For decades, water softeners have been used to remove the hardness caused by minerals --- most often calcium and magnesium --- dissolved in the water table. Ion exchange resins in the softeners remove the mineral content from the water, releasing sodium in its place. Although a typical ion exchange resin will last 20 to 25 years, it frequently needs to be refreshed with salt so it continues to operate properly. Knowing when to regenerate the resin will keep your softener working satisfactorily for many years.
Take a sample of your water, and send it to a laboratory to obtain an analysis of the content of the water. Specifically, request the amount of iron in the water and the total hardness of the water. These numbers are reported in ppm (parts per million) or mg/L (milligrams per litre).
Find the documentation on your water softener (often in the owner's manual), and discern the size of your resin bed. Typical values reported for the bed size are grains capacity or cubic feet of resin. This gives the amount of hardness that the resin can remove.
Calculate the adjusted total hardness per gallon for your water supply. Calculate this value by adding the total hardness value reported by the laboratory analysis and three times the iron amount in the water. For example, if the total hardness reported was 24 ppm and the total iron reported was 3 ppm, then the adjusted total hardness per gallon is equal to 24 + 3 x 3 = 33 ppm.
Convert the ppm or mg/L hardness to grains. Grains are the units used to measure the capacity of the resin. To convert to grains, divide the number of ppm by 17.1. For the example given, the water is 33 ppm / 17.1 = 1.93 grains. Therefore, the water supply contains 1.93 grains of hardness per gallon of water.
Find the capacity of your softener resin in gallons of water before resin regeneration is required. Convert the resin bed to grains capacity by using the identity that 1 cubic foot of resin equals 32,000 grains capacity. For the purposes of the example, you can use a 48,000 grains capacity. Recommendations are designed to regenerate the resin at 80 per cent capacity of the resin, so the number of gallons of water that are available before regeneration is given by (water softener grains capacity x 80%) / adjusted total hardness in grains = (48,000 x 0.8) / 1.93 = 19,896 gallons.
Figure out the number of days between regeneration cycles based on daily water usage. For purposes of the example, use 450 gallons per day. Divide the gallons between regenerations by the daily usage to get the number of days between regeneration cycles. Therefore, 19,896 / 450 = 44.2 days between regeneration cycles. Always round down so that the regeneration takes place before there is any chance of obtaining hard water from the softener. In this example, you would set the regeneration timer on the softener for 44 days.