Much of Utah’s water picks up calcium and magnesium from the rock and soil it passes through. Provo’s springs, for example, draw from limestone. Those two minerals are what “hardness” measures, and they are what turn into scale when water is heated. This guide pulls together the published figures for the cities we cover, explains where the water comes from, and shows how it compares with what tankless makers allow.
How hardness is measured
Water reports give hardness in one of two units:
- mg/L (milligrams per liter), sometimes written as ppm (parts per million). The two are the same for water. Reports state it “as calcium carbonate”.
- gpg (grains per gallon), the unit softener makers and many cities use.
The ratio between the two is fixed. Where a source gives only mg/L, we converted it to gpg, and those figures say “about”.
The USGS hardness scale
The U.S. Geological Survey (USGS) sorts water into four classes:
| USGS class | Hardness in mg/L | About in gpg |
|---|---|---|
| Soft | 0–60 mg/L | Under the moderately hard range |
| Moderately hard | 61–120 mg/L | about 3.6–7 gpg |
| Hard | 121–180 mg/L | about 7.1–10.5 gpg |
| Very hard | more than 180 mg/L | more than 10.5 gpg |
The USGS notes that hard water can cause mineral buildup in plumbing, fixtures, and water heaters. Its national well study found hard water most common in the east-central and western United States.
Herriman City goes further and says that, according to the USGS, Utah is at the top of the list for hard water in the country. The USGS page we checked doesn’t name Utah, so read that as Herriman’s summary. The city figures below are the stronger evidence.
Where Wasatch Front water comes from
Most cities here mix two kinds of water: groundwater from their own wells and springs, and surface water bought from a regional water district. The two can differ a lot in hardness.
Central Utah Water Conservancy District (Utah County)
The Central Utah Water Conservancy District (CUWCD) treats surface water at its Don A. Christiansen Regional Water Treatment Plant. Its 2021 report lists Orem, Provo, Vineyard, Eagle Mountain and Lehi as customers of that plant. The same report puts the plant’s finished water at an average of 8.2 gpg. Orem’s newer report lists 112–154 mg/L for that treated water, which is about 6.5–9 gpg.
CUWCD also runs the Central Water Project wells, about 1,500 feet deep, which serve cities such as Saratoga Springs, Eagle Mountain, Lehi and Vineyard. In 2021 that well water averaged 4.3 gpg, much softer than most city wells.
These are district figures. They describe the district’s share of a city’s water, not what comes out of every tap.
Jordan Valley Water Conservancy District (south Salt Lake Valley)
The Jordan Valley Water Conservancy District (JVWCD) supplies South Jordan, Riverton, Bluffdale and most of West Jordan.
It also supplies part of Herriman’s and Draper’s water. About twenty percent of its supply is groundwater; the rest comes mostly from Provo River reservoirs such as Jordanelle and Deer Creek.
JVWCD’s 2025 report gives an average total hardness of 182.4 mg/L, which is about 10.7 gpg and in the USGS very hard class. Samples ranged 16.0–381 mg/L. The district averaged 46 mg/L of calcium and 16.5 mg/L of magnesium. JVWCD’s website gives a simpler range of 1–12 gpg. The wide spread reflects the blend of reservoir water and wells at different times and places.
Local wells can be much harder
Some city wells are far harder than district water. Herriman says its local wells usually measure 25–60 gpg. Riverton stopped using its wells for drinking water in 2015 after residents said the water was too hard and tasted bad.
