Impacts of Water Quality on Residential Water Heating Equipment
Accumulation of scale and corrosion can decrease the lifespan and impact performance of water heating equipment; in fact, studies have shown corrosion to decrease equipment life by more than half if not properly mitigated.
Source: Pacific Northwest National Laboratory, Operated by Battelle for the United States Department of Energy under contract DE-AC05-76RL01830
Waslix, LLC was not a participant in this study. Reference to the study does not imply certification by the United States Department of Energy or the Pacific Northwest National Laboratory, nor does it constitute or imply endorsement, recommendation, or favoring of Waslix, LLC by the United States Department of Energy or the Pacific Northwest National Laboratory.
Author: SH Widder and MC Baechler
Date: November 2013
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Impacts of Water Quality on Residential Water Heating Equipment1.17 MBIn summary:
The U.S. Department of Energy commissioned a report to assess what impact water quality has on residential water heating equipment. The investigation found that mineral scale directly impacts performance, energy usage and longevity.Page: 23
The way all these constituents and indices work together and impact appliance life can be quite complex. It depends on their relative ratios, the concentrations of other organic and inorganic substances in the water, temperature, and other factors. Sections 2.1.3 and 2.1.4 provide a general description of how these characteristics typically influence scale and corrosion in residential water heaters.
2.1.3 Scale
Scaling and sedimentation occur when the water becomes saturated with minerals contributing to carbonate hardness, principally calcium. When the water is saturated with a mineral, if any more of that mineral is added it will precipitate out of the water as a solid. In the case of calcium, calcium precipitates as CaCO3, as shown in Equation1:
This precipitation is primarily a problem in water heaters and hot water piping because CaCO3 is inversely soluble, meaning it precipitates more with increasing temperature. At cooler temperatures found in cold water pipes, the existing CaCO3 may remain in solution since the water is not yet saturated. However, higher temperatures will cause CaCO3 to precipitate out of solution and cause scale and sedimentation problems in water heaters. Other issues that contribute to scale issues are high water usage and, of course, the specific water chemistry. For example, if the water has a high degree of non-carbonate hardness, such as significant concentrations of chlorate (ClO3-) or sulfate (SO42-), this will tend to increase the solubility of calcium and decrease scale formation. In addition, high water pressure will increase scale formation, as the higher pressures will force more CaCO3 out of solution (Weingarten and Weingarten 1992).
Scaling will typically occur in hard water conditions, since the concentration of calcium ions (in the form of carbonate hardness) will be high. Also, a high pH is characteristic of hard water, and can further lower the solubility of calcium carbonate in water.
The way all these constituents and indices work together and impact appliance life can be quite complex. It depends on their relative ratios, the concentrations of other organic and inorganic substances in the water, temperature, and other factors. Sections 2.1.3 and 2.1.4 provide a general description of how these characteristics typically influence scale and corrosion in residential water heaters.
2.1.3 Scale
Scaling and sedimentation occur when the water becomes saturated with minerals contributing to carbonate hardness, principally calcium. When the water is saturated with a mineral, if any more of that mineral is added it will precipitate out of the water as a solid. In the case of calcium, calcium precipitates as CaCO3, as shown in Equation1:
This precipitation is primarily a problem in water heaters and hot water piping because CaCO3 is inversely soluble, meaning it precipitates more with increasing temperature. At cooler temperatures found in cold water pipes, the existing CaCO3 may remain in solution since the water is not yet saturated. However, higher temperatures will cause CaCO3 to precipitate out of solution and cause scale and sedimentation problems in water heaters. Other issues that contribute to scale issues are high water usage and, of course, the specific water chemistry. For example, if the water has a high degree of non-carbonate hardness, such as significant concentrations of chlorate (ClO3-) or sulfate (SO42-), this will tend to increase the solubility of calcium and decrease scale formation. In addition, high water pressure will increase scale formation, as the higher pressures will force more CaCO3 out of solution (Weingarten and Weingarten 1992).
Scaling will typically occur in hard water conditions, since the concentration of calcium ions (in the form of carbonate hardness) will be high. Also, a high pH is characteristic of hard water, and can further lower the solubility of calcium carbonate in water.
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2.1.4 Corrosion
Corrosion occurs via an electrochemical reaction, similar to the operation of a battery. To complete the electrolytic corrosion cell, there must be an anode that gives up electrons, a cathode that receives the electrons, an electrically conductive connection between them that allows electrons in the form of electricity to flow from the anode to the cathode, and an electrolyte to accept electrons from the cathode, maintaining its positive charge and keeping the flow going. In the case of a water heater, typically one area of the ferrous metal tank acts as the anode and another area of the tank acts as the cathode. The tank itself is electrically conducting, connecting the anode and the cathode, and the water with its dissolved salts acts as the electrolyte, as shown in Figure 2.4.
2.1.4 Corrosion
Corrosion occurs via an electrochemical reaction, similar to the operation of a battery. To complete the electrolytic corrosion cell, there must be an anode that gives up electrons, a cathode that receives the electrons, an electrically conductive connection between them that allows electrons in the form of electricity to flow from the anode to the cathode, and an electrolyte to accept electrons from the cathode, maintaining its positive charge and keeping the flow going. In the case of a water heater, typically one area of the ferrous metal tank acts as the anode and another area of the tank acts as the cathode. The tank itself is electrically conducting, connecting the anode and the cathode, and the water with its dissolved salts acts as the electrolyte, as shown in Figure 2.4.
