Sewer Gas Smell in the House? Traps, Vents, and the One Test Pros Use
That sharp, rotten-egg odor isn’t “just the drain.” It’s sewer gas, and besides being nasty, it can signal a plumbing failure that deserves fast attention. The fix is usually straightforward once you confirm where the air is entering: a dried trap, a blocked vent, a leaky wax ring, or a cracked/loose drain component.
This guide walks you through safe DIY checks, explains why vents matter, and shows the pro test we use to prove the leak path in minutes. If you want same-day diagnostics, book Drain Cleaning. If a repair is needed (stack, vent, or under-slab), tap Sewer Repair.
First: Is it really sewer gas?
Likely sewer gas if:
Odor spikes after fixtures sit unused (guest bath, floor drain)
Smell is strongest near floor drains, laundry, or around the toilet base
Odor worsens when exhaust fans or the furnace/air handler run (they can pull air from leaks)
Less likely sewer gas if: the smell is only at the sink tailpiece and improves after cleaning—then it may be biofilm.
The fast homeowner checklist (10–15 minutes)
1) Refill dry traps
Every drain has a P-trap that blocks air with water. If it dries out, the house “breathes” the sewer.
Run water for 10 seconds at: guest sinks, showers, tubs, laundry standpipe, basement floor drains, and utility sinks.
For floor drains in low-use areas, add 1 cup of water plus a tablespoon of mineral oil to slow evaporation.
2) Look for toilet wax failures
Gently rock the toilet. Any movement + odor = likely wax ring leak.
Check for dampness at the base or staining on the floor below (if there’s a ceiling under it).
3) Sniff the laundry standpipe & air gap
A standpipe that’s too short or untrapped lets air in.
Make sure the dishwasher air gap (or high loop) is intact at the kitchen.
4) Check mechanical rooms
Open traps at condensate drains (furnace/AC, heat-pump water heater). Many have tiny traps that dry out—prime them.
If the smell clears after refilling traps, you’ve found it. If it returns quickly, move to venting and leak checks below.
Why vents matter (and how they fail)
Your vent system routes sewer air up the roof and outside so fixtures drain without glugging. Problems cause trap siphonage (traps pulled dry) or push odor through cracks.
Common Idaho issues:
Snow/ice caps or leaf debris blocking a roof vent
Bird nests or wasp nests in summer
Improper or aging AAVs (air admittance valves) stuck shut or open
Remodels that removed or undersized a vent
Quick checks:
If multiple fixtures gurgle when one drains, suspect venting.
Peek at the roof from the ground: is the vent cap iced over or buried in snow?
AAVs (usually under sinks) should be vertical and above the trap arm—replace if noisy, sticky, or decades old.
The one test pros use: Smoke testing
When traps are primed and the odor persists, we use non-toxic smoke pumped into the drain/vent system at low pressure. Wherever smoke exits is where sewer air was leaking.
What smoke testing reveals:
Cracked toilet flange or failed wax ring
Loose/hidden slip joints inside walls
Underslab breaks or open abandoned lines
Roof vent leaks or misconnected vents
Improperly capped cleanouts
Why it’s decisive: No guessing. We see (and photograph) the exact leak path, then give you a targeted fix instead of opening random walls.
Schedule smoke testing through Drain Cleaning; if we confirm a broken stack, offset, or underslab issue, we’ll outline repair routes via Sewer Repair.
When cleaning helps—and when it doesn’t
Jetting/cleaning helps if grease, lint, or roots are causing slow drains and siphonage, which pull traps dry.
Cleaning won’t fix a missing trap, failed wax ring, cracked pipe, or blocked roof vent. That’s repair, not maintenance.
Simple, lasting fixes (match the failure)
Dry traps: Add trap primers on rarely used drains (mechanical rooms, basements).
Wax ring failures: Reset the toilet with a new wax or waxless seal; fix any flange height issues.
Blocked vents: Clear the roof vent; replace damaged caps; re-establish full-size vent to roof.
AAV problems: Replace with rated valves, correct height, and access panel.
Cracked/loose piping: Repair/replace with proper fittings; pressure/flow test afterward.
Underslab breaks: Use camera + locate to mark the spot; choose spot repair or trenchless where appropriate.
Two quick “is it fixed?” checks you can run
Trap hold test: Prime all traps at night. If odor returns by morning, something is pulling water from a trap (venting issue) or there’s a leak path bypassing traps.
Fan test: Run bath/kitchen exhaust fans for 5 minutes. If odor intensifies, negative pressure is pulling sewer air through a leak—time for smoke.
Prevention you’ll actually keep up with
Monthly: Prime low-use traps; glance at the laundry standpipe.
Seasonal (spring/fall): Roof vent visual check; clear debris/snow caps.
After remodels: Verify that new fixtures have proper traps and vents; don’t rely on hidden, off-brand AAVs.
Annually: Consider a camera + jet on problem branches to stop siphonage from chronic slow drains.
FAQs
Is sewer gas dangerous?
In high concentrations it can displace oxygen and contains compounds like H₂S. Most home cases are low-level but unhealthy and unpleasant. See OSHA/CDC primers for health basics, then fix the source.
Can I pour bleach down the drain?
Skip harsh chemicals. They don’t fix venting or seals and can damage components. Prime the trap and diagnose.
The smell is only around one toilet.
Likely a wax or flange issue. If the toilet rocks even slightly, reset it—don’t caulk over the problem.
Do AAVs replace roof vents?
They supplement in specific situations. A healthy system still needs a full-size vent to roof.
Bottom line
Start with traps: prime them all.
Consider venting next: blockages and bad AAVs pull traps dry.
Use smoke testing to prove the leak path and fix it once.
If you want a clean, evidence-based diagnosis (with photos for your records), schedule same-day Drain Cleaning. If we uncover a cracked stack, failed flange, or underslab defect, we’ll move you seamlessly into Sewer Repair with a clear, line-item plan.