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The Anode Rod: The Part That Decides How Long a Tank Lasts

Most water heaters are replaced because of a part that costs a fraction of the tank and was never looked at.

Answer first

The anode rod is a consumable: it corrodes so that the steel tank does not. Once it is used up the tank itself begins to corrode, which is the point at which a heater starts dying. Almost nobody ever replaces one.

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A water heater with its anode rod intact, and one with the rod spentTwo storage water heaters drawn in section. In the first, a rod hangs most of the way down inside the tank and the steel shell is unmarked. In the second, only a stub of the rod remains and the shell is marked with pitting where corrosion has begun.ROD INTACTthe rod corrodes instead of the tankand it is a part you can changeROD SPENTnothing is left to give, so thesteel starts going instead
It is designed to be destroyed. Once it has been, the corrosion has to go somewhere else, and the only thing left is the tank — which is not a part you can change.
1

What it does

A water heater tank is steel with a glass lining, and the lining is never perfect. Wherever it is imperfect, bare steel meets water. The anode rod is a more reactive metal suspended in the tank so that it corrodes preferentially — it is sacrificed to protect the steel.

2

What happens when it is used up

Nothing, at first. Then the steel begins corroding instead, and the clock on the tank starts running. By the time it shows as a leak from the tank itself, the heater is finished and there is no repair.

3

Why so few are replaced

Because nothing tells you. The heater works normally with a spent anode, and unless somebody opens it up and looks, there is no symptom to notice until the tank fails.

4

Where it fits in the economics

DOE puts a storage tank's life at 10 to 15 years. A heater that reaches the top of that range and one that fails at the bottom of it often differ in how their water was treated and whether anyone ever checked the rod.

Source: US Department of Energy, Which Water Heater Is Right for You?.

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