What is an anode rod?
Updated August 2026
An anode rod is a sacrificial metal rod that threads into the top of a water heater and corrodes on purpose so the steel tank does not. It is a magnesium or aluminum shell formed around a thin steel core wire, roughly 3/4 inch thick and nearly as long as the tank is tall. Because that metal is more reactive than the tank wall, the water inside attacks the rod first.
Where it is
The rod lives inside the tank, screwed into a hex-headed port on the very top of the water heater. Sometimes the head sits out in the open; often it hides under a plastic cap or the sheet-metal lid, and on some models the rod is built into the hot-water outlet nipple. To inspect it, cut the power or gas, close the cold-water inlet, drain a couple of gallons to drop the level below the port, then break the 1-1/16 inch hex head loose with a socket and a breaker bar.
What it does
The rod's whole job is corrosion insurance. Every steel tank carries a small internal current that eats metal, and as long as reactive rod remains, that current consumes the rod instead of the tank. Once the rod is used up, the current turns on the tank steel, which rusts from the inside until it springs a leak, usually somewhere in years 8 to 12. A spent anode is the single biggest reason water heaters rust out. The classic warning sign is a rotten-egg smell in the hot water: a magnesium rod reacting with sulfur bacteria gives off hydrogen-sulfide gas, and switching to an aluminum-zinc rod almost always clears it.
Maintenance
Check the anode rod every 3–5 years, or every 1–2 years in hard water or on a home with a water softener, because softened and hard water are both more aggressive and wear anodes down faster. Replace it once the bare steel core shows along more than half its length or it is buried in scale. Time the check with your yearly tank service: while the heater is already drained, flushing the sediment and pulling the rod is one trip to the basement, not two.
What it costs to replace
A replacement anode rod runs $20–50 as a DIY part, or $200–300 installed by a plumber. Most of the professional price is labor, draining the tank and breaking a factory-tightened part loose, not the part itself.
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Frequently asked questions
How do I know if my anode rod needs replacing?
Pull it out and look. Replace it if more than half its length shows bare steel core wire, if it has thinned to the width of a pencil, or if it is caked in a thick calcium crust. You can also read it from the outside: rusty or discolored hot water, a persistent rotten-egg smell, or a tank more than six years old that has never been checked all point to a rod that is overdue.
Should I use a magnesium or aluminum anode rod?
Magnesium protects the tank best and is the right default for soft or normal water, but it wears faster and is the usual culprit behind rotten-egg odor. Aluminum (often sold as aluminum-zinc) lasts longer, handles hard water well, and clears the sulfur smell, though it sheds more sediment. A powered titanium rod is a set-and-forget upgrade that never wears out, for around $150.
Why does my hot water smell like rotten eggs?
That sulfur smell is hydrogen-sulfide gas, created when a magnesium anode rod reacts with sulfate-reducing bacteria living in the tank. It is a nuisance, not a health hazard, and it shows up only on the hot side. The durable fix is swapping the magnesium rod for an aluminum-zinc or powered rod; a quick stopgap is draining the tank and shock-treating it with hydrogen peroxide.