Q & A: Scale - Solar Water Heating’s Hidden Nemesis

Hard water is a problem with heat exchangers in solar water heaters and on demand water heaters. What’s the best way to prevent scaling in areas with hard water?

According to the US Geological Survey, 85% of the United States—and ironically most of the country’s best solar areas—has hard water. As shown on the map, the softest waters occur along most of the eastern corridor, along the Central Gulf coast, in the far Pacific Northwest and in Hawaii. The rest of the country has anywhere from moderate to very hard water.

Hard water contains high levels of calcium, with magnesium, bicarbonates and sulfates as other possible contributors. Scale is the precipitation or physical buildup from these minerals, although mineral content is only part of the equation. The water’s acidity or alkalinity (pH), its temperature, water flow characteristics and the physical composition of the water’s enclosure also greatly affect the extent of scale buildup. The science and calculation of scaling is complicated in that there are many variables, equations and real life situations that can be difficult to calculate and can vary greatly from site to site.

Over time, scale will accumulate on the inner components of a water heater’s heat exchanger or element and affect a system’s performance. Scale acts as an insulating barrier within the heat exchanger. According to a study from the Army Corps of Engineers, just a slither of scale buildup can reduce a heat exchanger’s efficiency by 15%. Since this buildup increases exponentially as temperatures within the heat exchanger or system rise, it is imperative to keep the water heater’s temperatures to a minimum and utilize a very efficient heat exchanger.

Effects on Solar Water Heating Systems

Assistant professor at Middle East Technical University in Ankara, Turkey, Dr. Derek Baker, as a University of Texas graduate student, conducted research regarding scale within solar water heating systems. After reviewing prominent scaling research papers over the past 25 years, he realized that the studies lacked a consistency and none included long-term trials. All of the studies were conducted by oversaturating water with calcium rather than testing the effects of hard water over time. He thinks that more research pertaining to real life scaling situations should be done.

The result of Dr. Baker’s research is a downloadable solar water-scaling calculator called Solscale. This straightforward program requires the user to input information such as dissolved calcium concentration (ppm as CaCO3), total alkalinity, pH and total dissolved solids (ppm). Users can purchase a water testing kit and enter the data to check scale potential for different systems in their area. Water quality varies greatly, even within the same geographic regions. Using generalized numbers for a specific location could be a mistake. Solscale does not allow the user to set a maximum high temperature for the solar system, a glaring omission from the program. Some solar water heating systems, such as drainback systems, allow the owner to set the maximum temperature for the solar storage tank. In retrofitting many systems in central Texas, I have seen considerably more scale buildup within glycol antifreeze systems as compared to drainback systems.

Dr. Baker recommends a solar system that utilizes a tank with a wraparound heat exchanger, which is isolated from the potable water since it is a copper pipe wrapped around the outer wall of the steel tank. The collector fluid within the copper pipe transfers its heat to the copper pipe wall, through the steel and ultimately to the glass lining of the tank wall. Dr. Baker’s logic is that even though it is not very efficient, the wraparound heat exchanger provides more surface area for the scale to attach. The overall efficiency is less affected than exhangers with smaller surface areas.

I agree with his general assertions, but I believe there are other variables to consider, including cost. The cost difference between a tank with a wraparound heat exchanger and a regular electric water heater, external heat exchanger and pump can be significant. An 80 gallon Rheem Solaraide HE, for example, costs approximately $1,400 wholesale and $1,750 retail. It includes a 6-year warranty with just 2 inches of insulation, and actually less around the heat exchanger. A high efficiency 80 gallon electric water heater can be purchased for approximately $600. Many models carry 12-year warranties and have 3 inches of insulation. Combined with an external heat exchanger and a small bronze or stainless steel pump, the total cost would be less than $900. The $500+ cost differential could be spent on a whole house water softener or on a technician to flush the external heat exchanger and check the system every 4 to 5 years—a wise thing to do regardless.

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