Quick Answer:
If you notice a sulfur or “rotten egg” smell only on the first draw of water in the morning, it’s usually from water sitting in pipes or the water heater overnight. Draw a first‑morning sample, then retest later in the day after running the tap. If the smell disappears after flushing, the cause is likely stagnation or a hot‑water issue; if it stays, further checks are needed.
Why This Happens
Water that sits in pipes or the top of a water heater can lose residual disinfectant and allow harmless bacteria to produce small amounts of hydrogen sulfide gas, which smells like eggs. Other causes include the reaction of the water with a sacrificial anode in the water heater, or an issue in well water chemistry. Recent plumbing work or a long absence can change conditions and bring the odor to the surface—see Sulfur smell after plumbing repair or Egg smell after long vacation for those specific situations.
Step-by-Step What to Do
1. Draw a first‑morning sample
- First thing in the morning, collect water straight from the tap without running it first. Note the strength and whether it’s on hot, cold, or both.
2. Retest later in the day
- Let the tap run for 30–60 seconds, then test again. Also check the same tap in the middle of the day after more household use. Comparing first draw to later draws helps confirm stagnation versus a persistent source.
3. Isolate hot vs. cold
- Test cold water and hot water separately. If only the hot water smells, the water heater is the likely culprit. If cold water smells, the issue may be in supply plumbing or source water.
4. Flush fixtures and clean aerators
- Remove and clean aerators and showerheads. Run cold water from the farthest tap for several minutes to flush stagnant lines.
5. Check the water heater
- If hot water is the source, try flushing a few gallons from the drain valve and consider checking or replacing the anode rod if the heater is older. If you’re not comfortable, include this in items to discuss with a technician.
6. If you have a well, test the source
- Well systems can develop low levels of hydrogen sulfide; have the well water tested and follow local guidance for treatment (chlorination or filtration) if needed.
7. Collect a sample for testing if the smell persists
- If flushing and the basic checks don’t stop the odor, collect a water sample after letting the system sit (first draw) and another after flushing. Send or take them to a certified lab for hydrogen sulfide and bacterial testing.
What Not to Do
- Do not consume first‑draw water if the odor persists.
- Do not ignore a persistent odor—continued use without identifying the source can mask an underlying issue.
- Do not pour concentrated chemicals into plumbing fixtures without guidance; improper use can damage systems or create hazards.
When to Call a Professional
- If the odor continues after flushing and cleaning, call a licensed plumber or water treatment professional.
- If both hot and cold water smell strong, or multiple fixtures are affected, get professional testing and diagnosis.
- If you have a private well, contact a well specialist or local health department for testing and treatment options.
Safety Notes
- Low levels of hydrogen sulfide in water are mostly an odor problem and not likely to cause illness, but persistent or strong smells should be addressed before drinking or cooking.
- If you smell natural gas (a strong sulfur or rotten‑egg odor that appears in the air, not just in water), leave the building immediately and call your gas company or emergency services—do not use electrical switches or phones inside the building.
- If you’re unsure how to flush or service the water heater safely, hire a pro rather than attempting risky repairs.
Common Homeowner Questions
Q: How long should I run the tap before the smell goes away? A: Usually 30–60 seconds at a single faucet; running a distant tap for a few minutes flushes more of the system.
Q: Is it safe to shower if the water smells only on first draw? A: Showering is generally safe, but avoid ingesting the water until the source is confirmed; if the smell is strong or causes irritation, stop and call a professional.
Q: Will a filter solve the problem? A: Some point‑of‑use filters and whole‑house treatments can remove hydrogen sulfide, but you should test the water first and match the treatment to the cause.
For more related articles, see the Sulfur / Rotten Egg Smell in Water hub.