The short answer
A rotten-egg smell from your hot water is usually hydrogen sulfide gas, created when bacteria react with the magnesium anode rod inside your water heater. It's typically not dangerous, but it's fixable — often by replacing the anode rod, sanitizing the tank, or addressing the water supply.
What causes the rotten-egg smell?
The culprit is hydrogen sulfide gas. Inside a water heater, naturally-occurring sulfur bacteria can react with the sacrificial magnesium anode rod (the part that corrodes to protect your tank), producing that distinctive sulfur smell. It usually shows up only on the hot side, which is the tell-tale sign it's the water heater and not your supply.
Hot side only, or both?
A quick diagnosis:
- Smell on the hot water only — almost always the water heater (anode rod / bacteria)
- Smell on hot and cold — more likely the water source or supply, which needs different treatment
- Smell that fades after running the tap — often bacteria in a little-used line or fixture
How is it fixed?
The common fixes target the cause: replacing the standard magnesium anode rod with an aluminum/zinc or powered anode reduces the reaction; sanitizing and flushing the tank clears the bacteria; and if the smell is on both hot and cold, whole-home treatment may be the real answer. Simply turning up the temperature briefly can knock back bacteria, but it's a stopgap, not a cure.
Don't just remove the anode rod
Pulling the anode rod to stop the smell will end the odor — and dramatically shorten your tank's life by removing its corrosion protection. Replace it with a different anode type instead.
What's the next step?
If your hot water smells like sulfur, we'll diagnose whether it's the anode, bacteria, or your supply, then fix it the right way — anode replacement, tank sanitizing, or treatment — and check that your water heater is otherwise healthy while we're there.
The rotten-egg odor from hydrogen sulfide in hot water is usually a nuisance rather than a health hazard at typical household levels, but it signals a reaction inside the tank worth addressing. If you smell sulfur and also suspect a gas leak, treat that separately and urgently.
Often yes — switching from a magnesium anode to an aluminum/zinc or powered anode, combined with sanitizing the tank, resolves most cases. If the smell is on the cold side too, the water supply needs treatment instead.
Terms in this guide
- Anode rod
- A sacrificial metal rod inside a tank water heater that corrodes on purpose so the steel tank does not. Replacing a spent anode rod on schedule is one of the cheapest ways to extend a water heater's life.
- Point-of-entry (POE) vs. point-of-use (POU)
- Point-of-entry treatment is installed where the main water line enters the home, so every tap benefits (whole-home filtration or softening). Point-of-use treats a single fixture, like an under-sink drinking-water filter. The right choice depends on what you are treating and why.