Hard water and your water heater: flushing, anode rods and tankless descaling

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What hard water does inside a water heater

Hard water carries dissolved calcium and magnesium. Heating it makes some of those minerals come out of the water. In a tank heater they settle as sediment on the bottom. A.O. Smith says this buildup can raise the risk of leaks, bad smells and other problems, cut the heater's energy efficiency, and clog water lines. In a tankless heater the minerals form scale inside the heat exchanger. Rinnai says scale build-up is caused by hard water and gets worse when the heater is set to a high temperature.

A report filed with the California Energy Commission reviewed the research on water quality and water heaters. It notes that regular flushing kept electric tank heaters efficient over their lifetime in the study it cites. Gas tank heaters were different. Cleaning out sediment did not fully restore their efficiency, because the burner surface at the bottom of the tank can't be completely cleaned, but regular flushing and deliming did significantly limit the loss. So the habit matters more than a one-time rescue.

Not sure how hard your water is? Test it at home or find it in your water report, then see where it falls on the hardness scale.

How often to flush a tank water heater

The sources agree on a floor of once a year and differ above that:

• EPA WaterSense says to flush your water heater once a year to reduce sediment buildup, and to check the temperature and pressure (T&P) relief valve at least once a year.
• A.O. Smith says most tank heaters need flushing at least once a year, and more often in hard water areas, because the higher mineral content raises the risk of sediment.
• The California Energy Commission report says draining is conservatively recommended every 6 months, but that this frequency probably isn't needed in most places.
• University of Georgia Extension suggests draining a gallon every few months, and says this is especially important in hard water regions.

A sensible way to choose: start at once a year, look at what comes out, and drain more often if you keep seeing a lot of sediment. If your manual gives a schedule, use that one.

On an old heater that has never been drained, go carefully. UGA Extension says to start the regular-draining habit only with new or fairly new heaters, because opening the valve on an older unit can lodge sediment in the valve and cause a leak. A.O. Smith says to call a plumber or service technician if sediment clogs the drain valve and you can't flush the tank.

How a tank flush works

This is the general outline from A.O. Smith's maintenance guide. Your model's manual has the exact steps and any warnings that apply to it.

1. Turn off the heat source to reduce the risk of scalds. On a gas model that means turning the gas control to off. On an electric model, switch it off at the breaker or disconnect.
2. Connect a garden hose to the drain valve at the bottom of the tank, and run it somewhere that can take several gallons of very hot water.
3. Open the drain valve and let the water and sediment run out.
4. Keep flushing with cold water until you no longer see sediment coming out.

A.O. Smith's annual checklist adds that you should open a hot water faucet and run it for about 3 minutes after refilling, to get the air out before returning the heater to service. It says this step is very important for electric heaters. In other words, make sure the tank is full of water before you turn the power or gas back on.

The anode rod, and why a softener changes it

Steel tank heaters have a sacrificial anode rod made of aluminum, magnesium or zinc. It corrodes so the tank doesn't. A.O. Smith says how long it lasts depends on the water's chemistry, how much hot water you use, and whether a water softener is installed, and it recommends inspecting the rod once a year.

• Without a softener: A.O. Smith's checklist says to check the rod at least every three years, depending on local water conditions. UGA Extension says every three to four years.
• With a softener: A.O. Smith's checklist says units with a water softener should be checked annually. The California Energy Commission report gives the reason: intentionally softened water uses up the anode faster.

When to replace it: UGA Extension says to replace the rod if more than six inches of its steel core wire is exposed. It also says a rod that is a few years old but shows no corrosion at all has probably stopped protecting the tank ("passivated") and likely needs replacing too. A.O. Smith says to replace it if it's significantly depleted, and notes that flexible rods exist for tanks with little headroom.

So a softener is not a reason to skip maintenance. It shifts the job from clearing scale toward checking the anode more often. If you're choosing a softener, the guides on softener sizing and softener salt cover the rest.

Tankless water heaters: descaling

A.O. Smith notes that tankless heaters don't need flushing like a tank does, because there is no tank, but many need descaling. Rinnai, for example, says to flush its tankless heat exchangers at least once a year. Its procedure circulates 4 gallons of undiluted white vinegar through the unit and says never to use chemical solutions of any kind.

Rinnai's manual for its Sensei RX residential models ties treatment to hardness:

• For water harder than 60 mg/L (the table puts this at 4 grains per gallon), it lists flushing once a year for residential units.
• For those same hard-water classes it recommends a water softener or its scale-control system, and the wording gets stronger as hardness rises, from recommended to required for the hardest water.
• When scale starts to affect performance, Rinnai units can display a diagnostic code (LC#). Some models also have a "Service Soon" (SS) reminder that the installer can set.

The manual also says water-quality damage is not covered by the warranty. Other brands have their own limits and procedures, so read the section on water quality in your manual before your first descale.

Do you need a softener for your water heater?

It depends on the heater type, according to the California Energy Commission report:

• Electric tank heaters (including heat pump models): the report says a softener isn't required for equipment performance, though you may still want one for taste or for hard water's effects on other fixtures.
• Gas tank heaters: the report calls a softener a last resort for the heater's sake, unless you want one for hard water's cosmetic effects. It adds cost and may shorten tank life, since softened water is harder on the anode.
• Tankless heaters in hard water: the report says a softener may be the only workable option, and manufacturers like Rinnai recommend or require treatment depending on hardness.

Most people decide on a softener for the whole house, not just the heater: spots, scale on fixtures, soap that won't lather. The decision guide and softener vs conditioner pages walk through that choice.

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When to call a pro

EPA WaterSense says it may be best to have a professional service your water heater at least once a year, and to look regularly for leaks, worn gaskets, loose connections and corrosion. Call one if the drain valve is clogged or leaks, the T&P relief valve keeps releasing water after you test it, you aren't comfortable relighting a gas unit, or a tankless unit shows a scale or service code. A.O. Smith also says to defer to your manufacturer's handbook for your model's maintenance.

Last updated: 2026-10-25