Quick Answer:
If you smell rotten eggs only from the cold taps, the source is usually in the supply side (city mains or a private well) rather than your water heater. First, smell cold water at multiple fixtures to confirm it is not heater-related. If the odor is present at several cold outlets, contact your water supplier or test your well water. Short-term flushing sometimes clears the smell; persistent cases need testing and professional treatment.
Why This Happens
The rotten-egg smell is caused by hydrogen sulfide gas. It can form when bacteria in pipes or groundwater react with sulfur compounds. Common reasons for cold-water-only odor include:
- Municipal supply changes or disturbances (repair work, main breaks, or after outages) that release gases into the lines.
- Private wells with sulfate-reducing bacteria or organic matter near the well.
- Stagnant water in rarely used lines that allows bacteria to grow.
Water heaters can also produce a sulfur smell, but that typically affects hot water. For heater-specific problems, see Rotten smell after water heater replacement.
Step-by-Step What to Do
Step 1 — Confirm it’s cold-water only
- Turn on a cold tap in the kitchen and a cold tap in a bathroom. Smell both. Then open an outdoor hose bib if you have one and smell that cold flow.
- Smell the hot water at one of the same fixtures. If the smell is only on cold taps and not hot, the heater is less likely to be the cause.
- Smell cold water at multiple fixtures to confirm it is not heater-related.
Step 2 — Check neighbors and utility notices
- Ask a neighbor if they notice the smell or call your water utility. If the problem affects multiple homes, it is likely in the public supply.
- Utilities sometimes post advisories after maintenance or outages; a sulfur smell can follow those events. Read any recent notices or call the utility if unsure. Information about similar issues after outages can be found in articles like Sulfur smell after city water outage.
Step 3 — Flush and retest
- Run cold water from taps for several minutes to clear stagnant water. Try the main cold tap and an outdoor hose bib.
- Recheck the smell after flushing. If it clears and stays cleared, the issue may have been stagnant water in the lines.
Step 4 — If you have a private well
- Stop using the water for drinking until you test it. Arrange testing for hydrogen sulfide and other common well contaminants.
- If tests show bacteria or hydrogen sulfide, contact a licensed well contractor. They can advise on shock chlorination, filtration, or other treatments.
Step 5 — If municipal supply is suspected
- Report the issue to your water utility. They can confirm whether the smell is coming from the mains and advise on next steps.
- Follow any boil or do-not-use advisories from your utility. For non-health-related sulfur smell, bottled water is a short-term option for drinking.
What Not to Do
- Do not treat cold-side odor as heater bacteria. The heater usually only affects hot water.
- Do not pour household chemicals into the plumbing to try to “kill” the smell. That can damage pipes, fixtures, or create unsafe reactions.
- Do not ignore municipal advisories. If your water utility issues a notice, follow their guidance rather than trying home fixes that may not address the real source.
When to Call a Professional
- If the smell is strong, persistent, or affects both cold and hot water after your checks.
- If you have a private well and a test confirms hydrogen sulfide or bacterial contamination.
- If multiple neighbors have the same smell and the utility cannot explain it — your utility or a licensed plumber can investigate the service line.
Safety Notes
- Hydrogen sulfide at low levels smells like rotten eggs. At high levels it can be harmful; if you notice a very strong odor, poor air quality, or symptoms like headache or nausea, leave the area and call your utility or emergency services.
- Do not rely on smell alone for safety. If your utility issues a boil or do-not-use notice, follow it until they clear the water.
- When in doubt about treating a well or installing filtration, hire a licensed professional. Improper well disinfection or poorly chosen treatment systems can make problems worse.
Common Homeowner Questions
- Why does only my cold water smell?
Because the source is usually before the heater — in the supply lines, well, or mains — rather than the water heater itself. - Will flushing my pipes fix it?
Flushing can clear stagnant water and sometimes removes the smell temporarily, but persistent odor needs testing and source treatment. - Is it safe to drink?
If the smell is mild and your utility has not issued an advisory, short-term drinking may be okay, but if the smell is strong or persistent, use bottled water until testing or utility guidance confirms safety.
For more related articles, see the Sulfur / Rotten Egg Smell in Water hub.