Short answer: If the smell is only in the hot water, the water heater is the likely source. The Minnesota Department of Health explains that a water heater can turn sulfate in the water into hydrogen sulfide gas, both by hosting sulfur bacteria and through a reaction with the magnesium anode rod. If the smell is in cold water too, the source is the water supply, not the heater.
Quick diagnosis
| What you notice | Most likely cause | What to check | Call a pro? |
|---|---|---|---|
| Smell only in hot water | Water heater anode and bacteria | Compare hot and cold at the same tap | Often |
| Smell in hot and cold | Well water or supply (hydrogen sulfide) | Test the water | Water professional |
| Smell after the heater was off a while | Bacteria growth in standing water | Flush the tank | Sometimes |
| Smell near a drain, not the tap | Drain or sewer gas | Smell the water in a glass away from the sink | See sewer smell guide |
Confirm the source
Fill a glass with cold water and another with hot, step away from the sink and smell each. If only the hot glass smells, it is the water heater. If both smell, the supply (often a private well) is the source.
Fixes the Minnesota Department of Health describes
- Replace the magnesium anode with an aluminum-type anode (ask the heater manufacturer or dealer what is compatible). Removing the anode entirely can shorten the heater’s life.
- Disinfect and flush the tank with a chlorine bleach solution; the smell can return if bacteria survive.
- Raise the temperature to 160°F for several hours to kill bacteria, then flush, only after confirming the relief valve works, and then lower the thermostat to prevent scalding.
Most homeowners have a plumber or water treatment professional do these steps.
When to call a professional
- The smell is in cold water too (water testing needed).
- You are not comfortable replacing an anode or shocking the tank.
- You are unsure whether it is gas: leave and call the gas utility.
What it usually costs
HomeGuide (2026) lists anode rod replacement by a professional at about $150 to $350; the part alone is about $20 to $50.
Frequently asked questions
Is rotten egg smelling hot water safe?
The Minnesota Department of Health says rotten egg smells rarely indicate sewage, but recommends testing well water for coliform bacteria and nitrate to be safe.
Why does only my hot water smell?
The water heater provides warm conditions for sulfur bacteria and a magnesium anode that drives the reaction producing hydrogen sulfide.
Related problems
Sources
- Minnesota Department of Health: Hydrogen sulfide and sulfur bacteria in well water
- Penn State Extension: Common drinking water problems and solutions
- HomeGuide: Water heater repair cost (2026)
This guide is general information, not a substitute for an inspection by a licensed professional. See our disclaimer.