Tankless water heater hard water problems in Las Vegas

How hard Las Vegas water is, next to what tankless makers allow

The Las Vegas Valley Water District’s 2026 annual report (based on 2025 data) gives the valley’s hardness as 280 parts per million, or 16 grains per gallon. Parts per million and mg/L are the same measure, so you can set that figure straight against the manuals.

We could not find a numeric hardness limit from Navien; its user manual gives none. Valley water, at 280 mg/L, sits in the “extremely hard” band of Rinnai’s table, the band where treatment is required rather than recommended.

Why scale hits a tankless unit harder than a tank

A tankless heater pushes water through a narrow heat exchanger and heats it fast. The U.S. Department of Energy’s Building America fact sheet on tankless heaters says that when the heater runs at higher temperature settings on water with a high mineral content, “scaling of the low-mass heat exchanger is likely”. Rinnai’s manual adds that scale build-up “can be accelerated if the water heater is set at a high temperature”, so a lower set point slows it down.

The same Department of Energy sheet says tankless heaters can last more than 20 years, against 10 to 15 for tanks. That longer life assumes the heat exchanger is looked after. See our guide to how long a water heater lasts in Las Vegas for the tank side.

Using a water softener with a tankless water heater in Las Vegas

Rinnai’s recommended-treatment table gives two options for every hardness band above “soft”: a water softener or its own scale-control system. Noritz’s descaling document includes a note on fitting a water softener, points to its own scale-control products, and adds that softeners “may be regulated by the local water jurisdiction”. We found no Southern Nevada rule on softeners in the water district’s service rules or the county’s plumbing amendments.

A softener takes the calcium and magnesium out of all the water in the house, so it also deals with spots, crust and scale in the rest of the home. If you’re weighing a “tankless water softener” or an anti-scale filter that uses no salt, read how salt-free systems differ from softeners first. Rinnai and Noritz name only the scale-control systems they offer themselves, so ask the heater’s maker before relying on any other.

If a softener is part of the plan, size it to the house, not the heater. You can work out your size from your household and your water’s hardness.

How do you flush a tankless water heater on valley water?

Flushing a tankless water heater means pumping a descaling solution through the heat exchanger to dissolve scale. Both Rinnai and Noritz say isolation valves on the water lines are needed to do it. Rinnai’s manual sets out the steps for its SENSEI RX models:

1. Shut off the gas supply and turn off the controller.
2. Close the shutoff valves on the hot and cold water lines.
3. Connect a pump’s outlet hose to the cold-side service valve, and a drain hose to the hot-side service valve.
4. Pour four gallons of undiluted, food-grade white vinegar into a pail, and put the drain hose and the pump’s inlet hose in it.
5. Open both service valves, start the heater’s descale mode as the manual describes, and run the pump so the vinegar circulates for at least one hour at about four gallons per minute.
6. Rinse: send the drain hose to a sink or outside, and let fresh water run through the heater for five minutes.
7. Clean the in-line filter at the cold water inlet, put it back, reopen the normal valves, and return the heater to normal mode.

Other models differ. Noritz says its descale instructions are meant for a qualified service professional, and Navien calls flushing “a somewhat complicated procedure” and warns that improper maintenance can void the warranty. Read your own manual, or have the work done.

Flush kits and descalers for a tankless unit in Las Vegas

A flush kit is the pump, hoses and pail used in the steps above. Rinnai’s manual uses plain white vinegar as the descaler. If you want to use a commercial descaler for a tankless water heater instead, check that your manual allows it first.

How often to flush in the valley

Rinnai’s table suggests flushing a home unit once a year where the water is treated. Its heaters can show a Service Soon (SS) code when flushing is due, and an LC code when scale has started to build. Rinnai warns that ignoring the LC code “will cause damage to the heat exchanger”. Noritz shows a “C” error code for scale build-up and excludes damage when that code was shown but the unit was not flushed.

An outdoor tankless heater in Las Vegas

If your tankless heater is mounted outside, note that Rinnai says its units will not freeze down to -4°F outdoors while they have power and are out of direct wind. The National Weather Service in Las Vegas counts an average of 9.8 nights a year at or below 32°F (1991–2020 normals). Rinnai also says to drain the heater by hand if power or gas fails during freezing weather, because freeze protection stops working.

Sources for this page

• Las Vegas Valley Water District · annual water quality report · 2026 (2025 data) · valley hardness, 280 ppm / 16 gpg.
• Rinnai · installation and operation manual for its residential SENSEI RX tankless heaters · 2025 · 200 mg/L guideline, treatment table, warranty wording, flushing steps, freeze protection.
• Noritz · descaling procedure for its Group B tankless models · 2017 · 12 gpg warranty exclusion, isolation valves, professional-use note.
• Navien · user’s information manual for its NPE-2 series · 2020 · warranty exclusion for hard-water deposits, flushing caution.
• U.S. Department of Energy · Building America fact sheet on tankless water heaters · 2009 · scaling at high temperatures and mineral content, lifespan.
• National Weather Service, Las Vegas · temperature day counts · 2026 (1991–2020 normals) · nights at or below freezing.