Quick Answer:
If you notice a sulfur or rotten-egg smell when your washer fills, it often comes from hydrogen sulfide produced by bacteria in warm, low-oxygen water or from sulfur compounds in the supply. Start by isolating where the water is coming from: collect the laundry fill water separately to see whether the cold supply, the hot side, or a recirculation loop is involved. Simple checks can tell you whether it’s a washer-only issue, the water heater, or a hot recirculation loop.
Why This Happens
- Bacteria in warm, stagnant water can produce hydrogen sulfide, which smells like sulfur or rotten eggs.
- An anode rod in a water heater can react with naturally occurring sulfur compounds and create a smell in the hot water.
- Hot water recirculation loops that stay warm and low in oxygen can become a breeding ground for odor-producing bacteria.
- Supply-side mineral or well-water issues can also cause a persistent smell that appears during laundry fill cycles.
Step-by-Step What to Do
Step 1 — Isolate the source by collecting fill water
Use a clean bucket to collect water from the washer’s fill hose during a normal fill. Collect one sample from the cold fill and one from the hot fill (run only the respective valve). Label them and note which smells. This directly checks whether the problem follows the hot line, cold line, or appears only when both mix.
Step 2 — Test cold vs. hot and watch timing
- Smell only on hot: points to the water heater or a hot recirculation loop.
- Smell only on cold: points to supply or washing machine inlet components.
- Smell only when both mix: may indicate a reaction in the washer or a mixed plumbing problem.
Step 3 — Check the water heater and recirculation loop
If the hot sample smells, turn off any recirculation pump and take a fresh sample from a hot faucet close to the heater. If the smell is present at the heater outlet but not at the cold inlet, the problem is likely inside the heater or loop. Flushing the heater and loop can often help — see Smell disappears after flushing system for more context on that outcome.
Step 4 — Inspect washer hoses and inlet screens
Remove and inspect the washer hoses and inlet screens for debris or biofilm. Clean or replace hoses that are old or contaminated. A washer-only odor sometimes comes from buildup in hoses or the machine itself.
Step 5 — Try a controlled rinse or treatment
- Run a hot-water rinse through the washer with no laundry to see if the smell clears from the machine.
- For well water, shock-chlorination of the well or treatment with a qualified technician may be necessary.
- If you treated the heater (for example by flushing or replacing the anode) check a fresh sample to confirm the odor is gone. If the issue was from the hot side, you may also relate to guidance on Rotten egg smell in hot water.
What Not to Do
- Do not ignore laundry-only odor.
- Do not pour large amounts of bleach into the water heater — that can damage tank components and plumbing.
- Do not run a recirculation pump continuously while testing; that can mask source location.
- Do not use flammable chemicals or strong acids to try to “fix” plumbing smells yourself.
When to Call a Professional
- If the smell persists after you collect samples and flush lines, especially from the hot side or a recirculation loop.
- If you suspect contamination of a private well or persistent bacterial growth in a hot recirculation system.
- If you’re uncomfortable disassembling hoses, flushing the heater, or handling water-treatment chemicals — call a plumber or water-treatment specialist.
Safety Notes
- Work in a well-ventilated area when checking or flushing lines.
- Wear gloves when handling hoses, anode rods, or standing water that may contain bacteria.
- Turn off power and gas to the water heater before doing any work that requires access to the tank’s interior or anode rod.
- Avoid mixing household cleaners; never mix bleach with ammonia or acids.
Common Homeowner Questions
- Is this dangerous to my health? Short answers: low-level hydrogen sulfide from plumbing smells is usually unpleasant more than harmful, but persistent high levels or well contamination needs professional testing.
- Could it just be my washer? Yes. Collect separate hot and cold fill samples to confirm whether the machine or the supply is the source.
- Will bleach permanently fix it? Bleach can temporarily remove odors in the washer or plumbing but it is not a permanent fix for heater or well issues and can damage components if misused.
For more related articles, see the Sulfur / Rotten Egg Smell in Water hub.