Water Softener Sizing
Compensated hardness, daily grain load, days between regenerations, and salt per regen and per year for a water softener.
Formula and source
comp_hardness = hardness_gpg + iron_ppm x 4; daily_gal = people x use_per_cap; grain_load = daily_gal x comp_hardness; days_between = floor(capacity / grain_load); annual_salt = salt_per_regen x (365 / days_between). 1 gpg = 17.1 ppm.
NSF/ANSI 44 and the Water Quality Association (WQA) hardness/iron-compensation practice by name; first-principles ion-exchange grain budget.
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