http://waterheatertimer.org/How-to-replace-water-heater-element.html
Effect of hard-water WITH water-softener:
Efficiency of gas and electric water heaters is unchanged over time when hard water is softened, however softened water corrodes anode rod and will reduce life of water heater unless anode rod inspected and replaced every 2 years. Once anode rod is 50-75% corroded, water heater tank begins to rust. This condition leads to leaking water heater which cannot be repaired. Rusted water heaters can rupture and cause massive damage as water runs until problem is discovered.
Effect of hard-water with NO water-softener:
Efficiency of gas heater drops 6-8% after two years > cleaning tank of sediment build-up will not restore full efficiency > however, regular flushing to prevent sediment maintains efficiency
> Tankless gas heater will fail when pressure sensor clogs with sediment within 2-3 years depending on hardness of water > regular de-liming will not restore efficiency > softener and filter are required
> Efficiency of electric heater stays at 99% until sediment reaches bottom element > cleaning tank and replacing lower element restores full efficiency > regular flushing to prevent sediment build-up also maintains efficiency
