How to tell a dry drain from a real sewer gas problem
Most plumbing smells come from one of three places: sewer gas getting past a trap or seal that’s supposed to block it, gunk building up inside a drain, or a chemical reaction inside the water heater. The smell tells you which one, and a couple of them you can fix yourself in five minutes. The others need a plumber, and one of them shouldn’t wait.
What causes sudden plumbing odors in a home?
Every drain in your house has a trap, the U-shaped bend under the sink that holds a little water. That water is the only thing between you and the sewer. The system also has vent pipes running up through the roof so air can move and the traps stay full. When a trap dries out, a seal fails, or a vent gets blocked, sewer gas comes in. The usual culprits:
- A dry trap in a guest bathroom, basement floor drain, or utility sink that hasn’t been used in weeks.
- A blocked, cracked, or badly installed vent stack.
- A failed wax ring under a toilet.
- A loose lid or bad seal on the basement ejector pit, which is very common in North Shore homes with a bathroom below the sewer line.
- A cracked or loose sewer cleanout cap.
- Biofilm, the slimy buildup of bacteria, hair, and soap inside bathroom and kitchen drains.
- The anode rod inside the water heater reacting with the water.
Five plumbing smells you should never ignore
1. Rotten eggs or sulfur from a drain, toilet, or the basement
That’s sewer gas, mostly hydrogen sulfide. It’s coming past a dry trap, a failed toilet seal, an ejector pit lid, a cracked pipe, or a blocked vent. If it’s a drain you rarely use, pour a gallon of water down it and see if the smell goes away. If it’s near a toilet that rocks or has a stained floor around the base, it’s the wax ring. If it’s strongest in the basement, check the ejector pit lid and the floor drain. If none of that clears it, the leak is somewhere you can’t see, and it needs to be found.
2. Musty, damp, or earthy
That’s a leak you haven’t found yet. Water behind a wall, under a floor, or around a tub is feeding mold in the drywall or subfloor. It gets worse the longer it runs, and mold remediation costs more than the plumbing repair. Look for soft spots, stained ceilings below bathrooms, and a water meter that moves when nothing’s running.
3. Sewage smell in the yard, by the foundation, or in the basement after it rains
That’s the main sewer line, the pipe carrying everything from the house out to the street. A cracked or root-damaged line leaks sewage into the soil, and the smell comes up through the ground or in through the foundation. If it shows up after heavy rain, or along with soggy spots in the lawn, slow drains, or a backup at the basement floor drain, the line is failing. This is the one that shouldn’t wait, because the next step is sewage in the basement. A camera inspection tells you exactly where the break is and whether it can be lined or has to be dug.
4. Rotting food or garbage from the kitchen sink
Grease, food, and biofilm in the disposal and the drain line. Run the disposal with cold water and ice, scrub the rubber splash guard underneath, and flush the drain with hot water and a little dish soap. If it comes back within days, the buildup is farther down the line and the drain needs to be cleaned professionally.
5. Rotten eggs only when the hot water runs
That’s the water heater, not the sewer. See below.
Why does my hot water smell like rotten eggs?
If the smell is only in hot water, at every faucet, the water heater is the source. Inside the tank is a magnesium anode rod, a metal rod that corrodes on purpose so the steel tank doesn’t. Certain harmless bacteria in the water react with the magnesium and produce hydrogen sulfide gas, which dissolves into the hot water and comes out at the tap.
The fix is to flush the tank and replace the magnesium rod with an aluminum-zinc rod, which protects the tank just as well without feeding the reaction. In stubborn cases the tank also gets treated with peroxide. This is not a dangerous smell, but it does mean the anode rod is being used up, and once it’s gone the tank starts rusting.
Is sewer gas dangerous?
At the levels you’d get from a dry trap or a bad seal, sewer gas is unpleasant and irritating rather than dangerous. Low-level exposure over time can cause eye and throat irritation, headaches, nausea, and fatigue. Ongoing exposure isn’t something to live with.
The thing to know: at high concentrations, hydrogen sulfide deadens your sense of smell. The rotten-egg odor disappears, not because the problem went away, but because you can no longer smell it. If a strong sewer smell suddenly stops and nothing changed, open windows and get the source found. A sewer gas leak from a broken line under a slab, or in a home with a septic system, is the case where that actually matters.
Can cold weather make drain smells worse?
Yes. In a hard freeze, frost can close off the vent pipe where it exits the roof. With the vent blocked, water draining through the system creates suction that pulls water out of the traps, and sewer gas comes up through the empty drains. The tell is a gurgling toilet or drain along with the smell, usually during or right after a deep cold snap. Once the vent thaws the traps refill and it goes away, but a vent that frosts over every winter is undersized or poorly placed and can be fixed.
How we find a smell you can’t trace
Sewer gas travels through wall cavities and drop ceilings, so where you smell it is often not where it’s coming from. When the simple checks don’t turn it up, we start by checking every trap, seal, and vent in the house, then run a camera down the main line. A sewer camera inspection shows cracks, offset joints, and root intrusion in the underground pipe, including under the slab, which is where the hardest-to-find leaks usually are.
Pouring bleach down the drain won’t fix any of this. It knocks down the bacteria at the top of the drain for a day and does nothing for a dry trap, a wax ring, or a cracked pipe.
Frequently asked questions
Why does my sink smell like sewage only when the water runs?
Usually biofilm in the overflow channel or the top of the drain. Running water pushes the stale air in the pipe up at you. Clean the overflow with a bottle brush and flush the drain with hot water and soap. If the smell is sewer gas rather than gunk, the trap or vent is the problem.
Will pouring bleach down the drain get rid of sewer gas smells?
No. Bleach kills surface bacteria briefly and does nothing for the seals, traps, and vents that actually let sewer gas in. It’s also hard on older pipes and on septic systems.
Is a faint rotten egg smell from my water heater dangerous?
No, but it means the anode rod is reacting and wearing out. Replacing it clears the smell and keeps the tank from rusting through early.
How do I keep a basement floor drain from drying out?
Pour a gallon of water down it once a month. A cup of mineral oil on top of the water slows evaporation. Many homes have a trap primer, a small line that automatically adds water to the floor drain trap; if yours has one and the drain still dries out, the primer has failed.
How often should I have my drains inspected for gas leaks?
You don’t need routine testing. Have it looked at when a smell keeps coming back after you’ve run water in the unused drains, or when it comes with gurgling, slow drains, or a toilet that rocks.
Sewer gas and drain odor diagnosis on the North Shore
Biagi Plumbing has been solving plumbing problems in Highland Park and across the North Shore since 1989, including Highwood, Lake Forest, Deerfield, Northbrook, Glencoe, Winnetka, Wilmette, Evanston, Glenview, Libertyville, Buffalo Grove, and the surrounding communities. If a smell keeps coming back and the simple fixes haven’t worked, call Biagi Plumbing at (847) 512-8921. We’ll track it to the source, whether that’s a wax ring, an ejector pit seal, a frozen vent, or a cracked line under the slab, and fix that instead of masking it.