Hot Water Smells Like Rotten Eggs: Causes and Fixes

The Short Answer: If only the hot water smells like rotten eggs, it’s hydrogen sulfide made inside the tank, not a gas leak. Sulfate in the water, bacteria, and hydrogen from the anode rod combine to make it. It isn’t a fault in the heater. Manufacturers’ fixes are chlorinating the tank and hot lines, and switching to a less active anode, done by a plumber.

Gas leak or water odor?

The test is where the smell is:

• In the room, whether or not water is running: treat it as a gas leak.
• Only when hot water runs, and stronger at hot taps than cold: water odor from the heater.
• In both hot and cold water: the source is the water supply, not the heater. Have the water tested.

Why it happens

A. O. Smith’s manual (opens in a new tab) says four things must all be present:

1. Sulfate in the supply water.
2. Little or no dissolved oxygen in the water.
3. Sulfate-reducing bacteria in the water heater. A. O. Smith notes these are harmless and nontoxic to humans.
4. Excess active hydrogen in the tank, made by the anode’s corrosion-protection reaction.

Rheem’s bulletin 1206 (opens in a new tab) calls it “a water chemistry condition, rather than a water heater problem.” It says the problem is more common with softened water, and that a more active anode may add to it, because it releases more hydrogen. The smell is noticeable at very low levels, around 0.5 parts per million.

Rheem’s bulletin links it to softened water, water that has sat and lost its oxygen, and untreated well water.

What fixes it

The manufacturers agree that the answer is to deal with the bacteria and reduce the anode’s activity:

In Bradford White’s procedure, chlorinating a tank means draining part of it, removing the anode (on Bradford White tanks it’s part of the hot water outlet fitting), pouring bleach in through the hot water outlet opening, installing the replacement anode, letting it stand, then flushing every hot line until the chlorine smell is gone. Rheem’s bulletin lets the bleach stand for an hour; Bradford White’s, at least three hours. Bradford White publishes its procedure in a bulletin for professionals (bulletin 109 (opens in a new tab)). Because it involves the anode fitting and strong bleach in the drinking water system, have a plumber do it.

What you can do now

• Run the hot water after time away. If the heater sat unused for two weeks or more, Rheem’s gas manual (opens in a new tab) says to open the hot faucet at the kitchen sink for several minutes before using electrical appliances on the hot water system, because flammable hydrogen gas can build up. Don’t smoke or have an open flame near the faucet while it runs; a sound like air escaping from the pipe means hydrogen was present.
• Note when it started. A new softener, a new heater, a well, or a vacation are useful clues for the plumber.
• Don’t remove the anode. Running a tank without one voids the tank warranty and shortens its life. Rheem’s manual says operating with the anode removed “will greatly shorten the life of the glass lined tank.”
• Don’t rely on turning up the temperature. Bradford White says the bacteria tolerate heat up to about 225°F, well above any safe tank setting.

More on the rod itself: water heater anode rod.

Other water heaters

• Tankless heaters don’t store water, so the smell usually comes from the supply or from water sitting in pipes. See tankless water heater sulfur smell.
• RV heaters: see RV water heater smells like rotten eggs. Suburban’s manual (opens in a new tab) gives owners a bleach treatment for the RV water system.
• Other odors, such as fishy, musty or burning smells, have different causes. See hot water heater smells fishy and water heater smells like a dead animal.

Frequently Asked Questions

Q: Is rotten-egg smelling hot water dangerous? A: The bacteria A. O. Smith describes are harmless to humans, and the smell is noticeable at very low concentrations. It’s unpleasant rather than an emergency. The exception is a rotten-egg smell in the air near a gas appliance, which may be a gas leak.

Q: Will a new water heater fix it? A: Usually not by itself. A new tank comes with a new anode, often magnesium, and the water that caused the smell is the same. Ask the installer which anode type suits your water.

Q: Does a water softener cause the smell? A: It can make it worse. Rheem says the problem is more common in softened water, and softened water uses up magnesium anodes faster.