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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.

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What it usually is

  1. 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.

  2. 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.

  3. 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.

  4. 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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