Rotten Egg Smell in Hot Water

The smell is hydrogen sulfide gas, produced when naturally occurring sulfate-reducing bacteria in the water react with the hydrogen given off as a magnesium anode rod sacrificially corrodes inside the tank. Magnesium is the most reactive metal commonly used for anode rods, which is exactly why manufacturers favor it for tank protection and exactly why it is also the most common cause of this specific odor. The reaction happens inside the tank itself, which is why the smell tends to appear only at hot taps.

Run cold water from an outdoor spigot ahead of any softener or filter to test the raw incoming supply directly before drawing a conclusion.

An anode rod is designed to corrode preferentially, sacrificing itself so the tank's steel does not, and magnesium is chosen for many tanks because it is a highly reactive metal that protects effectively. That same reactivity releases hydrogen ions readily as the rod corrodes, and in water chemistry that includes sulfate-reducing bacteria, that hydrogen becomes the fuel those bacteria use to produce hydrogen sulfide right inside the tank. Two tanks with the same magnesium rod, in different water chemistry, can produce very different results, from no smell at all to a strong one.

There are two real fixes, and both work by removing the magnesium reaction rather than by treating the smell after the fact. Replacing the magnesium rod with an aluminum-zinc alloy rod is the lower-cost option: it is a far less reactive metal, produces less hydrogen, and commonly resolves the smell while still protecting the tank sacrificially. Installing a powered anode instead protects the tank electrically rather than sacrificially, so nothing is corroding to produce hydrogen in the first place, eliminating the reaction entirely rather than just reducing it. The guide on replacing an anode rod covers the swap itself, and the compare pages on magnesium versus aluminum anode rods and powered versus sacrificial anode rods go deeper on choosing between the two.

An aluminum-zinc rod is cheaper up front and commonly resolves the smell, but like any sacrificial rod it wears out over years and eventually needs checking and replacing again. A powered anode costs more initially, but it never sacrifices any metal, so it produces no hydrogen sulfide reaction at all and needs no periodic rod replacement, only an occasional check that the power module itself is functioning. A household on well water, or one that has already gone through more than one magnesium or aluminum rod fighting the same recurring smell, is where the powered anode's higher upfront cost tends to pay for itself in avoided repeat visits.

A sacrificial anode rod is designed to corrode away completely over its service life, and the anode rod life calculator estimates roughly how long that takes for a specific tank and water condition. Once a magnesium rod is fully consumed, the reaction that produces the smell can actually lessen, since there is less reactive metal left to generate hydrogen, but that is not good news: with the rod gone, the tank's own steel is what starts corroding unprotected, which shortens the tank's remaining service life. A fading smell on an old rod is not a sign the problem solved itself; it is closer to a sign the rod needs checking and likely replacing before the tank starts taking the damage the rod was there to prevent.

Then the anode rod is not the cause, since the reaction that produces this smell happens inside the water heater tank, not in the cold supply line. A smell present at both hot and cold points to sulfate-reducing bacteria or dissolved sulfur in the incoming water itself, which is particularly common on private well water. Testing and treating the water supply is outside what a water heater page can responsibly cover; the water quality testing guidance at HomeWaterCalc is the place to start for that side of the problem, and no anode rod swap on the water heater will resolve a smell that originates in the supply itself.

Either route below removes the magnesium reaction causing the odor; a powered anode removes it permanently, an aluminum-zinc rod removes it at a lower upfront cost.