Hard water and your water heater

What hard water is

Hard water carries dissolved calcium and magnesium picked up as it passes through limestone, chalk and other mineral-rich ground. It is measured in grains per gallon (gpg) or milligrams per liter (mg/L, the same as parts per million). One grain per gallon is about 17.1 mg/L.

Hard water is safe to drink. The trouble is what happens when it is heated. Calcium carbonate is less soluble in hot water than cold, so heating drives it out of solution as solid scale. A water heater is a machine for heating water, which makes it the place in the house where hard water does the most damage.

Where water is hardest in the US

Hardness follows geology and water source. Cities that draw from mountain snowmelt and granite watersheds tend to have soft water. Cities that pump groundwater from limestone aquifers, or draw from rivers that cross mineral-rich plains, tend to have hard water.

• Generally soft: much of the Pacific Northwest, New England, New York City, the Southeast Piedmont around Atlanta and the Carolinas, and parts of Northern California fed by Sierra snowmelt.
• Generally moderate to hard: much of the Midwest and Great Lakes, the Mid-Atlantic, Colorado's Front Range, and the Texas Gulf Coast.
• Generally hard to very hard: the desert Southwest (Phoenix, Las Vegas, Tucson), Southern California areas that rely on Colorado River water, much of Texas inland, Florida's aquifer-fed cities, and the Plains states.

Hardness can vary block to block within a metro area when it mixes sources, and it changes over the year as utilities shift between wells and surface water. Your water utility's annual Consumer Confidence Report, required by federal law, lists hardness or calcium levels. Every city page on Warmspan also notes the local water source and its general hardness.

What hard water does to a tank heater

In a tank, the scale settles to the bottom as sediment. On a gas heater, it forms a layer between the burner and the water, which wastes fuel and overheats the steel. Water trapped beneath it boils and pops, which is the classic rumbling noise. On an electric heater, scale coats the heating elements, which then run hotter until they fail. The lower element can end up buried in sediment altogether.

Hard water also wears the anode rod faster in many cases, and sediment can clog the drain valve so that flushing gets harder each year. The combined effect is a shorter tank life. A tank that lasts twelve years on soft water may last eight or fewer on very hard water without maintenance.

See noisy water heaters and flushing for how flushing works and when it is too late, and element and thermostat repair for what scale does to electric heaters.

What hard water does to a tankless unit

Tankless heat exchangers are narrow passages that heat water very quickly. Scale builds on their walls the same way it builds on the inside of a kettle. As it thickens, the unit has to fire harder to reach the set temperature, the outlet temperature swings, and eventually an overheat or scale code shuts it down.

Regular descaling prevents this. In very hard water, many plumbers recommend a scale-prevention cartridge on the cold inlet as well. Without either, a tankless heat exchanger in Phoenix or Las Vegas can be badly scaled within a couple of years. See tankless repair and descaling for suggested intervals.

How to protect a water heater from hard water

A softener is the most complete fix and protects every fixture in the house, but it adds maintenance and salt costs. Softened water also tends to wear anode rods faster, so check the anode more often after installing one.

Well water is its own case

Homes on private wells do not get a utility water report, and their water can be very different from the city next door. Wells drilled into limestone are often extremely hard, sometimes above 20 grains per gallon. Some wells also carry iron, which stains fixtures orange and builds a reddish sludge in the bottom of a water heater, or sulfur, which reacts with magnesium anode rods to produce the rotten-egg smell many well owners know.

If you are on a well, test the water at least once for hardness, iron and pH. A local water treatment company or a county extension office can usually run the test for a small fee. The result tells you whether a softener, an iron filter or a different anode rod is worth the money, and it helps a plumber choose the right heater when it is time to replace.

Sand and silt are the other well issue. A worn well pump or screen can push fine grit into the house, and it settles in the water heater just like mineral sediment. A spin-down sediment filter on the main line is an inexpensive fix.

Hard water and energy bills

Scale is an insulator. Every layer that forms on a tank bottom, an electric element or a tankless heat exchanger slows the transfer of heat into the water, so the burner or element has to run longer for the same result. Industry and government estimates commonly cite efficiency losses in the range of a few percent for a thin layer of scale and much more for heavy buildup.

That makes maintenance in hard-water cities pay for itself twice: once in a longer-lived heater, and again in lower gas or electric bills. It is one more reason to flush on schedule rather than waiting for the heater to start making noise.

Signs hard water is affecting your heater

• White crust on faucet aerators and showerheads.
• Popping or rumbling from a tank while it heats.
• Electric elements failing every few years.
• Grit or white flakes in the water from hot taps.
• Tankless error codes for overheating or scale.
• Hot water running out sooner than when the heater was new.

If you see two or more of these, ask a plumber to check the sediment level and the anode on the next visit. Catching it early is far cheaper than replacing a tank early.

Questions people ask