If you have to replace your water heater,or, if you have replaced it in say the last year, heres a way to make it last almost indefinitely.(not including elements and thermostats) Write down the name,model number, serial number, size(gallons) and either 2 elements, 1 elements and any other info on the name plate. Google the manufacturer and fine their phone number, and or web address. What you are wanting is a "sacrificial rod" It's similar to the Zinc plate on salt water boats. In water heaters, they are either A) A large nut looking device in the top center of you heater, in between the clod and hot water hook-ups. or; B) they are made into the cold water fill tube it's self (where the cold water enters the water heater, a tube below hookup decends down to just above the bottom of the tank. After about 2 to 3 years , depending on how good/bad the chemical content of your water is, these rods will stop collecting the matter that is released from water as it is heated with electricty. When the rod is shot,the chemicals begin to attach themselves to the tank and begin to destroy it. Changing this rod every few years will stop this. These rods can only be purchased from the manufacturer.