Softening Domestic Hot Water
As a general rule of thumb, domestic hot water should rarely be completely softened, but why?
Domestic potable water distribution systems are installed in almost every building that is built, and not much attention to pretreatment is typically required. After all, the majority of commercial buildings utilize local municipal water treatment facilities to provide water according to the Primary Drinking Water Regulations. However, not all potable water is equal when it comes to large water distribution.
From a maintenance standpoint, two main problems typically arise regarding domestic water: corrosion and scale formation. Cold water, being maintained at a lower temperature, typically won’t have issues relating to corrosion, or issues are often slow to proceed. Most of the corrosion issues/failures of copper domestic water piping have been the result of either erosion-corrosion or cold-water pitting. Water softening is typically not required although provides certainty that calcium-carbonate scale formation will not occur within pipes and point-of-use locations. Softeners also have the ability to remove iron and unwanted discoloration.
Scale-formation and corrosion become increasingly more of a problem when water temperatures are elevated and maintained. Because domestic hot water systems are much more subject to corrosion and scale problems, it is not to be unexpected that plugging of piping from scale and corrosion product buildup, discoloration of water, inefficient heat transfer in heating and cooling equipment, and serious corrosion causing penetration of piping and leakage may occur in building water systems (Lane 1993). Water stagnation in hot water environments becomes another area of concern due to the presence of waterborne pathogens. Corrosion byproducts can become noticeable as brown staining and rust tubercles. Some corrosion can be attributed to the complete softening of the makeup water. Using softened water in domestic hot water systems typically will increase the corrosion tendency as evidenced by using the LSI (Langelier Saturation Index) or another credible index. Most corrosion-related issues can be resolved by partial blending with hard water to obtain a hardness residual between 60 to 120 mg/L (as CaCO3) depending on influencing parameters such as temperature, pH, alkalinity, and conductivity. But of those variables, hardness is probably the easiest to manipulate.
While scale/corrosion tendencies can be prevented, oftentimes these issues are dealt with as they arise. Both problems can be quantifiable with water samples or corrosion coupons. Figure 1 illustrates the LSI, a calcium carbonate saturation index that is quite effective at predicting scale formation. While it doesn’t take into account the presence of iron or other contaminants, the LSI will allow water treaters to better prescribe a ratio to which hard water can be blended to prolong the lifespan of potable water equipment. It is my opinion that scale/corrosion inhibitors for potable water should be avoided at all costs, but chemicals such as sodium silicate and sodium carbonate can be approved for these applications. These issues can also be solved with thermostatic mixing valves or blending hard/soft water before building water heaters. At the end of the day, local water treatment experts should be utilized beforehand to identify potential problems associated with potable water. A small water analysis fee can prove to be a worthwhile investment down the road.
List of Resources:
 Lane, R. W. (1993). Control of Scale and Corrosion in Building Water Systems (1st Ed). McGraw-Hill.
 Kotz, F. (2020, May 22). Does Water Softening Promote Corrosion of a Hot Water Heater? Kotz Heating, Cooling & Plumbing. https://kotzheating.com/blog/does-water-softening-promote-corrosion-of-a-hot-water-heater/
 Saturation and Water Balance. (n.d.). CHEMTROL Australia. https://chemtrol.com.au/knowledge-centre/saturation-water-balance/