Rotten-egg smell: sewer gas, natural gas or the water heater?
Updated October 30, 2026 · 3-minute read · Old House Sewer Lines
A rotten egg smell in the house can come from natural gas, from sewer gas or from the water heater, and natural gas has to be ruled out first. If you cannot be sure it is not gas, treat it as gas: everyone leaves now, and the call is made from outside.
Natural gas has no smell of its own. Con Edison says a chemical called mercaptan is added so people can detect a leak, and it describes the warning signs as an odor "similar to rotten eggs," a hiss or roar, or bubbles in standing water (Con Edison gas safety). The utility also says to leave even when the odor is faint, and not to assume a neighbor has called.
When it is sewer gas
Hydrogen sulfide carries the same rotten-egg smell. ATSDR describes it as a flammable, colorless gas "often referred to as sewer gas," and notes that it is heavier than air, so it collects low and in enclosed spaces such as basements (ATSDR ToxFAQs). It can be smelled at levels far below dangerous ones. At about 100 ppm, though, people lose the ability to smell it within roughly 2 to 15 minutes, so a smell that fades is not proof the gas has gone.
The usual entry points in an old house are dry traps, loose cleanout caps and blocked vents. NDSU Extension says dried-out traps in basement floor drains and little-used fixtures let sewer gas in, and that a gurgling or poorly flushing toilet can be the first sign of a vent problem (NDSU Extension). Con Edison advises keeping indoor sewer-line caps in good shape and attached tightly. Pouring about a quart of water into each seldom-used drain every three to four weeks keeps the seals full. More is in sewer gas smell in the house and hydrogen sulfide and sewer gas.
When it is only the hot water
A smell that appears only when the hot tap runs points to the water heater, not the sewer. NDSU Extension explains that sulfates in the incoming water react with the heater's magnesium anode rod to form hydrogen sulfide. Its suggested fix is removing the anode or replacing it with an aluminum one, a job for a plumber who knows that heater's warranty terms (NDSU Extension, Water Quality). If the cold water smells too, the source is the supply rather than the heater. If your home is on a public water supply, a supply-side odor is worth reporting to your water provider.
| Where you notice it | Likely source | First step |
|---|---|---|
| Throughout the house, near appliances or the meter, or you cannot tell | Possible natural gas | Leave now; call 911 or the gas utility from outside |
| Near a basement floor drain, unused sink or cleanout | Sewer gas through a dry trap or loose cap | Refill the trap; check the cap; call a plumber if it persists |
| Only from the hot tap | Water heater anode reacting with sulfates | Have the anode checked |
A musty, sewage-like smell from kitchen or bath drains can also mean a leaky or backed-up pipe, Poison Control notes. If drains are slow or gurgling at the same time, see gurgling toilet and gas smell or sewer smell.
Key facts
- Con Edison adds mercaptan to natural gas so leaks smell like rotten eggs (Con Edison).
- Con Edison says to leave even if the odor is faint, then call (Con Edison).
- Hydrogen sulfide is heavier than air; at about 100 ppm the sense of smell fails within 2 to 15 minutes (ATSDR).
- A hot-tap-only smell comes from sulfates reacting with the heater's magnesium anode (NDSU Extension).
- About a quart of water every three to four weeks refills a dry trap (NDSU Extension).