City-by-city figures
| City | Hardness | USGS class | Source and year | Note |
|---|---|---|---|---|
| American Fork | 12.38 gpg | Very hard | City FAQ | A recommended softener setting, not a lab result |
| Saratoga Springs | 142 mg/L (8.3 gpg) | Hard | City culinary water page, undated | Average; city wells plus CUWCD water |
| Highland | 14 gpg | Very hard | City hardness page, undated | All from the city’s own deep wells |
| Orem | 120–428 mg/L (7–25 gpg) | Moderately hard to very hard | Orem report, 2025 data | Surface water blended with springs and wells |
| Provo | 10–15 gpg | Hard to very hard | Provo 2025 water report | Provo calls this moderately hard; by the USGS scale it is harder |
| Spanish Fork | 231–349 mg/L (13.4–20.24 gpg) | Very hard | Archived city page, no sample year | Springs and wells |
| South Jordan | 7–10 gpg | Moderately hard to hard | City water page | All JVWCD water |
| West Jordan | 182.7 mg/L (about 10.7 gpg) | Very hard | West Jordan 2026 report | Average; mostly JVWCD water |
| Riverton | 182.4 mg/L (about 10.7 gpg) | Very hard | JVWCD 2025 report | JVWCD’s average; Riverton buys all its water from JVWCD |
| Herriman | 12–15 gpg for JVWCD water; 25–60 gpg for local wells | Very hard | Herriman City, 2025 | JVWCD itself says 1–12 gpg |
| Draper | 184.1 mg/L (about 10.8 gpg) | Very hard | WaterPro 2026 report | WaterPro serves only part of Draper; low confidence |
| Lehi | Not published | Unknown | Lehi 2025 report | Wells and springs plus CUWCD water |
The Utah area hub has a shorter version of this table, and each city page explains its own supply in more detail.
Hardness is not the same as TDS
Many reports list total dissolved solids (TDS) but not hardness. TDS counts every dissolved mineral and salt, including the calcium and magnesium that cause scale, plus sodium, sulfate and others. Lehi’s 2025 report, for example, lists total dissolved solids of 120–388 mg/L. That tells you the water carries minerals, but not how much of it is hardness. We don’t use TDS as a stand-in for hardness anywhere on this site.
Why scale forms in a heat exchanger
Calcium and magnesium stay dissolved in cold water. When water is heated, some of the calcium comes out of the water as calcium carbonate, a hard white solid. In a tank heater it settles to the bottom as sediment. In a tankless heater, the water is heated in seconds as it races through narrow tubes, so the scale coats the tube walls instead.
That coating causes three problems:
- It insulates. Scale sits between the flame and the water, so more heat goes up the vent instead of into your shower.
- It narrows the tubes. Flow drops, and the heater may struggle to reach temperature.
- It creates hot spots. The metal behind the scale runs hotter, which is why overheating codes often show up on scaled units.
Hotter settings make it worse. Rinnai’s manual says scale build-up is accelerated by high temperature settings.
What the Battelle study measured
The clearest data on scale and tankless heaters comes from a lab study Battelle ran for the Water Quality Association, reported by Contractor Magazine in 2010. It ran water heaters on hard and softened water and sped up the scaling to simulate years of use. The unsoftened test water was 26 gpg, which is harder than most Utah County city water but inside Herriman’s well range.
- Gas tankless heaters on hard water fell from 80% to 72% efficiency before they had to be delimed.
- That deliming was needed after 1.6 years of equivalent field service.
- After deliming they recovered to only about 77%, so some loss was permanent.
- On softened water, efficiency kept remaining constant over the life of the unit.
It is one lab study with accelerated scaling, so the exact timeline won’t match your home. The direction is clear, though: hard water costs efficiency, and descaling recovers most but not all of it.
Brand limits against Utah water
Each maker publishes a maximum hardness. Here is how the city figures above compare, brand by brand.
| Brand | Published limit | Utah cities in the table above that exceed it |
|---|---|---|
| Takagi | 7 gpg for single-family homes; 4 gpg for other uses | Every city with a figure, at least at the top of its range |
| A.O. Smith (non-X3) | 7 gpg for single-family homes | Every city with a figure, at least at the top of its range |
| Bosch | 100 ppm | Every city with a figure |
| Navien | 12 gpg | American Fork, Highland, Spanish Fork, Herriman’s JVWCD figure, and the upper part of Orem’s and Provo’s ranges |
| Noritz | 12 gpg | Same as Navien |
| Rinnai | 200 ppm | Same as Navien |
| Rheem | 200 ppm | Same as Navien |
Even where a city sits under the Navien line on average, its range can cross it. JVWCD’s highest 2025 sample was well above it. And a heater rated for harder water still scales; it just takes longer.
What to do with this
Find your city in the table, then check your heater’s brand. If your water is above the limit, the makers expect treatment: see scale prevention for softeners and scale-prevention cartridges. Either way, plan a regular flush and descale. Our guide on how often to flush turns these figures into a schedule.