Fix it
Why does my hot water smell like rotten eggs?
A sulfur smell only on the hot side is chemistry happening inside your water heater: harmless bacteria reacting with the tank’s sacrificial anode rod to make hydrogen sulfide gas. Unpleasant, common on well water, and very fixable.
What it usually is
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Sulfate-reducing bacteria plus a magnesium anode
The magnesium rod that protects the tank feeds electrons to otherwise-harmless bacteria, which exhale the rotten-egg gas. Hot-side-only smell is the giveaway.
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A tank that sat unused
Vacation homes and guest bathrooms grow the smell in stagnant warm water; a full hot flush often clears mild cases.
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The smell is on both hot and cold
Both sides smelling means the source is the supply — common on wells — and the heater is innocent. Treatment moves to the wellhead or a whole-house filter.
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A dying anode rod shedding heavily
A nearly consumed rod accelerates the reaction; the smell strengthening over months tracks the rod’s decline.
What you can check yourself
First isolate: fill two glasses, one hot one cold, and sniff outdoors. Hot-only implicates the heater. Flush the tank fully and re-test after a day. Turning the thermostat to 140°F for a day pasteurizes the tank (mind scald risk at taps, and turn it back).
When it’s a job for a pro
The durable fix is swapping the magnesium anode for an aluminum-zinc or powered anode — plumber work if the hex head has never been broken loose. On wells with both-sides smell, water treatment is the actual product.
An anode swap typically runs $150 to $350 with the part; powered anodes are $200 to $400 but end the chemistry permanently.
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