Highland’s water has a hardness of 14 gpg, according to the city’s Water Hardness page. That is over the published limit of every major tankless brand. If your heater runs on untreated Highland water, it needs a professional flush at least once a year, and a softener or scale-prevention system is worth serious thought.
Highland’s wells: clean, untreated and hard
Highland City supplies water from Highland City's own deep wells. Its About Our Water page says there are 6 deep wells inside city limits and that the water needs no additional treatment before it reaches homes.
That is good news for drinking water. It also means nothing between the aquifer and your water heater removes calcium or magnesium, the two minerals that form scale. Scale is a hard, chalky crust. Inside a tankless heater it coats the heat exchanger, the narrow metal passage where the burner heats water, and makes the burner work harder to reach the same temperature.
The city’s 2024 Water Quality Report lists total dissolved solids (TDS) of 240–348 mg/L, sampled in 2022. TDS is a broader count that includes scale-forming minerals and others that aren’t, so it isn’t a second hardness reading. The hardness page is the number to use, and because it comes from the city itself rather than from a company selling softeners, it’s a fair basis for decisions about your heater.
Where Highland sits against each manual
Manufacturers set hardness limits in their installation manuals and warranties. Highland’s figure clears none of them:
- Navien’s NPE-2 manual allows up to 12 gpg, and says water above 12 gpg requires a softener or PeakFlow.
- Noritz’s warranty says hardness above 12 grains per gallon voids coverage for resulting damage.
- Rinnai lists 200 ppm and Rheem 200 ppm. Highland’s figure works out above both.
- Takagi stops at 7 gpg, A.O. Smith at 7 gpg, and Bosch at 100 ppm.
The warranty side
Over the limit, warranty language turns against you. Rheem says damage caused by lime, mineral build-up, or scale is excluded. Navien says hard water deposits and lime or mineral build-up are excluded. Rinnai says damage caused by lime build-up is not covered. None of these mention Highland by name, of course, but they apply to any home whose water is over the line. A softener, or a manufacturer’s scale-prevention system, plus a record of regular flushes, is how Highland owners keep their warranty on solid ground.
Choosing treatment for water this hard
There are three main routes. A whole-house softener treats everything, and HomeGuide puts one at $1,200–$3,800 installed. HomeGuide also notes that salt-free conditioners, at $1,500–$4,500, are less effective in homes with extremely hard water, which is worth weighing at Highland’s level.
A point-of-use scale-prevention unit protects only the heater. Rinnai says most homes replace the cartridge in its Scale Control System every 10 to 12 months. Navien adds one additional year of labor warranty when a new Navien heater goes in with a PeakFlow A or PeakFlow C system.
Keeping the heater’s temperature setting moderate helps too. Rinnai notes that scale build-up is accelerated by high temperature settings.
Highland’s water system by the numbers
Highland is a self-contained system. The 2024 report says the city supplied 414 million gallons of water in 2024 through its culinary connections, and the supply is the city’s own wells, with no district water blended in.
That has two consequences for a tankless owner. No softer source arrives in some seasons to give the heater a break, and there’s no seasonal switch to a different water either. A flush schedule set once tends to hold, and the heater’s own reminder can carry it. Rheem’s RTGH maintenance notice can be set for 500, 750 or 1,000 hours of operation, and Navien’s descaling reminder can be set to no alarm, 6, 12 or 24 months.
Service options in Highland
- Flush and descale: the yearly job for untreated Highland water.
- Annual maintenance: flush, inlet filter and a full check in one visit, which also builds the service record a warranty claim may need.
- Repair: for overheat codes, fluctuating temperature or a unit that has quit.
- Installation: when replacing, ask about models with built-in scale protection.
- Scale prevention: softeners, PeakFlow, Rinnai Scale Control and similar systems, sized for water this hard.
HomeGuide’s 2026 cost guide gives $150 to $350 for a professional flush.
Alpine
Alpine, Highland’s neighbor to the northeast, also runs its own groundwater system. The city’s water comes from Alpine City's own spring and wells, named in its 2021 Annual Drinking Water Quality Report as Grove Spring, 300 East Well and Silver Leaf Well.
That 2021 report is the newest one we could find online, and it doesn’t publish a hardness figure. It does list total dissolved solids of 204 mg/L, sampled in 2019. That is lower than Highland’s TDS range, but TDS and hardness aren’t the same measurement, so it doesn’t prove Alpine water is softer.
An Alpine homeowner with a tankless heater should test the tap with a hardness strip and compare the result with the manual. Because the report is several years old, it’s also worth asking Alpine City whether newer sampling exists. Until then, a yearly flush is the safe default. Alpine requests go through this page.