Rotten Egg Smell from Water Heater
Updated February 25, 2026
Hot water smells like rotten eggs or sulfur -- caused by sulfate-reducing bacteria reacting with the magnesium anode rod inside the tank. This is typically a diy fix. Estimated cost: $20 - $40 for an aluminum/zinc anode rod.
Overview
Rotten egg smell only in the hot water (cold water smells fine) points directly at the water heater. Sulfate-reducing bacteria live in the warm, oxygen-depleted environment inside the tank. They feed on sulfates in the water and produce hydrogen sulfide gas -- the rotten egg smell. The magnesium anode rod accelerates this reaction by releasing electrons that the bacteria use as an energy source. The fix is straightforward: replace the magnesium anode rod with an aluminum/zinc rod, which does not fuel the bacteria. Flush the tank to clear the existing hydrogen sulfide.
Symptoms
- Hot water smells like rotten eggs or sulfur at every faucet
- Cold water smells normal -- the odor is only in hot water
- Smell is strongest when the hot water first turns on after sitting unused for hours
- Smell may come and go -- worse after vacation or periods of low usage
- No visible discoloration -- water looks clear but smells terrible
Common Causes
- Sulfate-reducing bacteria reacting with a magnesium anode rod -- the most common cause by far
- High sulfate levels in the water supply (common in well water) feeding the bacteria
- Water heater temperature set below 120°F -- bacteria thrive in warm water between 77°F and 113°F
- Stagnant water in an infrequently used water heater (vacation home, guest bathroom) allowing bacteria to multiply
- Water softener using sodium that replaces calcium but leaves sulfates available for bacteria
What You'll Need
How to Fix It
- Confirm the Source Is the Water Heater
Run cold water at a faucet for 2 minutes. Smell it. Run hot water at the same faucet for 2 minutes. Smell it. If only the hot water smells, the water heater is the source. If both smell, the problem is in the water supply itself (well water with hydrogen sulfide) and needs treatment at the source, not at the water heater.
- Flush the Tank with Hydrogen Peroxide
Turn off the heat source. Turn off the cold inlet. Drain 5-10 gallons from the tank through the drain valve. Disconnect the cold water inlet at the top of the heater. Pour 1-2 gallons of 3% hydrogen peroxide (drugstore grade) into the tank through the cold inlet opening. Reconnect the cold inlet. Open the cold valve to refill the tank. The hydrogen peroxide kills the existing bacteria and neutralizes the hydrogen sulfide. Let it sit for 2-4 hours. Then open hot faucets and run water until you no longer smell peroxide.
- Replace the Anode Rod
The long-term fix. Turn off the heat source. Run a hot faucet to relieve pressure. Use a 1-1/16 inch socket on a breaker bar to unscrew the anode rod from the top of the tank. Pull it out -- if it is mostly bare wire with little magnesium left, it needed replacing anyway. Install an aluminum/zinc anode rod ($20-$40). Aluminum/zinc does not react with sulfate-reducing bacteria the way magnesium does. The smell stops.
When to Call a Pro
Call a plumber if both hot AND cold water smell like sulfur (source is the water supply, not the heater -- need water treatment). Call a water treatment specialist if you have well water with high sulfate or hydrogen sulfide levels. A whole-house treatment system ($500-$2,000) addresses the root cause.
Prevention Tips
- Use an aluminum/zinc anode rod from the start if you have well water or high sulfate levels. Prevents the problem before it begins.
- Keep the water heater temperature at 120°F or above. Below 120°F, bacteria multiply faster.
- Flush the water heater annually. Stagnant sediment at the bottom harbors bacteria.
- Run hot water regularly in seldom-used fixtures. Stagnant hot water in pipes and the tank allows bacteria to proliferate.
Frequently Asked Questions
What are the signs of a rotten egg smell from water heater?
Hot water smells like rotten eggs or sulfur at every faucet Cold water smells normal -- the odor is only in hot water Smell is strongest when the hot water first turns on after sitting unused for hours Smell may come and go -- worse after vacation or periods of low usage No visible discoloration -- water looks clear but smells terrible
What causes a rotten egg smell from water heater?
Sulfate-reducing bacteria reacting with a magnesium anode rod -- the most common cause by far High sulfate levels in the water supply (common in well water) feeding the bacteria Water heater temperature set below 120°F -- bacteria thrive in warm water between 77°F and 113°F Stagnant water in an infrequently used water heater (vacation home, guest bathroom) allowing bacteria to multiply Water softener using sodium that replaces calcium but leaves sulfates available for bacteria
When should I call a plumber for a rotten egg smell from water heater?
Call a plumber if both hot AND cold water smell like sulfur (source is the water supply, not the heater -- need water treatment). Call a water treatment specialist if you have well water with high sulfate or hydrogen sulfide levels. A whole-house treatment system ($500-$2,000) addresses the root cause.
How do I prevent a rotten egg smell from water heater?
Use an aluminum/zinc anode rod from the start if you have well water or high sulfate levels. Prevents the problem before it begins. Keep the water heater temperature at 120°F or above. Below 120°F, bacteria multiply faster. Flush the water heater annually. Stagnant sediment at the bottom harbors bacteria. Run hot water regularly in seldom-used fixtures. Stagnant hot water in pipes and the tank allows bacteria to proliferate.
When should I call a plumber for rotten egg smell from water heater?
Call a plumber if both hot AND cold water smell like sulfur (source is the water supply, not the heater -- need water treatment). Call a water treatment specialist if you have well water with high sulfate or hydrogen sulfide levels. A whole-house treatment system ($500-$2,000) addresses the root cause.
Can I fix rotten egg smell from water heater myself?
This problem is rated "DIY". Most homeowners can handle this with basic tools and patience.
How serious is rotten egg smell from water heater?
This is a low-severity issue. It is not urgent but should be addressed when convenient.
How much does it cost to fix rotten egg smell from water heater?
The estimated repair cost is $20 - $40 for an aluminum/zinc anode rod. Costs vary by location, severity, and whether you hire a professional.
How can I prevent rotten egg smell from water heater?
Key prevention tips: Use an aluminum/zinc anode rod from the start if you have well water or high sulfate levels. Prevents the problem before it begins.. Keep the water heater temperature at 120°F or above. Below 120°F, bacteria multiply faster.. Flush the water heater annually. Stagnant sediment at the bottom harbors bacteria..