Quick Answer:
If a bad odor vanishes after you flush the plumbing, that often means the smell is coming from stagnant water, a particular fixture, or a temporary build-up in a part of the system. Flushing can clear the immediate source, but it does not always fix the underlying cause. Follow a few checks to find the source and decide whether a professional is needed.
Why This Happens
When you run water through the pipes you push out standing water and force fresh water in. Any odor tied to that standing water—decaying organic matter, low-use traps, or a localized bacterial colony—can disappear while water is flowing. Common causes include:
- Stagnant water in a rarely used fixture or trap.
- Sediment or organic material in the hot water heater or a filter.
- A backflow or cross-connection that briefly introduced contamination.
In some cases the smell is similar to rotten eggs or sulfur. If you notice patterns (for example, only after a backflow device was installed) check guidance on Egg smell after backflow install and on issues that affect only cold supply like Sulfur smell only in cold water.
Step-by-Step What to Do
1. Note where and when the smell appears
Check every fixture: kitchen, bathrooms, outdoor spigots, and the water heater. Smells in hot water but not cold usually point toward the water heater; the opposite suggests the cold supply or municipal source.
2. Flush all taps simultaneously and time it
Open every tap in the house (hot and cold where present) at once and let them run. Note how long the odor stays gone after you stop. Write down the time in minutes. This test helps show whether the smell is from a single branch of piping, a storage tank, or the main supply.
3. Isolate hot vs. cold
Run only cold taps and see if the smell returns. Then run only hot taps. If the odor is only on hot water, the heater is a likely source. If it’s only on cold, focus on incoming supply and filters.
4. Check fixtures and traps
Smells can come from dry or contaminated P-traps, garbage disposals, or aerators. Remove and clean aerators and run water down each drain to refill traps. If a trap is dry, run water into it and re-test.
5. Inspect simple filters and softeners
Replace or rinse sediment filters and check softener tanks. Sometimes a clogged filter holds organic material that causes odors until flushed away.
6. Re-test after a controlled flush
After any cleaning or filter change, repeat the “flush all taps simultaneously” test and note whether the smell returns and how long it stays away. Keep the timing notes for reference if you call a professional.
7. When disinfection is appropriate
If you suspect bacterial growth and the source is local (like a private well or internal tank), consider a controlled disinfection following manufacturer or local authority instructions. If you’re unsure how to proceed, stop and consult a professional rather than experimenting with chemicals.
What Not to Do
- Do not assume flushing permanently fixes bacteria. Flushing may only remove the immediate smell without addressing the source.
- Do not mix household chemicals to treat water or drains; that can create dangerous fumes.
- Do not ignore a smell that returns quickly or is accompanied by cloudy water, discoloration, or illness in the household.
When to Call a Professional
Call a plumber or water specialist if:
- The odor returns consistently soon after flushing or is present in multiple fixtures.
- You detect a rotten-egg (sulfur) smell that you cannot trace, or you have a private well and suspect contamination.
- There are signs of contamination: discolored water, sediment that won’t clear, or household members feeling unwell after using the water.
- You’re not comfortable performing tank flushing or disinfection yourself.
Safety Notes
- If water smells strongly of sewage or chemicals, avoid drinking it and use bottled water until the source is identified.
- Do not attempt complex repairs on the water heater or backflow devices without proper training.
- When disinfecting, follow official instructions and safety precautions—ventilate the area and avoid mixing products.
Common Homeowner Questions
- Why does the smell go away after I flush?
Because flushing replaces standing water and dislodges the material that caused the odor, at least temporarily. - How long should the smell stay gone?
If the source is cleared, it should not return; if it returns within minutes or hours, the underlying problem remains. - Can I fix it myself?
Often you can isolate and fix simple causes (clean traps, replace filters), but persistent or widespread smells merit a professional inspection.
For more related articles, see the Sulfur / Rotten Egg Smell in Water hub.