Why does my hot water smell like rotten eggs but the cold water does not?

The smell is hydrogen sulfide gas produced inside the water heater. Sulfate-reducing bacteria in the water take electrons from the magnesium anode rod that protects the tank from rust, and hydrogen sulfide is the byproduct. Because the reaction happens in the tank, only the hot water carries the smell.

Is the rotten egg smell in hot water dangerous?

At the concentrations that make household water smell, hydrogen sulfide is a taste and odour nuisance rather than a health hazard. It is worth treating as a signal about the condition of the water heater and its anode rod.

How do I get rid of the rotten egg smell in my hot water?

Flush the tank first, then disinfect it if the smell returns. The lasting fix is replacing the magnesium anode rod with an aluminium or zinc rod, which protects the tank without feeding the reaction. Never simply remove the anode rod: an unprotected tank rusts through quickly.

Should I remove the anode rod to stop the smell?

No. The anode rod is the main reason a tank lasts as long as it does. Removing it trades a smell for a leak. Replace it with an aluminium or zinc rod instead.

What if the rotten egg smell is in the cold water too?

Then it is the water entering the house, not the heater. On a private well that usually calls for source treatment such as chlorination or aeration, and a lab test first. On a public system, check what your utility reported.

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3-minute read · My Free Water Report

Reviewed by Tyler Griffin, Founder, SwiftPro Heating, Cooling & Plumbing

Hot water smells like rotten eggs? Start with the two-tap test

Hot only means the tank. Hot and cold means the water. Ten seconds tells you which problem you have.

If the rotten-egg smell is only in the hot water, it is almost never the city's water. It is a reaction happening inside your water heater. Sulfate-reducing bacteria in the water meet the magnesium anode rod that protects the tank from rust, and the byproduct is hydrogen sulfide gas. That gas is the smell. Run the cold tap and it is usually gone.

The two-tap test, before anything else

Hot only? The water heater. Keep reading.
Hot and cold? The water coming into the house, which points at a well or a source-water issue rather than the tank.
One fixture only? That fixture's aerator or drain, not the water.

Why the smell exists at all

Every tank water heater contains a sacrificial anode rod, usually magnesium. Its whole job is to corrode so that the steel tank does not, which is the single biggest reason a heater reaches double digits in age instead of rusting through. It is doing exactly what it should.

The trouble starts when the water carries sulfate and sulfate-reducing bacteria, which are harmless to drink and common in groundwater. As those bacteria process sulfate, they take electrons from the corroding magnesium, and hydrogen sulfide gas comes out the other side. The tank is warm, low in oxygen and undisturbed. It is close to an ideal habitat.

The smell is a nuisance, not a poison. At the concentrations that make water smell in a house, hydrogen sulfide is an odour and taste problem rather than a health one. What it is worth treating as is a signal about the tank.

The fixes, in the order worth trying them

Flush the tank. Clears sediment the bacteria live in. Worth doing annually anyway, and sometimes it is enough on its own.
Disinfect the tank, typically with a peroxide or chlorine treatment held in the tank and then flushed. This kills the current population but does not change the chemistry that invited it.
Swap the anode rod for aluminium or zinc. This is the lasting fix. An aluminium or aluminium-zinc rod protects the tank the same way without feeding the reaction the same electrons. Do not simply remove the rod: a tank with no anode rusts out fast, and you trade a smell for a leak.
Turning the thermostat up to around 140 degrees suppresses the bacteria, but it raises the scald risk and the standing energy cost, so it is a stopgap rather than a plan.
On a well, treat the source if the smell is in the cold water too. Chlorination or aeration ahead of the house, not a change at the tank.

When the smell is telling you something bigger

An anode rod that has been consumed enough to drive this reaction is often an anode rod near the end of its life, which means the tank has been unprotected for some part of its recent history. If the heater is also past eight years, rumbling, or running out of hot water sooner than it used to, the smell is worth treating as one symptom among several rather than a standalone annoyance. What actually shortens a water heater's life covers the rest of that picture.

And if the smell is in the cold water as well, stop treating it as a heater problem. On a public system, check what your utility reported. On a well, it is time for a lab test: what to test for and how often.

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