A softener protects your water heater from scale, and it can also make the tank’s sacrificial anode rod wear out faster. Both things are true, and which one matters more depends on what kind of water heater you have and how hard your water is. This guide summarizes a U.S. Department of Energy (DOE) laboratory report, two DOE guides, and A. O. Smith’s own technical bulletin on softeners (all checked Nov 4, 2026). We haven’t inspected or serviced a water heater ourselves, so this is a summary of what those sources say. Your water heater’s manual and warranty come first, and a licensed plumber is the right person for anything involving gas, electricity, or pressure relief valves.
The short answer
- Hard water leaves scale in a water heater. Scale and sediment build up on heating elements, on heat exchangers, and at the bottom of the tank.
- Tankless heaters are the most exposed. DOE says scale can restrict flow through the heat exchanger and shorten the unit’s life, and that manufacturers may require a softener once hardness passes roughly 8 to 14 grains per gallon.
- Tank heaters are more forgiving. The DOE laboratory report (PNNL-22921) says an electric tank heater doesn’t need a softener for performance reasons, and that gas tank heaters should treat a softener as a last resort unless the water is extremely hard.
- Softened water uses up the anode rod faster. A. O. Smith says to expect more rapid anode consumption with a softener and to inspect more often.
- If you have a softener and a tank heater, inspect the anode rod about once a year (A. O. Smith’s recommendation for units with a softener), and follow your own manual.
What hard water does inside a water heater
Heating water makes dissolved calcium and magnesium drop out as scale. The DOE laboratory report (Pacific Northwest National Laboratory, 2013) says the main effect is scale on heating components such as heat exchangers, heating elements, and the tank bottom. Sediment particles are heavier than water, so loose scale also sinks and collects at the bottom of the tank.
What that costs depends on the type:
- Electric tank heaters usually keep their rated efficiency. The report says scale and sediment don’t hurt performance or lifetime until the sediment reaches and buries the bottom element. That’s why it says softening is “generally not recommended” for electric tanks from an equipment standpoint.
- Gas tank heaters are hit harder because the burner heats the tank bottom from below and sediment acts as insulation. One study the report cites found gas storage efficiency fell from 70.4 percent to 67.4 percent over the equivalent of two years of hard water. For extreme cases the report says scale can cut efficiency by up to 10 percent and lifetime by 25 to 40 percent, and that a softener may be justified, with the cost of anode maintenance factored in.
- Tankless heaters have the least room for error. DOE’s tankless guide says scale builds up in the copper heat exchanger, restricts flow, and diminishes the unit’s life, and that scaling has “much less effect” in a tank because the scale settles on the bottom. The PNNL report says softening may be required in most hard water conditions for tankless units, and that in extremely hard water the heat exchanger life can fall to less than two years without frequent deliming.
A. O. Smith’s own bulletin lists scale effects in the same terms: clogged pipes and reduced recovery efficiency.
Why a softener can shorten a tank’s life if the anode isn’t kept up
A steel tank heater has a glass lining and a sacrificial anode rod, usually aluminum, magnesium, or zinc, that corrodes in place of the tank. A. O. Smith’s maintenance guide says how long the rod lasts depends on the water chemistry, how much hot water you use, and “whether or not a water softener is installed.”
A. O. Smith’s Technical Bulletin 44 explains the mechanism:
- A softener swaps calcium and magnesium for sodium. That reduces hardness but “does not reduce the conductivity of the water,” and the bulletin says higher conductivity speeds up anode consumption.
- If the softener’s rinse is incomplete and residual salt reaches the heater, that raises conductivity further.
- A softener also removes the thin film of scale that would otherwise coat the steel inside the tank. A. O. Smith’s lab found a thin scale film cut the steel corrosion rate by 33 percent in moderate-conductivity water (17 grains hardness) after three weeks.
- The bulletin’s conclusion: “Customers using water softeners should expect more rapid anode consumption. Anodes should be inspected more often and replaced when nearing the end of their useful life.”
The PNNL report puts a number on the risk without proper upkeep: it cites a 1992 study finding tank lifetimes in extremely soft water can drop by 50 percent or more, and says that if the anode is properly maintained or replaced with a powered anode rod, the effects of soft water on tank integrity can be “minimized or eliminated.” Its recommendation for tank heaters in softened homes is annual anode inspection or a powered anode.
How often to check: the schedules in the sources
| Situation | Anode inspection | Source |
|---|---|---|
| Tank heater, normal water | Every 3 to 4 years, then annually | PNNL-22921; DOE fact sheet (every 3 to 4 years) |
| Tank heater with a softener | Annually (A. O. Smith: “Units with a water softener should be checked annually”) | A. O. Smith maintenance guide; PNNL-22921 |
| Tank flush | About a quart every 3 months per DOE; A. O. Smith says most tank heaters need flushing at least once per year, more in hard water | DOE fact sheet; A. O. Smith |
| Tankless descaling | Twice a year to once every three years depending on the maker, model, and water | DOE tankless guide |
PNNL says to replace an anode when more than 6 inches of bare core wire shows, when it has corroded to less than half its original diameter, or when it is heavily pitted or split. It also notes a rarer failure called passivation, where a scale coating limits the anode’s protection, which happens with high carbonate hardness.
