The City of San Diego reports its tap water averages about 16 grains per gallon (276 parts per million) of hardness, which counts as very hard. That mineral content ends up inside water heaters.
Tank heaters: sediment
In a tank heater, minerals settle at the bottom as sediment. It insulates the water from the burner, makes popping and rumbling noises, and can shorten the tank's life. Periodic flushing helps.
Tankless heaters: scale
In a tankless heater, minerals build up as scale in the heat exchanger. The Department of Energy says scale restricts flow and shortens the unit's life, recommends flushing from twice a year to every three years, and notes manufacturers may require a softener above about 8 to 14 grains per gallon.
The softener question
The City of San Diego says its water is balanced and does not need a softener, and that softeners remove minerals in a way that makes water more corrosive to plumbing. Tankless manufacturers may still require one for warranty purposes. Read your warranty, and ask an installer about scale protection options.
General information, not professional or legal advice; follow your manufacturer's manual, and your installer and local permit office have the final word. Sources:City of San Diego, Water Quality (hardness); City of San Diego, Water Quality FAQ; U.S. Department of Energy, Building America: Transitioning to a Tankless Water Heater.
Water Heater Repair in San Diego →