Quick Answer:
If you only notice a smell when you run hot water, the hot side of the system is releasing or concentrating the odor — often from bacteria in warm plumbing, a reaction inside the water heater, or trapped debris in hot-side fixtures. Start by comparing odor strength between hot and cold runs and testing at several fixtures to narrow the source.
Why This Happens
Warm temperatures encourage bacteria and organic growth inside the water heater and in hot-water lines. When the heater or hot pipes heat that water, gases like hydrogen sulfide (the “rotten egg” smell) or other odors can be released into the water and carried out of the tap. Corrosion or reactions between water and anode rods or piping materials can also create smells.
Sometimes the odor comes from nearby drain lines or trap issues that only get noticed when hot water changes flow patterns. If the problem began after a brief supply interruption, check guidance for Sink smells after city outage as that situation can leave trapped water or sediments that produce odor.
Step-by-Step What to Do
1. Compare odor intensity between hot and cold water cycles
Run cold and then hot water at the same fixture for a minute each, smelling the stream at the faucet. Note which is stronger and whether the smell appears immediately or after a minute of running. Do this at multiple fixtures (kitchen, bathroom sink, shower) so you know whether the issue is housewide or localized.
2. Identify whether it’s a fixture or the whole hot system
- If only one faucet smells, clean the aerator, remove and check the supply hoses, and inspect the drain for trapped debris.
- If several taps smell, the heater or shared hot piping is more likely the source.
3. Flush the water heater
Sediment buildup can trap bacteria and odors. Flushing the tank can clear debris: shut off the heater power or gas, connect a hose to the drain valve, and run until the water runs clear. If you’re unsure how to do this safely, call a plumber.
4. Consider a controlled temperature adjustment
Raising the heater temperature temporarily to about 140°F for a few hours can reduce bacterial growth, but this increases scald risk. If you try this, warn household members, then lower the setting afterward. If you’re uncomfortable with this, ask a professional.
5. Check the anode rod and heater components
Some anode rod materials react with water and create odors. Inspect or have a technician check the anode and the tank for corrosion. Replacing the anode or replacing parts may remove the source.
6. Clean fixtures and traps
Remove aerators, shower heads, and P-traps where practical. Clean out biofilm and debris, then re-test. If the smell seems related to drains rather than the water itself, look into solutions for sink drain odors and troubleshooting the P-trap and venting. See guidance about Bad smell from bathroom sink drain for common drain fixes.
7. For well water, consider professional disinfection
Well systems can develop sulfur bacteria that require shock chlorination or other treatments. This should be done by or under the guidance of a professional to avoid damaging plumbing and to ensure safe water afterward.
What Not to Do
- Do not assume hot-water odor is always the water heater.
- Do not pour bleach or other strong chemicals into the water heater unless instructed by a professional — this can be dangerous and damage the tank.
- Do not raise the temperature to eliminate smell without understanding scald risks and informing household members.
- Do not ignore a new sulfurlike smell if you also detect gas odor; treat possible gas leaks as an emergency and leave the building.
When to Call a Professional
- The smell persists after flushing and cleaning fixtures.
- You find black/gray particles, unusual discoloration, or corrosion in the tank or piping.
- Your home uses well water and you suspect bacterial contamination or sulfur bacteria.
- You’re not comfortable performing any of the steps above or can’t access heater components safely.
Safety Notes
- Hot water at elevated temperatures causes scalding. Warn others before increasing the heater setting and restore a safe temperature afterward.
- If you smell natural gas (strong rotten-egg-like odor combined with other signs), evacuate and call emergency services — do not try to diagnose indoors.
- Avoid mixing bleach with ammonia or other cleaners; chemical reactions can create hazardous gases.
Common Homeowner Questions
- Why does the smell stop after running water for a bit? Running flushes the gas or bacteria from the line; the immediate water may carry the trapped smell, then fresher water follows.
- Can I fix this by myself? You can try cleaning aerators, flushing the heater, and comparing hot vs cold, but complex issues or well disinfection are best left to professionals.
- Is the water unsafe to drink? An odor alone doesn’t always mean dangerous contamination, but persistent unusual odors should prompt testing or professional help to be safe.