Why does only my hot water smell like rotten eggs?

When cold water is odor-free but hot water smells sulfury, the source is almost always your water heater, not your well. Water heaters contain a magnesium or aluminum anode rod that protects the tank from corrosion. Sulfate-reducing bacteria can colonize the warm water inside the tank and react with the anode rod to produce hydrogen sulfide gas. The solution is usually replacing the anode rod with an aluminum/zinc alloy rod.

How do I know if the smell is from my well or my water heater?

Run your cold water for a few minutes. Smell it. If cold water has no odor but hot water smells sulfury, the water heater is the source. If both cold and hot water smell, the problem is upstream in your well or pipes. Both can happen at the same time, but isolating cold vs. hot is the first diagnostic step.

Will replacing the anode rod fix the smell?

In most cases, yes. Replacing the standard magnesium anode rod with an aluminum/zinc alloy anode rod (also called a zinc anode) eliminates the bacterial reaction that produces hydrogen sulfide. The zinc is toxic to the sulfate-reducing bacteria. This fix is well-established for the rotten egg smell in hot water specifically.

Can I just remove the anode rod instead of replacing it?

You can, but it's not recommended for long-term use. The anode rod protects the water heater tank from corrosion. Without it, the tank will corrode significantly faster, shortening its life. Replacing with a zinc anode rod is the better solution, since it eliminates the smell while keeping tank protection in place.

Does this problem happen with tankless water heaters?

Tankless heaters don't use anode rods and don't have a standing reservoir of warm water, so the hydrogen sulfide reaction that happens in tank-style heaters doesn't apply. If you have a tankless heater and your hot water smells, the source is in your well water or supply pipes, not the heater itself.

Why Your Hot Water Smells Like Rotten Eggs (and Cold Water Doesn't)

The key diagnostic question is this: does your cold water smell, or only your hot water? If it’s only hot, the problem is almost certainly your water heater, not your well. This distinction matters because the fix is completely different for each source, and most guides lump them together in a way that wastes your time and money.

The Anode Rod Reaction Explained

Every tank-style water heater has an anode rod. It’s a long metal rod, usually magnesium or aluminum, that hangs inside the tank. Its job is to sacrifice itself to corrosion so the steel tank doesn’t rust out. Without it, most water heater tanks would fail in a few years.

The problem is that magnesium anode rods react with certain bacteria. Sulfate-reducing bacteria are naturally present in many water supplies. They’re harmless in your well water at normal concentrations. But when they enter your water heater, they find a warm, low-oxygen environment with a reactive magnesium surface, which is ideal for producing hydrogen sulfide gas.

The reaction produces a sulfury, rotten egg odor specifically in the hot water. Cold water passes straight through your pipes without entering the heater, so it doesn’t pick up the odor. If your hot water smells and cold water doesn’t, that’s almost diagnostic on its own.

Not every home with a magnesium anode rod develops this problem. It depends on your water chemistry, particularly sulfate levels and whether sulfate-reducing bacteria are present. Some well water conditions trigger it, and some don’t. Chlorinated municipal water typically prevents the bacterial colonization, so this problem is more common with private wells.

How to Confirm Your Water Heater Is the Source

Run your cold water from a kitchen or bathroom tap for 2-3 minutes. Smell it at the faucet and in a glass. No sulfur odor means cold water is fine.

Now run your hot water from the same tap and smell it. If the odor is present in hot but absent in cold, the water heater is the source. You’ve confirmed the diagnosis.

A few other confirming signs: the smell gets stronger after the water heater hasn’t been used for a while. If you’ve been away for a few days or the heater sat unused overnight, the hydrogen sulfide builds up in the standing hot water. Running the tap for a minute or two after a period of non-use produces the strongest odor. This is typical of the anode rod reaction.

Also check whether the smell is worse from some taps than others. If you have a recirculating hot water system that keeps hot water moving through the pipes constantly, the smell may be more diffuse. But if your hottest tap (usually closest to the heater) smells strongest, the heater is the source.

One more check: go outside and smell your hose bib or an outdoor cold water tap. If that water has no odor, your source water is fine and the problem is internal to the heater.

The Fix: Replace the Anode Rod

Replacing the magnesium anode rod with an aluminum/zinc alloy anode rod is the standard fix. Zinc is toxic to sulfate-reducing bacteria, so the new rod doesn’t support the same reaction. The smell typically disappears within a few days of the swap, once the old hydrogen sulfide clears out of the tank.

The anode rod is accessed through the top of the tank. On most water heaters, it’s under a hex head bolt, either exposed or under a plastic cap. The rod is about an inch in diameter and runs most of the length of the tank. You’ll need a 1-1/16 inch socket and a long extension bar. The rod is usually torqued in fairly tight, so a breaker bar is often helpful.

Before you start: turn off the heater. Shut off the cold water supply to the heater. Attach a garden hose to the drain valve and drain a few gallons to relieve pressure. Then loosen and remove the anode rod with your socket.

Take the old rod with you when you buy the replacement. Replacement aluminum/zinc rods are sold at plumbing supply stores and home improvement stores. Make sure it matches the thread size and length of your original.

If your water heater is still under warranty, check the warranty terms before doing this yourself. Some manufacturer warranties require professional service.

When Flushing the Tank Also Helps

Sediment builds up at the bottom of tank water heaters over time. This sediment layer creates an anaerobic (low-oxygen) zone at the bottom of the tank, which is exactly the environment sulfate-reducing bacteria prefer. If your tank has heavy sediment buildup, replacing the anode rod alone may not fully eliminate the odor.

Flushing the tank removes the sediment and the bacterial colony living in it. Connect a hose to the drain valve at the bottom of the tank, run it outside or to a floor drain, and flush until the water runs clear. This is good maintenance regardless of odor issues, and it’s worth doing at the same time you replace the anode rod.

Electric water heaters sometimes develop more severe bacterial colonization because they heat water to lower temperatures than gas heaters. If your electric heater smells, you can also try temporarily boosting the thermostat to 140 degrees F for a few hours to pasteurize the water inside the tank. Do this carefully. Water at 140 degrees F causes scalding burns faster than most people expect. Use the high-temperature setting only temporarily and return to 120 degrees F afterward.

When the Problem Is in Your Well

If both your hot and cold water smell like rotten eggs, the source is hydrogen sulfide in the well water itself, not the heater. This is a different problem that needs treatment at the point of entry to the house.

The hydrogen sulfide hub covers the causes and treatment options for sulfur odor coming from your well. Your well water hub is also a starting point for understanding what testing and treatment options apply to your specific well.

The symptom page for hot water that smells like rotten eggs covers both the heater and the well-source scenarios together if you want a broader overview.

Sources

Water Quality Association. “Hydrogen Sulfide in Water.” WQA Technical Fact Sheet. https://www.wqa.org
Rheem Manufacturing / Water Heater Manufacturer Documentation on Anode Rod Function and Replacement.