Quick Answer:
An intermittent sewer-type odor that appears and disappears during the day usually means the problem is linked to how and when fixtures are used, a briefly failing vent, or occasional backflow from a shared drain. Start by tracking when the smell happens and which appliances are running. A few controlled tests will usually show whether the issue is a dry trap, negative pressure, or a venting problem you should call a plumber about.
Why This Happens
- Traps (P-traps) hold water to block sewer gas. If a trap is momentarily siphoned, evaporated, or bypassed, smells can appear only when other fixtures are used.
- Negative pressure can form when multiple drains run at once; that pressure can pull water out of a trap briefly and let gas through.
- Venting that works most of the time can fail intermittently because of wind, downdrafts, or partial blockages (for example, birds or nests), letting sewer gas push back into the house at certain times.
- Intermittent backflow from a clogged shared drain or main can push odor into branch lines only when a particular appliance runs.
- If it tends to show up at night or during specific conditions, check timing and usage patterns or see Odor only at night for related causes.
Step-by-Step What to Do
1. Log times and appliance use
Keep a short log for several days. Note the exact time you notice the smell and what was running in the house in the previous 15–30 minutes: dishwasher, washing machine, shower, toilet flushes, garbage disposal, or HVAC fans. This record is the most useful tool for spotting a repeatable trigger.
2. Run fixtures sequentially to reproduce the smell
With someone else helping if possible, run fixtures one at a time in a steady order: kitchen sink, bathroom sink, shower, tub, toilet, dishwasher. Pause a few minutes between each and check for odor. Note which fixture or sequence triggers the smell. Reproducing the smell on command is the key to diagnosis.
3. Check for negative pressure using a second fixture
While someone holds their nose near the floor drain or suspect fixture, have another person run a second fixture (for example, run a sink while you watch a nearby floor drain). If the odor appears when the second fixture runs, negative pressure or siphoning may be pulling trap water out or drawing gas through a leak.
4. Inspect the vent stack for downdrafts or obstructions
From safe ground level, look toward the roof vent for signs of birds, nests, or debris. On a calm day, a brief visual check or a pair of binoculars can show obvious obstructions. Wind-driven downdrafts can also push gas back down. Do not climb the roof if you are uncomfortable; hire a pro to inspect the vent and clear any blockages.
5. Test for intermittent sewer gas with a bathroom fan
Turn the bathroom fan off, note whether the odor is present, then switch it on. If the fan suddenly draws the odor into the living space when running, that suggests a venting or pressure issue. Try the same with range hood or attic fan to see if air movement changes where the smell shows up.
What Not to Do
- Avoid repeatedly masking intermittent odors with sprays or air fresheners — this hides patterns and delays diagnosis.
- Do not keep returning roof vents or dampers to a closed position to stop a smell; closing vents can create pressure problems and hides the underlying issue.
- Call a pro when you can reproduce the smell reliably or suspect intermittent venting failures. Repeated DIY masking or temporary fixes wastes time and can make problems worse.
When to Call a Professional
- You can reproduce the smell reliably by running a specific fixture or sequence.
- The odor is strong, persistent, or affects multiple rooms or fixtures.
- You find an obvious vent blockage you cannot safely clear, or you suspect intermittent backflow from a shared drain or sewer main.
- Household members experience headaches, nausea, or dizziness when the smell appears.
Safety Notes
- Sewer gas can contain hydrogen sulfide and other compounds that smell bad and can be harmful in high concentrations. Brief low-level exposure is usually unpleasant rather than dangerous, but persistent high concentrations are a health risk.
- Ventilate the area by opening windows and doors when you notice the odor. Avoid open flames if you suspect any combustible gas, and if the smell resembles natural gas (rotten-egg scent used by gas utilities), contact your gas company immediately.
- Do not put your head into drains or crawl into confined spaces alone. Use caution on roofs and ladders; hire a professional if you are not experienced working at height.
Common Homeowner Questions
- Why does the smell only happen when appliances run?
Running appliances changes drain flow and pressure, which can briefly open a path for gas if traps or vents aren’t working right. - Can I fix this by pouring water in the drains?
Adding water may temporarily refill a dry trap, but it won’t solve negative pressure, vent blockages, or intermittent backflow—those need testing and sometimes a professional repair. - Is this an emergency?
If the smell is mild and occasional, it’s not an immediate emergency, but if it’s strong, widespread, or causing symptoms, call a professional right away.
More in this topic
For more related fixes and similar symptoms, see Sewer Gas Smell from Floor Drains.