These are the sources’ recommendations, not a substitute for your heater’s manual. Anode rod removal means shutting off power or gas and the water supply and relieving tank pressure, so call a plumber if you aren’t comfortable doing it.
Tankless heaters and hardness limits
DOE’s tankless guide says manufacturers vary on softener requirements. If hardness exceeds 8 grains per gallon (136 ppm) to 14 grains per gallon (239 ppm) or more, “manufacturers may require that a water softener be installed to maintain efficiency or prevent unit failure.” Flush frequency also varies by maker, model, use, and water.
That matters beyond performance: A. O. Smith’s maintenance guide says many tankless heaters require descaling, and an older A. O. Smith preventive maintenance guide (2003) says that in most cases where a heater fails because of minerals dropping out of the water, the manufacturer’s warranty does not apply. We haven’t read every maker’s current warranty, so read yours for the exact wording and any hardness limit before you buy or install. Our hardness guide shows how to measure the number to compare against the manual.
If you’re also choosing between treatments, the PNNL report notes electronic or electromagnetic scale devices have shown “mixed effectiveness” when independently evaluated, and our softener versus salt-free conditioner guide covers what salt-free options do and don’t do.
Hot water that still feels hard
If the softener is working but your hot water seems hard, the Fleck 5600SXT manual’s troubleshooting chart lists “hot water tank hardness” and says repeated flushing of the hot water tank is required. Morton’s guide adds that water used during a regeneration can fill the water heater with hard water. Our troubleshooting guide walks through the other causes, and our salt settings guide covers rinse and salt settings, since incomplete rinsing is the case A. O. Smith flags for anode wear.
A practical checklist
- Find your heater type, age, and warranty. The model’s manual has the maintenance schedule and any water quality limits.
- Know your hardness. Use your utility’s water report or a test, as described in our hardness guide. For a well, use a state-certified lab.
- If you have a softener and a tank heater, put an annual anode check on the calendar, and flush the tank on the schedule your manual gives.
- If you have a tankless heater, compare your hardness to the manual’s limit and follow its flush or descale schedule. If hardness is above the limit, treatment is likely required.
- Check the softener’s regeneration and rinse settings if the anode is wearing very quickly. See our sizing guide and resin life guide for how settings and resin condition fit together.
- Ask a plumber or the manufacturer about powered anode rods, expansion tanks, or any gas, electrical, or pressure relief work.
Where a softener fits
If your water is very hard, a softener is the most common way to keep scale out of a water heater and the rest of the plumbing, and it is often the only practical protection for a tankless unit. For a tank heater on moderately hard water, the sources suggest regular flushing and anode care may be enough, so a softener decision can rest on the other benefits (fixtures, appliances, soap use, taste of the water). Use our sizing calculator to see what size your household and hardness call for. The Fleck 5600 SXT Metered is one common example, and the sizing guide has its price and specs.
NSF’s listings show no certified US Water Systems softener system (only NSF/ANSI 42 filter components), so if certification matters to you, compare certified softeners in NSF’s own listings; our NSF guide shows how. For questions about your drinking water, check your utility’s Consumer Confidence Report or ask your local health department.
Bottom line
A softener protects against scale, which matters most for tankless and gas heaters, but it also speeds up anode wear in a tank heater. If you have both, check the anode about once a year, flush the tank, and follow your heater’s manual. If you have a tankless heater, read its hardness limit before deciding.
Sources (all checked Nov 4, 2026)
- S. Widder and M. Baechler, Pacific Northwest National Laboratory for the U.S. Department of Energy, Impacts of Water Quality on Residential Water Heating Equipment (PNNL-22921), November 2013
- K. Brozyna and A. Rapport, IBACOS for the U.S. Department of Energy, Measure Guideline: Transitioning to a Tankless Water Heater, September 2012 (section 2.5, water quality and maintenance)
- U.S. Department of Energy Building America, Water Heating technology fact sheet (quart flush every three months, anode inspection every three to four years)
- A. O. Smith, Technical Bulletin 44: Water Softeners
- A. O. Smith, Water Heater Maintenance Guide and Maintenance & Troubleshooting Guide (PDF)
- A. O. Smith, Preventive maintenance plan, Form 4800 Rev. 9 (PDF) (warranty note on scale; older document, 2003)
- Pentair, Fleck 5600SXT downflow service manual, troubleshooting pages (checked Nov 3, 2026)
- Morton Water Softeners, Troubleshooting Guide (checked Nov 3, 2026)