In response to requests from Plumbing Manufacturers Intl. (PMI) and its members, as well as from other supporters of the U.S....
Manufacturers are often asked to size a softener system for a “typical” residential application. I would like someone to please define the attributes of the influent water supply and makeup of this so-called “typical” residence. Is it the one with three residents, 10 grains per gal of total hardness and 2 mg/L of chorine? Or does it have seven residents, 25 grains per gal hardness, plus 2 mg/L of iron? How about the 4,000-sq-ft home with two permanent occupants and the occasional temporary increase to nine residents?
The point is that there is no “typical” residential application, with the exception of specific geographic regions where sizing requirements and contaminant levels are very consistent. Let’s examine some of the parameters used to determine the most efficient and “best” system for various “typical” residential point-of-entry (POE) applications.
Calculating softener capacity requires a basic understanding of salt to hardness removal factors. For example, it is common to see water softeners advertised with 30,000 to 35,000 grains removal capacity per cu ft. Based on favorable conditions, 35,000 grains per cu ft can be obtained by regenerating with about 20 lb of salt. This calculates to an efficiency rating of 35,000 grains divided by 20 lb of salt, which results in 1,750 grains of hardness removed per pound of salt. This is atrociously inefficient.
Many regulations now require an efficiency rating of 4,000 grains removed per pound of salt. The efficiency curves show significantly higher efficiency ratings as salt levels are reduced. Under favorable influent and proper equipment conditions, you can expect to achieve in excess of 4,000 grains per pound of salt in the 3 to 5 lb per cu ft range. In most conditions, hardness leakage will remain less than 1 grain per gal even at these lower, environmentally friendly salt settings. This level of hardness leakage will still allow excellent soft water performance for residential applications.
In most applications, plan to use 5 lb of salt per cu ft of resin to achieve about 20,000 grains of capacity per cu ft. Use a quality hardness test kit to calculate the hardness. Add 3 to 4 grains for every part per million (ppm) of iron and manganese. Thus, for a hardness of 14 grains per gal with 1.5 ppm of iron, calculate your capacity based on about 20 grains per gal compensated hardness. For a 1-cu-ft system regenerating with 5 lb of salt, you will generally expect about 20,000 grains of capacity divided by 20 grains of hardness, which results in a capacity of about 1,000 gal between regenerations.
Standard single meter delayed systems may reach the calculated capacity nearly 24 hours before regeneration will be initiated. These systems will require a reserve capacity to be calculated, usually based on one day’s water use. This can significantly alter the efficiency of the system, as it will normally regenerate far earlier than actually needed.
A softener with a 1,000-gal capacity being used by a household of five using an average of 75 gal per person daily will need to be set to regenerate about every 625 gal. Using a higher capacity softener, a softener with an automatically adjusted reserve capacity or one with immediate regeneration will help mitigate this inherent inefficiency with single systems. Twin alternating systems that theoretically use the entire capacity of each unit before regenerating are an excellent option.
POE residential softener systems have both maximum and minimum flow rate capabilities. The peak flow should be maintained in the range of about 15 to 20 gpm per sq ft of bed area. This calculates to a peak service flow of about 9 gpm for a 9-in. diameter mineral tank. Excessive flow rates will cause increased pressure drop and can actually damage or crush the resin beads. In the most extreme cases, there may not be enough contact time to accomplish adequate ion exchange.
On the low end, most meter-initiated control valves can accurately monitor flow rates well under 0.5 gpm. However, flow rates of less than about 3 gpm per sq ft of bed area will result in channeling within the mineral tank. In excessively low flow conditions, the water enters the mineral tank following the path of least resistance. This is typically right through the center of the resin bed, with the pattern of flow resembling a funnel. This leads to poor distribution of the hardness removal, lower than expected capacity and loss of efficiency.
Much of the flow even in larger homes is at or below 2.5 gpm. To maintain good flow characteristics at 2.5 gpm, the system would need to be no larger than 12 in. in diameter. At about 0.78 sq ft of bed area, a 12-in. diameter mineral tank can theoretically flow relatively consistently down to about (3 x 0.78) 2.34 gpm. Compare this to a 16-in. diameter tank that will flow consistently down to about (3 x 1.39) 4.17 gpm. A flow rate of 2.5 gpm through a 16-in. diameter tank would likely channel within the resin bed requiring an “adjustment” to the calculated capacity setting. When a larger residential system is required, consider using multiple smaller systems that can be brought online based on flow rate needs. These demand flow style systems are perfect for commercial and some large residential applications that have wide swings in water flow demand.
Poorly sized POE water softener systems often take care of the hardness removal duties, and in many cases, the residents will not even notice the otherwise poor and inefficient overall performance. These incorrectly sized systems generally work because they are regenerated too often, using exorbitant amounts of salt and water.
An undersized unit may regenerate daily, putting excessive strain on the resin and valve components. Under sized systems are salt hogs, using a large portion of their capacity to cover the required reserve capacity. Pressure drops can be drastic, and system longevity is significantly compromised.
Oversized systems have lower than calculated service runs and may need to regenerate far ahead of actual capacity capabilities. Specifying the correct system for a particular job is one of your best defenses against the low-price cookie- cutter softeners found in so many large retail outlets. Learning to size properly will help you sell more systems, focusing on efficiency, environmental concerns and overall value.