People with well water usually assume the well is to blame. In this case, that assumption is almost always wrong.

If your cold water smells clean and only the hot tap produces that rotten egg odor, the source is sitting in your utility room or garage. It’s your water heater, and the fix is straightforward once you understand what’s actually happening inside the tank.

What’s Actually Going On

Water heaters contain a part called a sacrificial anode rod. It’s usually made of magnesium or aluminum, and its job is to corrode instead of the steel tank walls. The rod oxidizes slowly over time, protecting the tank from rusting from the inside out.

Most water supplies contain sulfate-reducing bacteria at low levels. These organisms naturally occur in groundwater and aren’t pathogens. They don’t cause illness. But inside a water heater tank, they find ideal conditions: warm water, low oxygen, and a magnesium anode rod to react with. The bacteria use the sulfate dissolved in your water and the anode as part of their metabolic process, and hydrogen sulfide gas is the byproduct.

Hydrogen sulfide is what smells like rotten eggs. Even in tiny concentrations, the human nose detects it easily, which is one reason evolution kept that ability sharp.

The smell is typically strongest in the first few gallons of hot water after the tank has been sitting unused for a few hours. That’s when the bacteria have had time to produce gas in a closed, warm environment.

The Confirming Test

Run your cold tap. No smell. Now run the hot tap. Smell.

That result is conclusive. Your well water doesn’t selectively produce hydrogen sulfide in the hot water side of your plumbing. Wells produce it in all the water or none of it. The hot-only pattern is the signature of a water heater problem.

If both your hot and cold taps smell like sulfur, that’s a different problem entirely: actual hydrogen sulfide in your well water. Read about hydrogen sulfide in well water for how to approach that.

The Softener Connection

If you have a water softener, pay attention to this.

Softened water accelerates anode rod deterioration. The softening process replaces calcium and magnesium in the water with sodium. The higher sodium content in softened water is more aggressive on magnesium anode rods and speeds up the chemical reaction that produces hydrogen sulfide. Homes with softeners consistently report stronger and earlier onset of sulfur smell in hot water compared to homes without softeners.

If you have both a softener and rotten egg hot water, a standard magnesium rod replacement will work temporarily but the problem will likely return sooner than expected. Go straight to a zinc/aluminum alloy anode rod instead.

Fix Options, From Easiest to Involved

Option 1: Replace the anode rod with a zinc/aluminum alloy rod.

This is the right first move for most people. Zinc/aluminum anode rods (sometimes marketed as “odor-resistant” or “sulfur-resistant” anode rods) don’t drive the same hydrogen sulfide reaction that magnesium rods do. The smell stops because the chemical reaction that causes it is eliminated.

The part costs $20 to $40 at a hardware store or online. Replacing it takes about 30 minutes if you have a socket wrench with an extension and can access the top of the water heater. The rod is located at the top of the tank, usually under a plastic cap, and is tightened in with a 1-1/16 inch socket. Turn off the heater and cold water supply first. Drain a few gallons to reduce pressure in the tank before loosening the rod.

This solves the problem in the majority of cases.

Option 2: Flush and shock the tank with hydrogen peroxide.

If replacing the rod doesn’t fully eliminate the smell, or if you want to knock down the bacteria population while you’re at it, flush the tank fully and treat it with food-grade hydrogen peroxide (3% solution, diluted further). This kills the sulfate-reducing bacteria inside the tank. Your local plumber can walk through the specific procedure, as the dilution ratio and flush sequence vary by tank size.

The bacteria may reestablish over time, especially if the hydrogen sulfide problem is partly driven by sulfate content in your well water. In that case, the anode rod replacement addresses the ongoing reaction and the peroxide treatment addresses the existing bacterial population.

Option 3: Raise tank temperature to 140°F for 24 hours.

Running the water heater at 140°F pasteurizes the bacteria living inside the tank. Set the thermostat to 140°F, leave it for 24 hours, then return it to your normal setting (typically 120°F for safety). This kills bacteria effectively.

One caution: 140°F water at the tap causes scalding in seconds. If children or elderly people are in the home, make sure nobody uses hot water during the treatment period. Some plumbers add a mixing valve after the heater to deliver lower temperatures to fixtures while maintaining higher tank temperatures. That’s a more permanent safety solution if you want to keep the tank consistently hotter.

This approach treats the symptom (existing bacteria) but doesn’t stop the underlying reaction if your anode rod is still magnesium. Combine it with the anode rod replacement for better long-term results.

How to Know if the Anode Rod Is Due for Replacement

If your water heater is more than 5 years old and you’ve never replaced the anode rod, it’s likely overdue regardless of the smell issue. Most manufacturers recommend inspection every 2 to 3 years and replacement every 4 to 5 years.

A fully depleted anode rod looks like a thin wire with some remaining coating. If the rod is worn down to less than half its original diameter, replace it. The manufacturer’s documentation for your heater will specify the rod’s location, thread size, and the replacement part number.

The Simplest Path Forward

Try the zinc/aluminum anode rod first. It’s $20 to $40, it takes 30 minutes, and it permanently changes the chemistry that drives the smell. Most homeowners who do this once report the problem never comes back. If you have a water softener, this replacement is even more important because softened water will burn through a magnesium rod faster than the manufacturer’s warranty expects.

If the smell comes back, or if it’s still present after the anode replacement, have a plumber do a peroxide flush and check whether your well water itself has elevated sulfate levels. At that point, a whole-house water quality test is worth doing. The well water testing guide covers what to order and how to read the results.

Sources: EPA Private Wells | Water heater manufacturer documentation for anode rod specification and replacement intervals