Like a conventional water heater, the longevity of an all-in-one system can be maintained by the simple and regular replacement of the anode rod, a sacrificial piece of metal that is suspended in the steel water-heater tank. Although the inside of the tank is glass-lined, microscopic cracks can develop, leading to rusting of the steel shell without an anode rod. An electrochemical reaction corrodes the anode rod instead of the steel tank, helping to extend its life. Corrosion will eat away at the anode rod over time, necessitating its replacement. Replacing the anode rod every three to five years can extend the tank’s life. Magnesium anode rods cost about $20 and are very simple to replace.
Some tanks have electric anodes, which use a small DC current to prevent corrosion. This technology is usually found in commercial hot water tanks, where high usage can accelerate corrosion.