Theoretically a water heater should rust out from the inside in a matter of months. But this doesn’t happen thanks to water heater anodes, the “secret ingredient” that keeps water heaters up and running and virtually rust-free throughout their existence.
Water Heater Anodes 101
Water heater anodes, which are more commonly known as sacrificial anodes, are long rods of magnesium or aluminum wrapped around a steel wire core. When placed inside a water tank they will gradually corrode and would eventually rust down to nothing if not replaced within two or three years. Because the sacrificial anode rusts away the steel interior of the tank remains untouched, and it will stay that way as long as new sacrificial anodes are added as needed.
It is a basic principle of science— when two metals are in contact inside a watery solution they will not rust at the same time. Instead, whichever one is more reactive (prone to rusting) will corrode away completely before the second metal is touched. As it happens magnesium and aluminum are both far more reactive than steel, and that is the secret to the sacrificial anode’s success.
Water Heater Anode Options
Sacrificial anodes come in one of two models. Hex-head anodes screw in at the tank and extend down into the water, while combo anodes are an integrated component of the hot-water delivery assembly, extending into the tank through the hot water outlet. Water heaters are generally sold with either a hex-head anode exclusively or with one anode of each type, which explains why some new heaters are sold with six-year warranties and others with 12-year guarantees (two sacrificial anodes are twice as effective as one).
For those who prefer an option that will keep on working continuously and without degredation, a powered anode can be used in lieu of the sacrificial type. This version of the protective anode is powered by electricity and stops interior tank corrosion by releasing a small but continuous flow of electric current into the water. This interrupts the electrochemical reaction that normally causes metallic corrosion.
Because it never rusts at all a powered anode can last indefinitely. Of course it is a far more expensive than the traditional option, generally running in the $250 range (a conventional sacrificial anode will cost from $40 to $75).
Magnesium sacrificial anodes are more common now than the aluminum versions. As it rusts aluminum leaves behind significant waste product that can clog water pipes and fill tank bottoms with nasty gunky sediment, which can threates a water heater’s long-term survival. Aluminum anodes also swell a bit as they rust and at times can be incredibly difficult to remove.
Meanwhile powered anodes have great utility when water is highly soft. Soft water is more corrosive than hard and will tend to wear out sacrificial anodes so frequently they’re no longer a cost-effective choice. Powered anodes will also kill bacteria that can infect water heater tanks and create terrible odors that follow the water wherever it flows.
Making a Change
Homeowners and landlords should remove sacrificial anodes from water heater tanks at least once per year to check for excessive wear and tear (this is an easy task, all that’s required is an adjustable wrench). If several inches of the steel wire core is visible too much corrosion has taken place and it’s time to get a new anode.
Water heater anodes are not automatically interchangeable, so anyone who needs to buy a new one should check with the dealer or the manufacturer first to find out which anodes are compatible with their heater.
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