· Michael Blair
A sulphur or rotten egg smell that appears only when you run hot water is not a problem with the supply. It is a reaction happening inside the water heater, between the sacrificial anode rod and sulphate-reducing bacteria living in the tank. The first thing to establish is which tap it comes from.
Hot side only, or both
Run a cold tap and let it go for a minute. If the cold water smells too, the issue is in the supply rather than the heater, which on city water is worth reporting and on a well points at the well itself.
If only the hot side smells, the tank is the source. The smell usually gets worse after a period away from the house, because the water has sat still and the bacteria have had time to work.
What is actually happening in the tank
Every tank heater has a magnesium or aluminium anode rod in it. Its job is to corrode instead of the steel tank, which is the only reason a glass-lined tank lasts as long as it does. The rod gives off electrons as it does that work.
Harmless sulphate-reducing bacteria use those electrons to convert naturally present sulphates into hydrogen sulphide gas, which is the smell. The water is not unsafe to drink and the tank is not failing. It is a side effect of the corrosion protection doing its job.
What actually fixes it
Shocking the tank clears the bacteria for a while. Turning the thermostat up to 160°F for a few hours kills them, but you have to run that water out through every hot tap to reach the branch lines, and then turn it straight back down to 120°F because anything higher scalds.
The durable fix is swapping the magnesium rod for an aluminium-zinc rod, which produces far less of the gas. On a heater that already smells, replacing the rod and chlorinating the tank in the same visit is what stops it coming back within a month.
- Confirm the smell is on the hot side only
- Chlorine shock or a high-temperature flush to clear the bacteria
- Replace a magnesium anode with aluminium-zinc
- Return the thermostat to 120°F afterwards
The one thing not to do
Do not simply remove the anode rod. It is a common piece of advice online and it does stop the smell, because it removes the reaction. It also removes the only thing protecting the steel tank from corroding, and it voids the warranty on every tank heater sold.
A heater run without an anode in Middle Tennessee water will not last. If the smell has returned after a rod swap and a shock, the honest answer is a powered titanium anode or, on an older tank, putting the money toward the replacement instead.