Hot Water Smells Like Rotten Eggs? It's Almost Always Your Water Heater

If the rotten egg smell only comes out of your hot taps and the cold water smells clean, the smell is being made inside your water heater, not in your well. When someone calls me about a sulfur smell, the first thing I ask is whether they smell it on the cold, the hot or both, and a hot-only answer usually points to the anode rod in the water heater. The bacteria that make the smell are not the harmful kind. The fix is a different rod and a chlorinated tank, done by a plumber, not a filter.

Aidan "Is there any taste or odor in the water? Rotten egg odor?"

Caller "Not really. Mainly after it comes through a hot water heater, there's been a little bit."

Aidan "If it's only on the hot water side, it's usually the anode rod in the hot water heater."

That came up in August, while I went over a caller's well test. This page is the long version of my answer.

Smell the cold tap first

Smelling a cold tap and a hot tap on their own tells you where the smell is coming from, and it costs nothing.

1. Do it first thing in the morning, or after the house has been empty for a few hours, so the water has sat.
2. Run a cold tap and smell the water.
3. Run a hot tap and smell that.
4. Find what you smelled in the table.

A caller who only smelled it on the hot side asked me about the rod in the tank. My answer is the rule this table rests on: "If you smell it on the cold water side, it has nothing to do with the hot water heater."

Smelled both taps and still not sure?

Tell me what each tap smelled like

Text Aidan what the cold and the hot smelled like, and whether you have a water softener. He can tell you whether it is the heater or the well, and what to ask your plumber for.

Text Aidan443-277-2204 Call800-460-5810Aidan

8 AM to 5 PM Eastern, seven days a week. A real person, not a call center.

Why the heater makes the smell

Three things make the smell more likely: a water softener, well water, and water that sits unused for a long stretch. The heater makes the smell out of what your water brings into it.

The anode rod is a metal rod, 30 to 40 inches long, that hangs down inside the tank from a plug on top. The rod wears away slowly instead of the tank. A new one is smooth, and a worn one comes out corroded and pitted. Most are magnesium, and magnesium is the metal that feeds the smell.

A.O. Smith, the water heater maker, says the bacteria need four things in the tank at once:

• Sulfur in the water, which the bacteria turn into the smelly gas.
• Hydrogen, made in the tank by the rod.
• Little or no oxygen, as in water sitting in a closed tank.
• Water below 138 degrees, where the bacteria thrive.

The bacteria are a non-toxic kind. A.O. Smith says they often get into a water system during construction or through a break in an underground pipe, and that the smell is not caused by the heater itself.

Change the anode rod in the tank

The fix that lasts is swapping the magnesium rod for an aluminum or zinc rod and chlorinating the tank. The chlorine has to reach every hot water line too. As I put it in one of my videos: "take out the one that came with it, replace it with say an aluminum one or another type that doesn't give off that smell and you solve the problem."

I recommend a licensed plumber for the rod change. The job means turning off the water, letting the pressure out of the tank and unscrewing a plug on top. Minnesota's health department says to have a plumber do it unless you know water heaters well, and Bradford White says it takes a plumbing professional.

Before you call, read the label on the heater for the brand and the gallons. Most tanks take the rod through a plug on top; most Bradford White tanks have it in the hot water outlet instead, which takes a different rod.

I don't sell anode rods, and you don't need anything from me for this fix. Home Depot and plumbing parts sellers carry them.

Have the plumber bleach the tank

A new rod takes away what feeds the reaction, and chlorine kills the bacteria already in the tank and the hot lines. Ask for the new rod and the chlorine on the same visit. A.O. Smith's procedure is below, shortened, so you know what your plumber is doing and what you are paying for.

If the smell comes back after both, see When the cold water smells too.

Quick fixes that wear off in weeks

These are the things people try first. Each one either wears off or puts the tank at risk.

• Turning the heater up to 160 degrees for a few hours. Minnesota's health department lists it, and it kills the bacteria. Researchers who studied smelly water heaters at 14 sites report that a one-time heat treatment holds the smell off for only 1 to 2 weeks. Water that hot scalds: check that the relief valve works, and turn the heater back down afterward.
• Chlorinating the tank without changing the rod. It kills what is there now, but if any bacteria survive, the smell can be back within a few weeks. A.O. Smith says heavy cases often take repeated tries.
• Pulling the rod out and leaving it out. The smell stops, but nothing protects the tank. A.O. Smith says that voids the tank warranty, and Minnesota's health department says it can significantly shorten the heater's life.
• Running the hot tap until it clears. That flushes out the water that sat overnight. The rod and the bacteria are still in the tank, so the smell can come back the next time the water sits.

When the cold water smells too

This part is for a smell on both taps. If only your hot water smells, start with the new rod and the chlorine, not a filter.

If the cold water smells at all, the gas is already in the water before it reaches the heater, and a new rod will not fix it. A faint well smell can seem to live only in the shower, because heating water drives the dissolved gas out of it. A caller in Washington State told me the water did not smell coming out of the well, and my answer was: "It's just sulfur in the ... hot water side. When you heat it up, it ... does intensify it."

A smell that is in the well water has to be treated where the water comes into the house, before it reaches the heater or any tap. A.O. Smith's advice for a smell that comes back after a new rod is aeration or chlorination of the water supply, and the filter I recommend for it works by air. I explain how it works in where the rotten egg smell comes from, and how your level decides the fix in what your hydrogen sulfide level means.

A caller in Texas described it well: "And then the sulfur, when it hits the hot water heater, it stinks." That well had iron too, and the air-injection filter I recommended "takes out iron, sulfur, and manganese" in one tank. The sulfur filters I sell run $2,095 to $3,295 by the size of the house. Each ships with the tank empty and the media in separate bags for loading at install; drain tubing is not included, and I recommend a licensed plumber for the install.

In this 3-minute video I start with the cold, hot or both question at 0:32, then show what takes the smell out of well water.

Both taps smell? Hydrogen sulfide lab test, $139

Measure the gas before you buy anything

One certified lab test reports how much hydrogen sulfide is in your well water, plus pH, and that number sets the size of the fix. The gas escapes from an ordinary sample, so this kit is built to hold it. If you already have a lab report with a sulfide number, skip this.

Get the hydrogen sulfide test

Results in about 10 business days, with a free call to go over them.

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