If your water is average in terms of hardness, then every 14-16 years id
replace the water heater. Once it starts banging when firing (indicating
that it has a load of sediment in the bottom )...its time to replace it
--- normally, its at about 12-16 years before this takes place to the
point where its banging alot, and loudly. Once you install a new water
heater, drain 5 gallons out every month, under pressure by hooking up a
hose to the drain valve. This will 'theoretically' extend its life.