Rotten smell after running water

Drain channel with residue leading to rotten odor

Quick Answer:

If you get a rotten smell the moment you run a faucet, it usually comes from bacteria, biofilm, or a reaction in the hot-water system — not fresh food or trash being stirred up. Run hot and cold water separately to see which side triggers the odor. That first check quickly points to either the fixture/drain or the hot-water system.

Why This Happens

  • Biofilm and bacteria can build up in the faucet, aerator, or drain and release a rotten or sulfur-like smell when disturbed.
  • If the smell appears only with hot water, the water heater can produce hydrogen sulfide (a rotten-egg odor) when certain minerals and bacteria interact with the tank’s anode or sediments.
  • Stagnant water in a rarely used trap or line can grow odors. If a sink sits unused, the trap can dry slightly and allow sewer gas to sneak in.
  • If the smell comes back after you run water and then stop, consider both the drain and the hot-water source as possibilities. For cases where the sink sits unused for a while, see Odor only when sink unused. For smells that appear after time away, see Smell after long vacation.

Step-by-Step What to Do

1. Identify hot versus cold

  • Run the cold tap for 30–60 seconds. Note whether you smell anything at the spout or drain.
  • Do the same with hot water. If the odor appears only on hot, the water heater or hot plumbing is likely involved.

2. Check the faucet and aerator

  • Turn off the water, remove the aerator, and rinse it. Clean out any debris or slimy residue from the faucet spout.
  • After reassembling, run water again to see if the smell improves.

3. Flush the drain and P-trap

  • Run a strong flow of hot tap water down the drain for a minute to flush loose biofilm. Avoid pouring boiling water into PVC pipes; use the hottest tap setting if unsure.
  • If the smell persists, try a gentle home remedy: 1 cup baking soda, followed by 1 cup white vinegar. Wait 10–15 minutes, then flush with hot water.
  • If you have a garbage disposal, run it with cold water and a handful of ice plus coarse salt to knock loose buildup (do not use harsh chemicals in the disposal).

4. Check the hot-water heater

  • Open a hot faucet and let it run; if only hot water smells, the heater may have bacteria or a corroded anode producing hydrogen sulfide.
  • Flushing the water heater tank can help remove sediments. If odor persists after flushing, consult a plumber about replacing the anode or treating the tank.

5. Test other fixtures

  • Run other sinks, showers, and outdoor spigots. If multiple fixtures have the same odor, the problem may be in the main plumbing or sewer connection rather than one sink.

What Not to Do

  • Do not assume food or trash is the source when odor is from drain.
  • Don’t mix household cleaners (for example, bleach and ammonia) — mixing chemicals can produce dangerous fumes.
  • Avoid pouring large amounts of strong chemical drain cleaners into a system without understanding pipe materials and potential damage. Try gentler cleaning steps first or call a pro.
  • Don’t stick your head directly into a drain to sniff; sewer gases can be harmful in concentrated amounts.

When to Call a Professional

  • If the smell is only on hot water and persists after flushing the heater, call a plumber — this can indicate a water heater issue that may need tank service or anode replacement.
  • If multiple fixtures smell, if you detect raw sewage odor, or if drains gurgle or back up, contact a professional right away.
  • If you’re uncomfortable disassembling faucets, flushing the heater, or using cleaning methods safely, get a plumber to inspect and diagnose.

Safety Notes

  • Work in a well-ventilated area when investigating and cleaning odors.
  • Wear gloves and eye protection when handling cleaning agents or removing aerators and debris.
  • Do not combine cleaning chemicals. If you use vinegar and baking soda, rinse thoroughly before using other cleaners.
  • If you suspect sewer gas or a gas leak (rotten-egg scent can sometimes be misinterpreted), leave the home and contact professionals immediately.

Common Homeowner Questions

  • Why does only my hot water smell? — Often a water-heater issue such as bacteria or anode reaction; flushing or service may be needed.
  • Will the smell go away if I run the water longer? — Sometimes flushing helps, but persistent odors usually need cleaning the trap/drain or inspecting the heater.
  • Is this dangerous? — Mild drain odors are usually unpleasant but not dangerous; strong sewer gas or persistent rotten-egg smell should be checked promptly.

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