Hard Water and Water Heaters: What Scale Really Costs You
Key finding: In the 2009 Battelle study, funded by the Water Quality Research Foundation (the research arm of the water treatment industry), each 5 grains per gallon of hardness cost gas tank water heaters about 4% efficiency, up to 24% at 30 grains per gallon. On 26 grains per gallon water, tankless heaters stopped working from scale after the equivalent of 1.6 years. Heaters on softened water kept their factory efficiency over a simulated 15-year life.
What the Battelle Study Found
The best-known research on hard water and water heaters is the 2009 Softened Water Benefits Study, conducted by Battelle Memorial Institute and funded by the Water Quality Research Foundation. According to the Water Quality Association's summary of the findings:
| Heater Type | On Hard Water | On Softened Water |
|---|---|---|
| Gas tank | About 4% efficiency lost per 5 grains per gallon, at 50 gallons of hot water a day; about 24% at 30 gpg | Kept original factory efficiency over a simulated 15-year life |
| Tankless (gas) | Failed from scale in downstream plumbing after the equivalent of 1.6 years on 26 gpg water | Kept original factory efficiency over 15 years |
| Electric tank | About 0.4 lb of scale per year for each 5 gpg; up to 30 lb of rock-like scale can build up. Battelle noted heating element life can be expected to shorten | Not reported as an issue |
What that means in practice, using the study's gas tank rule of thumb:
| Your Hardness | Estimated Efficiency Loss (Gas Tank) |
|---|---|
| 5 gpg (about 85 mg/L) | About 4% |
| 10 gpg (about 171 mg/L) | About 8% |
| 15 gpg (about 257 mg/L) | About 12% |
| 20 gpg (about 342 mg/L) | About 16% |
| 30 gpg (about 513 mg/L) | About 24% |
A fair caveat: this study was funded by the water treatment industry, which sells softeners. The physics, that scale insulates heating surfaces, is well established, but treat the exact percentages as estimates from one study, not guarantees for your home.
How Scale Forms in a Water Heater
Hard water carries dissolved calcium bicarbonate. Heat converts it to calcium carbonate, which does not stay dissolved, so it settles as white, rock-like scale. This is the same "temporary hardness" that crusts the inside of a kettle; see temporary vs permanent hardness. In a gas tank, scale collects on the bottom over the burner and insulates the water from the flame. In an electric tank, it coats the heating elements and builds up in the bottom. In a tankless unit, it lines the narrow passages of the heat exchanger, which is why tankless heaters are so sensitive to hardness.
Signs Your Water Heater Has Scale
- Popping, rumbling or crackling sounds as the burner or element heats
- Hot water runs out sooner than it used to
- Longer recovery time between showers
- White flakes or grit in faucet aerators and showerheads
- Tankless units showing scale or overheating error codes
- Higher energy bills without a change in usage
How to Flush a Tank Water Heater
Follow your heater's manual first. NYSERDA, New York's state energy agency, recommends draining the tank annually to clear built-up sediment. In hard water, many homeowners flush more often.
- Turn off the heat source. Gas: set the control to "Pilot" or "Vacation" per the manual. Electric: switch off the breaker.
- Let it cool for a few hours. Tank water can scald.
- Attach a garden hose to the drain valve and run it to a floor drain or outside.
- Close the cold water supply valve to the heater.
- Open a hot water faucet somewhere in the house to let air in.
- Open the drain valve and let the tank drain. Cloudy water and sediment are normal.
- Briefly open the cold supply with the drain still open to stir up and flush remaining sediment. Repeat until the water runs clear.
- Close the drain valve, remove the hose, open the cold supply and let the tank fill completely, with the hot faucet open until water flows steadily.
- Restore power or gas only after the tank is full. An electric element powered on in an empty tank can burn out.
Old tank? If your heater is many years old and has never been flushed, a stuck or brittle drain valve can leak once opened. Consider having a plumber do the first flush.
How to Descale a Tankless Water Heater
Tankless heaters need descaling, not just draining. To prevent scale in the first place, see our tankless scale prevention guide. Navien, for example, recommends descaling at least once a year, more often with hard water, higher temperature settings or heavy use. Navien says you can use an approved descaling solution or household white vinegar, circulated through the unit for 45 minutes or longer with a small pump and service valves, followed by a fresh-water flush of at least 10 minutes. Navien also notes that improper DIY descaling could void the warranty, so follow your manufacturer's instructions or hire a professional.
The Anode Rod
Most tank heaters contain a sacrificial anode rod that corrodes so the tank does not. NYSERDA recommends checking it every 1 to 3 years and replacing it as needed. Softened water can wear anodes faster, so if you install a softener, check the rod on schedule.
Preventing Scale
- Soften the water. In the Battelle study, heaters on softened water kept their factory efficiency over a simulated 15 years. See best water softeners and what they cost.
- Flush or descale on schedule, more often in very hard water.
- Avoid unnecessarily high temperature settings. Hotter water forms scale faster. The US Department of Energy suggests 120°F as a setting for most households. Households with someone immunocompromised, or a dishwasher without its own heater, should follow medical and manufacturer advice on temperature.
- Know your number. Test your hardness or check your city guide.
Frequently Asked Questions
In the 2009 Battelle study, funded by the water treatment industry, each 5 grains per gallon of hardness reduced gas tank water heater efficiency by about 4%, up to about 24% at 30 grains per gallon.
Yes, they are especially sensitive. In the Battelle study, tankless heaters on 26 grains per gallon water failed from scale after the equivalent of 1.6 years. Manufacturers such as Navien recommend descaling at least once a year.
Follow your manual. NYSERDA recommends draining the tank annually; homes with very hard water often flush more frequently.
For tankless units, Navien says household white vinegar or an approved descaler can be circulated through the unit. For tank heaters, follow your manufacturer's instructions, since many recommend flushing rather than chemical treatment.
Often because scale and sediment on the bottom of the tank trap water that boils in pockets as the burner or element heats. Flushing usually helps.
Softened water prevents scale. In the Battelle study, heaters on softened water kept their factory efficiency over a simulated 15-year life. Check the anode rod regularly, since softened water can wear it faster.
Last updated: October 2026. Sources: WQA summary of the Battelle study, NYSERDA, Navien.