15 grains per gallon: what it means for a household of 3

Notes

• Two units, one number, and everyone mixes them up. 15 gpg is 257 ppm — multiply by 17.1 to go one way, divide to go back. Water utilities usually report ppm, softener manuals want gpg, and a softener set on the wrong one is out by a factor of seventeen.
• 176 pounds of dissolved mineral a year is the number worth holding on to. That is calcium and magnesium carbonate passing through the plumbing of a 3-person house at this hardness. Most of it leaves with the water; the fraction that does not is what coats the heating element, narrows the pipe and dulls the shower glass.
• Where it lands first is the water heater. Scale settles on the bottom of a gas tank between the flame and the water, so the burner has to work through an insulating layer, and on the elements of an electric tank until they overheat and fail. Flushing a few gallons off the drain valve once a year is the cheapest maintenance in the house and almost nobody does it.
• Sizing the softener is about grains per day, not about the house. 3,375 grains a day is what has to come out. A 32,000-grain unit regenerating roughly every 7.1 days handles it; a unit that regenerates daily wears out early, and one that goes three weeks between cycles lets the resin bed channel.
• Iron changes the sum, and it is not on most reports. Every 1 ppm of dissolved iron adds roughly 4 gpg of load to a softener, so 3 ppm of iron on 15 gpg water behaves like 27 gpg. Above about 3 ppm — or at any level once the pH is over 7 — a softener is the wrong tool entirely and needs a filter ahead of it.
• Test strips give you one number badly. A lab test costs little and returns hardness, iron, manganese, pH and total dissolved solids together. Those five decide the equipment; the strip decides nothing.