Will hard water ruin a hot water heater?

For a storage tank, the news is better than the sales pitches suggest, and it depends on the fuel.

Electric tank. PNNL's review of the research found that efficiency stayed at its rated value until sediment reached and buried the bottom element. Hard water "did not appear to impact the efficiency of electric storage water heaters or the lifetime of the tank," although the heating element "would be expected to have a shorter life." Cleaning the tank and replacing the lower element restored full efficiency.

Gas tank. Sediment sits between the burner and the water and acts as an insulator. In the testing PNNL summarizes, gas storage efficiency dropped from 70.4 percent to 67.4 percent over the equivalent of two years, a reduction of 3 percent. The trapped heat can also stress the glass lining of the tank, and PNNL says that can lead to tank failure over time. In extreme cases the report puts the efficiency loss at up to 10 percent.

Both. Scale and sediment can clog the tank's drain valve and the temperature and pressure relief valve, which affects maintenance and safety. That is the practical reason to keep the tank clean. The same scale is what coats your faucets and showerheads, which our page on how hard water damages plumbing covers. If you hear popping or rumbling, sediment is the usual cause, and our guide to the popping and rumbling noise walks through it.

The picture changes for a tankless unit. PNNL notes that the test on gas tankless heaters found "the efficiency of tankless gas water heaters decreased from 80 percent to 72 percent over 1.6 years, after which the tankless water heaters failed due to a clogged pressure sensor or other control device and required deliming." After deliming, efficiency returned to 77 percent. The report adds that in extremely hard water, scale can reduce lifetime "to less than 2 years" unless deliming is frequent.

Those tests used harder water than most homes see (the test water was about 444 mg/L, and Phoenix runs 172 to 302 ppm), so treat the exact numbers as a warning, not a forecast. The lesson holds in Phoenix, where hardness is high. A tankless heater with no descaling plan is the type most likely to fail early. Our page on how often to descale a tankless water heater in Phoenix gives the schedule.

PNNL's report includes one number that surprises people. In water with a hardness of 170 mg/L, "0.06 inches of lime scale could accumulate in just 4 years in a 50-gallon storage water heater with a water temperature of 140°F." At 120°F, "it takes almost 30 years to generate 0.06 inches of scale under the same water draw volume."

The same report cites another study in which a 0.06-inch coating of lime scale on a boiler heat exchanger cut its efficiency "by an average of 11 percent," and notes that efficiency fell steadily as the scale got thicker. That is a boiler, not a home tank, but it shows why even a thin layer is worth avoiding.

Phoenix water starts at about 172 mg/L, so that example sits at the mild end of the local range. Lowering the tank temperature is on PNNL's list of ways to slow scale, along with reducing hot water use. The setting affects safety and comfort, so pick it deliberately, and see whether Phoenix heat damages water heaters and pipes for how location and temperature work together.

These steps do the most for the least effort:

1. 1Flush the tank every year. A. O. Smith says most tank heaters need flushing "at least once per year," and "if you live in a hard water area, you may need to flush your water heater more often." PNNL says draining "conservatively every 6 months" is a safe upper bound. See how often to flush a water heater in Phoenix.
2. 2Ask about a curved dip tube. PNNL describes it as a fitting that swirls the incoming cold water, "to keep the water and sediment moving and reduce scale buildup."
3. 3Descale a tankless heater on the maker's schedule. This is the step that changes its lifespan the most.
4. 4Check the anode rod. Hard water still corrodes the tank slowly, and A. O. Smith recommends inspecting the anode "once a year."

### Signs scale is already building up

You will usually notice scale before it causes a failure. Common clues are a popping or rumbling sound as the burner or element heats, hot water that runs out sooner than it used to, and a water heater that takes longer to recover. If your hot water seems weaker than it should be, our guide on why the water is not getting hot enough covers the other causes to rule out. Sediment is the easy one to fix. PNNL notes that draining a tank "is a simple practice that can be accomplished by homeowners with a small amount of education," though a stuck drain valve or a gas unit is a good time to have a plumber do it.

Sometimes, but not because of the tank alone. PNNL's conclusion is that for electric tanks, softening is "generally not recommended from an equipment maintenance perspective." For gas tanks, it says scale in extremely hard water "may justify" softening and its upkeep, and for tankless heaters it says softening "may be required in most hard water conditions."

A softener also changes how a tank ages, because it speeds up anode wear. Our page on whether salt-based softeners damage a water heater covers that trade. In short, the heater alone rarely justifies a softener, but the rest of the house often does. If you decide to soften, choosing the right size is the next step.

If you are not sure what is inside your tank, HQ Plumbing and Air offers free estimates and upfront pricing. A quick flush and a look at the anode is a normal first visit.