Every plumber has drained a water heater and heard the gravel. That is scale: the calcium and magnesium in hard water, dropped out of the water by heat and baked onto the hottest surface in the house. A national laboratory put numbers on what it costs. Some water quality reports print a line that says whether a town's water is going to do it.
Scale is hardness plus heat. The water heater is where they meet.
Short answer
Hard water carries dissolved calcium and magnesium. When that water is heated, the minerals come out of solution and stick to whatever is hot: the bottom of a tank, the heat exchanger of a tankless unit, the element of an electric heater. The layer is called scale. It insulates the metal from the water, so the burner runs longer to heat the same tank, and it builds until something fails or gets replaced.
Calcium carbonate is less soluble in hot water than in cold. The same water that leaves no mark on a cold line drops its minerals the moment it hits a heating surface. That is why the water heater fails first, and the cold side of the house last.
Scale does not announce itself. It builds for years inside a tank nobody opens. The heater that should have gone twelve years goes seven, the customer blames the brand, and the plumber who replaces it never mentions the water. That is the call this post is about.
The numbers
In 2013 the Pacific Northwest National Laboratory (PNNL) published a review of scale's effect on water heaters and appliances, report PNNL-22921, by Widder and Baechler. It reported the results of a Battelle study that ran heaters side by side on hard and softened water. Here is what it found.
| Equipment | On hard water | Source |
|---|---|---|
| Tankless gas water heater | Efficiency fell from 80 percent to 72 percent in 1.6 years | PNNL-22921 (2013), reporting the Battelle study |
| Gas storage water heater | Efficiency fell from 70.4 percent to 67.4 percent over 2 years | PNNL-22921 (2013), reporting the Battelle study |
| Appliances in general | Preventing scale extends appliance life 25 to 40 percent | PNNL-22921 (2013), citing New Mexico State University |
| Water heaters, scale | About 528 grams a year on unsoftened water against about 7 grams on softened water | WaterWorld, Battelle Benefits, 23 April 2010 |
| Showerheads | Clogged after an average of 3,203 gallons on unsoftened water | WaterWorld, Battelle Benefits, 23 April 2010 |
| Faucets | Clogged after the equivalent of 19 days of flow at 50 gallons of hot water a day | WaterWorld, Battelle Benefits, 23 April 2010 |
Neither source states the hardness level tested, so these figures show the direction and the scale of the damage, not a number for any one house. A tankless unit losing eight points of efficiency in under two years is a warranty call, a callback and eventually a replacement. Your company is already getting paid to show up for that failure. Nobody is getting paid to prevent it.
The line on the report
Some water quality reports print a row called Corrosivity, measured as a Saturation Index, usually the Langelier index. It is a calculation from the water's pH, temperature, hardness, alkalinity and dissolved solids, and it answers one question: does this water want to drop its minerals out, or dissolve them?
The water is saturated with calcium carbonate and wants to put it somewhere. The Carlsbad report's own footnote says it plainly: positive index means non-corrosive, with a tendency to precipitate and deposit scale on pipes. Good for the pipe. Bad for the water heater.
The water is hungry for minerals and will take them from the pipe. The same footnote: negative index means corrosive, with a tendency to dissolve calcium carbonate. Bad for the pipe, and a different conversation entirely.
| Report | Saturation Index printed | Hardness on the same page | What it says |
|---|---|---|---|
| Carlsbad, imported | Skinner plant, district report page 2: 0.62 to 0.75, average 0.70 | 165 to 291 ppm (9.6 to 17.0 grains) | Positive. Hard water that wants to deposit scale. |
| Carlsbad, desalinated | Desalination plant, same page: 0.04 to 0.62, average 0.28 | 43.7 to 79.6 ppm (2.6 to 4.7 grains) | Positive but low. Softer water, less to deposit. |
| Sachse, Texas | Not printed in the 2025 report | 96.0 to 200 ppm (5.6 to 11.7 grains) | Prints the ingredients (pH 7.3 to 8.8, alkalinity 43.6 to 138, hardness, TDS 182 to 455) but not the index. |
Most reports do not print a Saturation Index. When one does, read it. When one does not, the hardness row plus the water heater tells you the same story, one drain valve at a time.
On the call
Convert it to grains. Dallas prints 120 to 155 milligrams per liter, which is 7.0 to 9.1 grains, and Roger's field readings in Dallas homes run 8 to 12. Anything on the U.S. Geological Survey's hard or very hard rows is scale water.
If the report prints one and it is positive, the utility has told you in writing that its water tends to deposit scale. Carlsbad's does, in footnote j. That is a public document you did not write.
Hardness in grains at this kitchen sink. The report's range gets you close. The strip tells you what this heater has been drinking.
Hard water is hardness plus heat. Your heater is where they meet. Here is the city's number, here is your tap's number, and here is what a national lab measured it doing to a tankless unit. Then the homeowner decides, and the plumber has said nothing he cannot hand them on paper.
Hardness is not a health problem. It is an equipment problem, and the equipment is the plumber's. That is why the water heater call is where the water conversation belongs.
Watch it
Questions
Hard water. Dissolved calcium and magnesium come out of solution when the water is heated and stick to the hottest surface, the bottom of the tank, a tankless heat exchanger or an electric element. Calcium carbonate is less soluble in hot water than cold, which is why the heater scales before the cold lines do.
In the Battelle study reported by Pacific Northwest National Laboratory report PNNL-22921 (2013), a tankless gas water heater on hard water fell from 80 percent to 72 percent efficiency in 1.6 years, and a gas storage heater fell from 70.4 percent to 67.4 percent over two years. The hardness tested is not stated in the report.
A calculation, usually the Langelier index, from the water's pH, temperature, hardness, alkalinity and dissolved solids. A positive index means the water is non-corrosive and tends to precipitate and deposit scale on pipes. A negative index means the water is corrosive and tends to dissolve calcium carbonate. The Carlsbad Municipal Water District prints it; the City of Sachse does not.
Both. Positive means the water will not eat the pipe, which is why utilities aim for it. It also means the water wants to deposit its minerals, and the water heater is where it does. Carlsbad's imported water runs 0.62 to 0.75, positive, with hardness of 165 to 291 parts per million on the same page.
PNNL-22921 cites New Mexico State University for a 25 to 40 percent longer appliance life when scale is prevented. In the Battelle study the same report describes, water heaters on unsoftened water built up about 528 grams of scale a year against about 7 grams on softened water.
Sources
Checked in October 2026. Reports are reissued every year, so confirm with the source before you rely on a number.
Written by Roger Wakefield, Texas Master Plumber
45 years in the trade and LEED AP. Published . More about Roger or visit rogerwakefield.com.
Keep going
Day one of the HALO Water Tour is reading the report and testing the water. Day two is knowing what to put on the house. Day three is saying it like a plumber. Carlsbad October 27-29 and Dallas November 3-5, 2026. Twenty seats a city.