Quick Answer:
A faint or noticeable odor a few days after replacing a backflow preventer is often from disturbed sediment, trapped water, or air pockets in the plumbing — not automatic cross contamination. Do a simple inspection of the new device and flush nearby fixtures. If the smell is strong, persistent, or clearly sewage-like, stop using affected outlets for drinking and follow the steps below.
Why This Happens
Replacing a backflow device can disturb settled material and change how water flows through nearby pipes. Common causes:
- Disturbed mineral deposits and biofilm that release odors when loosened.
- Stagnant water trapped in low points, traps, or the new assembly that needs flushing.
- Air pockets that concentrate odors and later vent into fixtures.
- Residual material from the repair (packing, grease, or sealants) that has an odor until fully flushed.
Sometimes this is similar to what homeowners see with Sediment after repiping or the temporary discoloration you might notice after work on the main like Brown water after main shutoff. Proper inspection and flushing usually clears the issue.
Step-by-Step What to Do
1. Ventilate and avoid drinking from affected taps
Open windows and run exhaust fans if the smell is strong. Do not taste the water; use bottled water for drinking until the cause is identified.
2. Inspect the new backflow for trapped odor
Do a visual check of the replacement device and surrounding fittings for trapped water, debris or any dislodged material. Smell around test cocks, unions and nearby drain traps. Look for dampness, drips or visible residue.
3. Run and flush faucets systematically
- Start at the highest fixture and work down through the house, running cold then hot water for several minutes at each tap.
- Open outside hose bibs and flush toilets a few times.
- Remove and clean faucet aerators and showerheads, then flush again.
4. Check traps, vents and the hot water heater
Smells can come from a dry P-trap, blocked vent, or the water heater (if sediment is disturbed on the tank bottom). Run water through seldom-used drains and consider flushing the heater per manufacturer guidance if the hot side smells.
5. Isolate where the odor is strongest
Try to narrow it to one fixture, the hot or cold side, or the area near the backflow device. This helps determine if the smell is from the new device, a fixture, or deeper in the system.
6. Clean or disinfect cautiously
If the smell seems like organic buildup, cleaning aerators and traps often helps. For persistent bacterial odors, brief disinfecting of lines can be done with household bleach following product instructions or by a professional. Do not pour chemicals into the backflow assembly or open the device unless you are qualified.
7. Monitor for 24–72 hours
Many temporary odors dissipate with flushing and use. If the smell persists beyond a couple of days, especially if it’s sewage-like or accompanied by discolored water, proceed to call a professional.
What Not to Do
- Do not ignore delayed odors after backflow work. Persistent smells can indicate a problem that needs attention.
- Do not open or tamper with the backflow device unless you are trained and certified; that can void repairs or cause contamination.
- Do not pour strong chemicals into the backflow assembly or drains without guidance; improper chemicals can damage seals and create hazards.
- Do not assume a faint odor is safe to drink through; when in doubt, use bottled water until cleared.
When to Call a Professional
- If the smell persists after thorough flushing and basic cleaning (24–72 hours).
- If the odor is clearly sewage-like, strong, or accompanied by brown or discolored water, reduced pressure, or visible leaks.
- If you suspect the backflow device was installed incorrectly, is leaking, or if there’s any chance of cross connection or contamination.
- If you are unsure about disinfecting or need a shock chlorination — let a licensed plumber or water treatment professional handle it.
Safety Notes
- Do not taste water to test for contamination. Use smell and appearance only as indicators.
- Wear gloves and eye protection when handling cleaning agents. Follow label directions precisely.
- If you must shut off water as part of troubleshooting, know where the shutoff is and how to restore it safely.
- When in doubt about health risk, stop using affected outlets for drinking and cooking until cleared by a professional.
Common Homeowner Questions
- Why did the smell appear days after the work? A: Disturbed sediment or trapped water/air can release odors slowly after service; it often clears with flushing.
- Could this be cross contamination? A: If the backflow device was installed correctly, cross contamination is unlikely, but persistent sewage smell or discolored water should be treated seriously and checked by a pro.
- How long should I wait before calling a plumber? A: If flushing and basic cleaning don’t resolve it within 24–72 hours, or if the smell is strong or sewage-like, call a licensed plumber.