A washer that smells like sewage usually has a dry or missing trap at its drain, or buildup inside the machine. A water heater that smells like rotten eggs only on hot water usually has bacteria reacting with its magnesium anode rod. Run a cold tap and the washer alone to tell them apart.
The two-minute test
- 1Smell the hot and cold water separately. Minnesota's health department recommends smelling the water from the hot and cold taps after you have been away from home for a few hours. Hot water only points at the water heater. Cold water too points at the water supply or the plumbing.
- 2Smell the washer drain. Stand near the standpipe, the vertical pipe the washer's drain hose goes into, and smell. Then run a short rinse cycle and smell again. If the odor rises out of the standpipe, the drain is the source.
- 3Smell inside the washer. Open the door or lid and smell the drum, gasket and detergent drawer. A sour, musty smell there is the machine, not the sewer.
If the washer is the source
### A dry or missing drain trap
Every drain that connects to a sewer needs a trap: a bend in the pipe that holds water and blocks sewer gas. Mesa's wastewater guidance says to keep P-trap seals filled with water to block sewer gases, and notes that hydrogen sulfide gas is toxic and causes the rotten-egg smell. Wisconsin's health department gives the same basic advice: flush floor and sink drains with water so the traps do not dry out.
A washer's trap sits under or behind the standpipe. If the laundry room sits unused for weeks, the water in that trap can evaporate and the smell comes up. Pour a quart or so of water into the standpipe and see if the smell fades. Our page on what a P-trap is shows how the seal works.
If the smell does not fade, the trap may be missing, damaged or poorly vented. A washer drain that empties straight into a wall pipe with no trap lets gas straight into the room. That's a job for a plumber, because the fix involves cutting into the drain.
### Buildup inside the machine
Samsung's support page says mildew develops when moisture or other residue, such as leftover detergent or additives, is trapped in a dark place. Its fix is to run a self-clean cycle with chlorine bleach, wipe the tub, and then leave the lid or door open, and the detergent drawer open, so the machine dries. For front-load washers it also recommends cleaning the pump filter. Do this first if the smell is inside the drum but not at the standpipe.
Two habits keep it away. Leave the door open between loads, and use only the amount of detergent the machine calls for. Too much detergent leaves residue, and residue is what mildew feeds on.
### A drain hose pushed too far in
If the washer hose is pushed deep into the standpipe, it can seal the opening. Dirty water may then siphon out or sit in the hose, and the smell comes from there. The hose should hang loosely in the top of the standpipe, with air able to get around it. Pull it back and check whether the smell fades.
### Other laundry-room culprits
The washer is not the only drain in that room. A floor drain, a utility sink or a nearby bathroom can each have a dry trap of their own. Wisconsin's health department names floor drains and sink drains as places where traps dry out. Run water into each one and see whether the smell follows the water. If it does, you have found it. A cap left off a cleanout, or a loose one, can leak gas too, and so can a worn seal at a toilet.
### A blocked line or vent
If the washer drains slowly, gurgles, or the smell shows up in other drains too, the cause may be a blocked branch drain or vent rather than the washer. Wisconsin suggests checking roof vents for leaves or nests that block them. When several drains act up together, read can a sewer line problem make several drains back up. When the washer overflows the standpipe itself, see washing machine drain overflow.
If the water heater is the source
A rotten-egg smell that shows up only with hot water usually comes from the tank. Minnesota's health department explains that hydrogen sulfide gas can give water a rotten-egg smell, and that many water heaters contain a magnesium anode rod that can produce hydrogen sulfide. The anode rod is the "sacrificial" part that corrodes so the tank steel does not. Sulfur bacteria in the tank can play a part too. Its page is written for well owners, but the hot-versus-cold test works in any home.
For a hot-water-only smell, try these in order. The first two come from the state's advice:
- 1Replace the anode rod. Ask a plumber about a different rod material. This is the most common lasting fix.
- 2Disinfect the tank. Chlorination can kill sulfur bacteria. A plumber can flush and disinfect the tank.
- 3Flush the tank. A tank that has never been flushed collects sediment where bacteria live. Our page on how often to flush a water heater in Phoenix covers it.
Do not raise the thermostat as a fix. The state page mentions that raising the water heater temperature can kill bacteria, but a hotter tank raises the scald risk, and it is not a step to take casually.
If the smell is in the cold water too, the water heater is not the cause, and the next stop is a plumber or the water provider. If you cannot decide whether the smell is sewage, rotten eggs or gas, read a house that smells like gas or rotten eggs.
Three examples of how the test plays out
These are made-up examples to show the reasoning, not real jobs.
Sewer smell in the laundry room, nowhere else. The hot and cold taps smell fine. The smell is strongest at the standpipe and eases after you pour water in. The trap had dried out. Add water, and every few weeks run a little water into any drain you rarely use.
Sour smell in the drum, fine at the standpipe. Pouring water down the standpipe changes nothing, but the door gasket smells musty. That's the machine. Run the self-clean cycle, wipe the gasket, and leave the door open.
Rotten-egg smell in the hot water at every tap. Cold water is clean. The tank is the source. Have a plumber look at the anode rod, and ask about flushing and disinfecting the tank.
When none of these fit, or the smell is in both hot and cold water, stop guessing and have it checked.
How worried should you be
ATSDR says people can smell hydrogen sulfide at very low levels, and that at high levels the nose stops noticing it. A faint smell from a dry laundry trap is a nuisance to fix, not an emergency. A strong smell, or one with headaches, nausea or dizziness, is different. Read the first signs of sewer gas poisoning, and if the smell is strong or someone feels ill, leave and call 911.
For a smell that keeps coming back after you have filled the trap and cleaned the machine, a plumber can camera the drain and check the vent. HQ Plumbing offers free estimates and upfront pricing, and 24/7 emergency service if the smell is heavy.