Why Does My Hot Water Smell Like Sulfur?
QUICK ANSWER
A sulfur or rotten egg smell in only your hot water is usually caused by hydrogen sulfide gas, produced by a reaction between the magnesium anode rod in your water heater and bacteria in the water. The fix is replacing the anode rod with an aluminum-zinc alloy version.
A sulfur smell in your hot water is one of the few water heater issues that has a specific, well-understood cause and a clean fix. The smell almost always traces back to the anode rod inside the tank. Important safety note up front: if you smell sulfur at gas appliances or near the gas line itself, that is a gas leak emergency, not a water heater issue. Leave the house and call the gas company.
What causes the sulfur smell in hot water?
Hydrogen sulfide gas dissolved in the water creates the rotten egg smell. The gas forms when sulfate-reducing bacteria (naturally present in most water supplies) react with the magnesium anode rod inside your water heater tank. The anode rod is a sacrificial metal that protects the tank from corrosion by corroding instead, but the byproducts of that corrosion include hydrogen sulfide when bacteria are present. The smell only appears in hot water (not cold) because the reaction happens inside the heated tank.
Is the smell dangerous?
The hydrogen sulfide concentrations from a water heater reaction are not dangerous to drink or shower in, just unpleasant. Important: the rotten egg smell is also the odorant added to natural gas to make leaks detectable. If you smell sulfur at a gas appliance, near the gas meter, or throughout the house (not just from a hot water faucet), that is a potential gas leak. Leave the house, do not turn lights on or off, and call the gas company from outside. Test by running cold water from the same faucet; if cold water smells fine and only hot smells like sulfur, it is the water heater, not gas.
How do you fix the sulfur smell?
The proven fix is replacing the standard magnesium anode rod with an aluminum-zinc alloy anode rod. The aluminum-zinc rod still protects the tank from corrosion but does not react with sulfate-reducing bacteria. Replacement anode rods cost 25 to 50 dollars. You also drain the tank, remove the old rod (a 1-1/16 inch socket on a long breaker bar), and install the new one. The job takes 1 to 2 hours. Alternative: a powered (titanium) anode rod which has no sacrificial metal to react and lasts 20 years.
When should you call a pro?
Call a plumber or licensed pro if you cannot loosen the anode rod (often welded in place by years of corrosion), if the smell persists even after a new anode rod is installed (suggesting bacterial contamination requiring tank disinfection with chlorine), or if your water heater is over 10 years old and may not be worth the repair investment. Call the gas company (not a plumber) immediately if you suspect a gas leak rather than a water heater sulfur smell. Gas leaks need urgent professional response.
Sulfur smell in hot water is almost always the anode rod reacting with water bacteria, producing hydrogen sulfide. The fix is swapping the magnesium rod for aluminum-zinc. The smell is not dangerous when it comes from the water heater, but rotten egg smell at gas appliances or throughout the house is a gas leak that requires immediate evacuation and a call to the gas company. Test by checking cold water from the same faucet first.
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