The single most important factor in whether a water heater lives or dies is the condition of its sacrificial anode. For more than 60 years, it has been used as a key part of the rust protection of a tank, although few people know it’s there.

This is a rod made of magnesium or aluminum that’s formed around a steel core wire and is screwed into the top of the tank. A six-year-warranty residential tank will have one, while a 12-year-warranty tank will have two, or an extra-large primary anode. Commercial tanks have from one to five. Special aluminum/zinc sacrificial anodes or powered (impressed-current) anodes can be used to resolve odor problems caused by bacteria in some water. But if you have a vacation home where the water heater sits idle for long periods of time, using them may not be a solution.

When the tank is filled with water…