Water softeners and water heaters: what softened water does to your tank

What hard water does to a water heater

The U.S. Geological Survey explains that when hard water is heated, calcium carbonate comes out as solid scale, which can shorten equipment life, raise the cost of heating water, lower the efficiency of electric water heaters and clog pipes. It classifies water up to 60 mg/L as soft, 61 to 120 as moderately hard, 121 to 180 as hard and more than 180 as very hard. Divide mg/L by about 17.1 to get grains per gallon.

In a tank, scale settles as sediment, which is what makes a water heater pop and rumble. Our guides to water heater noises and flushing cover that.

What a softener fixes

A Battelle study for the Water Quality Research Foundation, a water treatment industry group, ran gas tank, electric tank and gas tankless heaters on softened and unsoftened water under accelerated conditions. It found:

• Gas tank heaters on unsoftened water dropped from 70.4% to 67.4% efficiency over the equivalent of two years. On softened water, scale buildup was almost negligible.
• Tankless heaters on unsoftened water had to be delimed at the equivalent of 1.6 years, after efficiency fell from 80% to 72%. Deliming only brought them back to about 77%.
• Low-flow showerheads and faucets on unsoftened water clogged in under seven days of the accelerated test.

Keep in mind who paid for it, but the direction matches what manufacturers say. Rheem's water treatment team says softeners can help a water heater last longer by keeping minerals out of the tank.

The catch: the anode rod

A tank water heater has a sacrificial anode rod that corrodes so the steel tank doesn't. A. O. Smith's Bulletin 44 explains why softened water speeds that up:

• A softener swaps calcium and magnesium for sodium. That lowers hardness but not conductivity, and higher conductivity makes the anode corrode faster.
• If the softener isn't rinsed properly after regenerating, leftover salt reaches the tank and does the same.
• A thin film of scale actually protects exposed steel, such as pipe connections and welds. A. O. Smith's tests found a 33% drop in steel corrosion with a thin scale film. Softened water prevents that film, so the anode does all the work.

A. O. Smith's conclusion: customers with softeners should expect faster anode consumption and inspect and replace anodes more often. Its installation manual says softened water requires an annual anode inspection. Rheem suggests switching from a magnesium anode to an aluminum, zinc or hybrid rod on soft water. See our anode rod guide.

Tankless heaters and hard water

For tankless units, treating hard water matters more, because scale builds inside a small heat exchanger. Noritz's warranty terms say the warranty becomes null and void if hardness above 12 grains per gallon (200 mg/L) contributes to a failure. Our guide to a tankless heater that won't heat covers scale codes such as Rinnai's LC.

Do you need one?

Look up your water report first. Some areas are very hard: the Manheim Area Water and Sewer Authority in Pennsylvania puts its average at 17.48 grains per gallon (299 mg/L), from two deep wells (see our Manheim page). Others soften at the plant. Westerville, Ohio says it softens its water to about 125 mg/L, or 7 grains per gallon (see our Westerville page).

If you add a softener to a home with a tank heater, put an anode check on the calendar for the first year and see how fast it's going. If you're replacing the heater anyway, our replacement page covers tank and tankless options.

Local pages mentioned

• Westerville, OH
• Manheim, PA