We at the Know Your H20 Team get a lot of questions. One common question always relates to scale formation or coatings related to hot water lines. One of the first question we always ask is: When is the last time you flushed your hot water heater ?

Hard water causes mineral sediment (mainly calcium carbonate from calcium and magnesium) to build up quickly in a tank-style water heater. As the water is heated, the carbon dioxide in the water outgas. When carbon dioxide leaves water, the water becomes less acidic. That change causes the dissolved bicarbonate to form carbonate. The carbonate than combines with calcium, magnesium, iron, and other divalent cations to form a chemical scale that will precipitate out of the water. Sometimes the scale is white, but sometimes the scale is bluish to green. Heating causes these dissolved minerals to precipitate and settle at the bottom of the tank. Over time this forms a layer of scale/sediment within the hot water tank and even the plumbing within the home that can create multiple problems.

Main reasons flushing is important with hard water

• Loss of efficiency and higher energy use: The sediment acts like insulation between the burner (gas) or heating element (electric) and the water. Heat transfer is impaired, so the unit runs longer and works harder. Estimates commonly cited (including references to U.S. Department of Energy figures) indicate that even about ¼ inch of buildup can reduce efficiency by roughly 25–40%.

• Reduced usable hot-water capacity: The sediment occupies space at the bottom of the tank, so you get less hot water than the rated tank size suggests.

• Premature wear, hot spots, and tank damage: In gas heaters, the trapped heat can create hot spots that stress and weaken the tank bottom, accelerating corrosion. In electric units, sediment can bury the lower element and cause it to overheat and fail. Overall, neglected sediment is a leading contributor to shorter tank life (sometimes dropping expected lifespan from the typical 10–15 years range down toward 6–8 years in hard-water conditions without maintenance).

• Noise and other symptoms: Popping or rumbling often occurs as water boils and steam bubbles force their way through the sediment layer. You may also notice reduced flow, cloudy or discolored hot water, or more frequent heating-element/burner issues.

Flushing drains the tank (usually with a hose attached to the drain valve) and removes the loose sediment before it hardens into a thick, rock-like layer that is much harder to clear. In hard-water areas this is preventive maintenance rather than optional—many plumbers and manufacturers recommend doing it at least once a year, and every 6–9 months (or more often) in very hard water.

Additional notes

A whole-house water softener greatly slows sediment formation and reduces how often you need to flush. Tankless water heaters do not collect sediment in a tank the same way, but they still require periodic descaling of the heat exchanger in hard-water conditions.

Regular flushing helps maintain efficiency, preserves capacity, reduces energy waste, and extends the life of the water heater. Check your model’s manual for the exact procedure (power/gas off, cold-water supply shut off, drain valve opened, etc.), and consider professional service if the unit is older or has never been flushed.

Most tank water heaters should be flushed about once a year. With hard water, do it more often—typically every 6 months.

Manufacturers such as A.O. Smith and Rheem generally recommend flushing at least annually to remove sediment, with more frequent flushing advised for hard water or high usage.

Additional guidance

• Tankless water heaters do not collect sediment in a tank the same way, but they still need periodic descaling (usually annually, or more often in hard water) to clear scale from the heat exchanger.

• Check your specific model’s manual, as some manufacturers list their own intervals.

• Signs you may need to flush sooner: rumbling or popping noises, reduced hot-water capacity or slower recovery, cloudy/discolored hot water, or rising energy use.

• If your water heater is older and has never been flushed (especially after 3+ years), have a professional assess it first. Flushing a long-neglected tank can sometimes loosen baked-on sediment and lead to leaks.

You can determine your water hardness by getting a DIY Testing Kit or conducting an informational water analysis, please consider using the NTL Mail Order Testing Service (Well Water / City Water).

Learn More About Your Drinking Water

Want to learn more about the water in your home? Order Our Drinking Water, our educational booklet for well-water and city-water users.

Other Websites

Know Your H2O
B.F. Environmental Consultants
Carbon County Groundwater Guardians
Keystone Clean Water Team (Donate)

Affiliate Disclosure: This article contains affiliate links. If you purchase a product through one of these links, we may earn a small commission at no additional cost to you. We only recommend products we believe in. This helps support our mission to educate consumers about water quality.