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2.2.1 Hard Water and Scale
The primary impact of hard water on water heater performance is the buildup of scale on heating components including heat exchangers, heating elements, or the bottom of the tank. This will decrease efficiency in some cases, but most likely will not lead to premature failure of the equipment. Lime scale buildup overtime can also clog hot water pipes, the drain valve on the water heater, or the temperature and pressure relief valve on the water heater, which impact the maintenance and safety-related performance of the water heater (Weingarten and Weingarten 1992). In addition, because the sediment particles are heavier than water, once a coating has been formed on available surfaces, they tend to sink to the bottom and accumulate at the bottom of the tank. This accumulation of free sediment at the bottom of the tank is more likely to result in reduced equipment lifetime due to overheating elements in electric storage water heaters and overheating tank material in gas storage water heaters at the bottom underneath the sediment, which can eventually cause the water heater to fail (Weingarten and Weingarten 1992).
2.2.1 Hard Water and Scale
The primary impact of hard water on water heater performance is the buildup of scale on heating components including heat exchangers, heating elements, or the bottom of the tank. This will decrease efficiency in some cases, but most likely will not lead to premature failure of the equipment. Lime scale buildup overtime can also clog hot water pipes, the drain valve on the water heater, or the temperature and pressure relief valve on the water heater, which impact the maintenance and safety-related performance of the water heater (Weingarten and Weingarten 1992). In addition, because the sediment particles are heavier than water, once a coating has been formed on available surfaces, they tend to sink to the bottom and accumulate at the bottom of the tank. This accumulation of free sediment at the bottom of the tank is more likely to result in reduced equipment lifetime due to overheating elements in electric storage water heaters and overheating tank material in gas storage water heaters at the bottom underneath the sediment, which can eventually cause the water heater to fail (Weingarten and Weingarten 1992).
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In addition to impact on energy efficiency, scale accumulation can have an impact on equipment longevity as well. New Mexico State University reports that by reducing and preventing mineral limescale buildup on pipes and appliances, households could realize longer life for their appliances varying between 25 and 40 percent (Konigsberg 2011).
2.2.3 Market Impacts
An assessment of homeowners’ insurance claims resulting from water heater failures conducted by the Insurance Institute for Business & Home Safety (IBHS 2007) found that water heater failures are one of the top five “water loss” claims, or claims in which monetary compensation is sought as a result of water damage to furniture, equipment, or the home itself. Of the 700 water-heater-related water loss claims analyzed, 69 percent were due to leaks in the water heater or to the tank bursting, as shown in Figure 2.7. The water heater failures cost an average of $4,444 per incident after the deductible was paid and the average age of failure was 10.7 years, but ranged from 1 to 30 years.
Figure 2.7 Percentage of Total Water Loss Claims Attributed to Each Failure Mechanism: Leak in Water Heater or Burst Tank, Unknown, Supply Line Failure, Valve Malfunction, or Other. Source:IBHS 2007.
In addition to impact on energy efficiency, scale accumulation can have an impact on equipment longevity as well. New Mexico State University reports that by reducing and preventing mineral limescale buildup on pipes and appliances, households could realize longer life for their appliances varying between 25 and 40 percent (Konigsberg 2011).
2.2.3 Market Impacts
An assessment of homeowners’ insurance claims resulting from water heater failures conducted by the Insurance Institute for Business & Home Safety (IBHS 2007) found that water heater failures are one of the top five “water loss” claims, or claims in which monetary compensation is sought as a result of water damage to furniture, equipment, or the home itself. Of the 700 water-heater-related water loss claims analyzed, 69 percent were due to leaks in the water heater or to the tank bursting, as shown in Figure 2.7. The water heater failures cost an average of $4,444 per incident after the deductible was paid and the average age of failure was 10.7 years, but ranged from 1 to 30 years.
Figure 2.7 Percentage of Total Water Loss Claims Attributed to Each Failure Mechanism: Leak in Water Heater or Burst Tank, Unknown, Supply Line Failure, Valve Malfunction, or Other. Source:IBHS 2007.Page: 35
3.0 Conclusions and Recommendations
When determining the appropriate selection and application of water heating equipment, especially when determining the cost-effectiveness of more expensive, high efficiency equipment options, the lifetime of equipment is very important. Increasing the lifetime of water heaters can improve the cost-effectiveness and increase the savings achieved, which can offset the initial investment in an energy-efficient water heater. Accumulation of scale and corrosion can decrease the lifetime and impact performance of water heating equipment; in fact, studies have shown corrosion to decrease equipment life by more than half if not properly mitigated (Weingarten and Weingarten 1992). In addition accumulation of scale will impact the efficiency of gas storage and tankless water heaters (Paul et al. 2010).
In Section 3.1, selection and installation recommendations are provided for electric storage water heaters, gas storage water heaters, and gas tankless water heaters based on information in the literature. Section 3.2 identifies gaps in the current body of knowledge surrounding the impact of water quality on residential water heating equipment and recommends opportunities for future investigation.
3.0 Conclusions and Recommendations
When determining the appropriate selection and application of water heating equipment, especially when determining the cost-effectiveness of more expensive, high efficiency equipment options, the lifetime of equipment is very important. Increasing the lifetime of water heaters can improve the cost-effectiveness and increase the savings achieved, which can offset the initial investment in an energy-efficient water heater. Accumulation of scale and corrosion can decrease the lifetime and impact performance of water heating equipment; in fact, studies have shown corrosion to decrease equipment life by more than half if not properly mitigated (Weingarten and Weingarten 1992). In addition accumulation of scale will impact the efficiency of gas storage and tankless water heaters (Paul et al. 2010).
In Section 3.1, selection and installation recommendations are provided for electric storage water heaters, gas storage water heaters, and gas tankless water heaters based on information in the literature. Section 3.2 identifies gaps in the current body of knowledge surrounding the impact of water quality on residential water heating equipment and recommends opportunities for future investigation.
