Rotten egg, hot side only
This usually means the anode rod in the water heater rather than anything in the supply.
The magnesium rod that protects the tank reacts with sulfate and bacteria in the warm tank and gives off hydrogen sulfide, which is the egg smell.
Look for the smell on the hot tap only, strongest first thing in the morning or after a holiday, and no smell at all on the cold line.
Do not buy a whole-house treatment system for this. Treating the supply does not change what is happening inside the tank, and the smell comes straight back.
Common questions
- Why does only the hot water smell like rotten eggs?
- A smell on the hot tap alone usually points at the water heater rather than the supply. The magnesium anode rod that protects the tank reacts with sulfate and bacteria in warm water and gives off hydrogen sulfide, which is the egg smell. If the cold line is clean, the well usually is not the source.
- Does changing the anode rod usually fix the smell?
- Swapping the magnesium rod for an aluminum-zinc rod is the usual repair and it usually clears the smell. The tank often needs disinfecting at the same time, and a rod seized into an older heater usually turns the job into a service call.
- Is a whole-house filter needed for a rotten egg smell on the hot side?
- Usually not. Treating the incoming supply does not change what is happening inside the tank, so the smell usually comes back. Checking the cold tap first is what separates a heater problem from a supply problem.
Related: rotten egg, hot and cold and rotten egg plus black slime.