reading is a decision for homewatercalc.com; this page only covers the water heater side of the schedule.
Gear for descaling on the schedule below
A flush kit and a descaler solution cover most tankless descaling jobs; a scale-protection filter is a preventive option for consistently hard water.
Chromex Flush Kit with NSF Descaler
$134.99A complete recirculation flush kit sized for common residential tankless heat exchangers.
Best for: A first descale with nothing already on hand
Check price on Amazon
Distilled White Vinegar, 5 Percent, 1 Gallon
$12.99A low-cost descaling solution suitable for routine flushes at the intervals below.
Best for: The honest budget answer, recommended by the manufacturers
Check price on Amazon
Aqua-Pure AP431 Scale Inhibition Cartridge
$53.82An inline filter that reduces scale-forming mineral content reaching the heat exchanger between flushes.
Best for: An existing 3M housing
Check price on AmazonFrequently asked questions
- What ppm is considered hard water?
- Water in the 120 to 171 ppm range, equivalent to 7 to 10 grains per gallon, is classified as hard. Below 51 ppm is soft, 51 to 120 ppm is moderately hard, 171 to 257 ppm is very hard, 257 to 342 ppm is extremely hard, and above 342 ppm is classified as severe. These bands come from the exact 17.1 ppm per grain per gallon conversion applied to commonly used hardness classification bands.
- How do I convert grains per gallon to parts per million?
- Multiply the grains per gallon figure by 17.1 to get parts per million, which is the same as milligrams per liter. This is an exact conversion factor, not an estimate, so a hardness test reading in gpg and a utility report reading in ppm can always be reconciled directly by multiplying or dividing by 17.1.
- How often should I descale a tankless water heater in hard water?
- In the hard band, 7 to 10 grains per gallon or 120 to 171 ppm, a common planning interval is about every 12 months at a 120 F setpoint, or about every 9 months at a 140 F setpoint. No test procedure publishes an official interval, so this is a convention, and a manufacturer's own service manual can specify something different for a specific model.
- Does raising my water heater's setpoint mean I need to descale more often?
- Yes. Higher storage temperature drives dissolved calcium and magnesium out of solution faster, so the same hardness band scales a heat exchanger more quickly at 140 F than at 120 F. This page's suggested intervals shorten the 120 F figure by roughly 25 percent at 140 F across every hardness band, with a 3 month practical floor at the most severe band.
- Is there an official test that determines descaling interval?
- No. Unlike UEF or first hour rating, there is no DOE, ASHRAE or NSF test procedure that measures or certifies a descaling interval for a water heater. Every interval published anywhere, including on this page, is a planning convention built from hardness and setpoint, not a measured result, which is exactly why a manufacturer's own service manual is worth checking for a specific model.
- Where do I find my water's hardness, in gpg or ppm?
- A municipal water utility's annual consumer confidence report typically states hardness in ppm or mg/L, while a home test strip, titration kit or a water softener's control valve typically reads in gpg. For questions about testing methods, softening or filtration to change that hardness reading, homewatercalc.com covers water treatment; this page only covers converting the reading into a descaling schedule.
No test procedure publishes a descaling interval, which is exactly why this chart exists. Treat every figure above as a starting schedule to check against a specific manufacturer's service manual, not as a certified number, and remember that treating the water itself is a decision for homewatercalc.com rather than this site.