If only the hot water smells like rotten eggs, the cause is usually inside the water heater: bacteria in the tank react with the magnesium anode rod and produce hydrogen sulfide. It's common with well water and softened water. Changing the anode rod type, and sometimes disinfecting the tank, usually fixes it. If you smell it near the heater rather than in the water, treat it as a possible gas leak.
Smell gas? Stop. Leave the house, don't use switches or phones inside, and call your gas utility or 911 from outside.
Water smell or gas smell?
Natural gas has an added odorant that also smells like rotten eggs. So first, work out where the smell is:
- Only when hot water is running, at the tap or in the shower: almost certainly the water.
- Near the heater, near the stove, or in the room even with no water running: treat it as gas. Leave and call from outside.
The 30-second test
- Run the cold water at a sink and smell it.
- Then run the hot water.
- Only hot smells: the source is the water heater.
- Both smell: it's in your water supply. With a private well, that's a well water question, so have the water tested. On city water, call your water utility.
Why it happens in the tank
Most tank water heaters have a magnesium anode rod, a sacrificial metal rod that corrodes so the steel tank doesn't. Sulfate-reducing bacteria, which are harmless in themselves, can live in the warm tank and react with the rod, giving off hydrogen sulfide gas. That's the smell.
It's more common when:
- The water comes from a private well.
- The water is softened. Softening changes the water chemistry and speeds up anode wear.
- The heater sits unused for a while, like after a vacation, or in a cabin or rental.
- The thermostat is set low.
Only the hot water smells? An anode rod change is a common fix. Ask a plumber whether it suits your heater.
Call nowHow it's fixed
Swap the anode rod
Replacing a magnesium rod with an aluminum/zinc rod, or a powered anode, usually stops the reaction. Don't just remove the rod. Without it the tank will rust through much faster and may void the warranty.
Disinfect the tank
Flushing the tank and treating it to kill the bacteria can clear an existing smell. It's often done along with the rod change.
Temperature
Raising the temperature can help control bacteria, but it raises the scald risk. Talk to your plumber about a safe setting and whether a mixing valve makes sense.
Is it safe to use?
The smell is unpleasant, but at the levels typical of a water heater, hydrogen sulfide is generally considered a nuisance rather than a health risk. If you're on a well, or the cold water smells too, get the water tested.
When the heater is old
If the tank is near the end of its life, has rust in the hot water, or is leaking, putting money into a new anode doesn't make sense. Read repair or replace first.