Water Heater Smells Like Rotten Eggs: Is It Gas Or The Water?

A rotten egg smell around a water heater is either natural gas or hydrogen sulfide in the hot water, and the two call for opposite responses. The Pipeline and Hazardous Materials Safety Administration describes odorized natural gas as having a distinctive “rotten egg” smell. A State Water Heaters manual describes a hot water odor with the same name, caused by a reaction involving the tank’s anode rod. This page starts with how to tell them apart, because the cost of confusing them is lopsided.

First: Is It Gas?

If the smell is in the air near the heater, treat it as a gas leak and leave. PHMSA’s guidance for an odor of natural gas is to make sure gas appliances are turned all the way off, leave the area, and telephone 911 from a neighbor’s house or another location well away from the gas leak. It says not to start an engine, strike a match or make a flame, use a telephone or cell phone, or turn light switches, garage door openers or other electrical switches on or off. Our emergency shut-off guide opens with the same rule.

A manufacturer’s manual adds a reason not to rely on your nose. State Water Heaters’ manual for its residential gas tankless units (models THR-160, THR-180 and THR-199) says the odorant added by the gas supplier “may fade over an extended period of time,” tells the reader not to depend on it as an indication of leaking gas, and recommends a fuel gas and carbon monoxide detector. If you cannot tell whether the smell is gas or water, treat it as gas.

When It Is Only The Hot Water

If the smell comes only from hot water at the tap, Penn State Extension says reactions inside the water heater may be the source. Its page on hydrogen sulfide in water wells says the gas may sometimes be noticeable only in the hot water, in which case chemical reactions within the water heater may be the source. It adds that water heaters are fitted with a magnesium rod to inhibit corrosion, and that the rod can chemically reduce sulfates to form hydrogen sulfide. That page is about well water; the State Water Heaters electric manual describes the same smell for water heaters generally, in a section headed “Smelly Water.”

What Has To Be Present For The Smell

The manual says four conditions must all be present. The first is a concentration of sulfate in the supply water. The second is little or no dissolved oxygen in the water. The third is a sulfate-reducing bacteria within the water heater, which the manual calls harmless and non-toxic to humans. The fourth is an excess of active hydrogen in the tank, which it says is caused by the corrosion-protective action of the anode. The manual calls the smell “the most common complaint” associated with the anode rod, and says the odor comes from hydrogen sulfide gas dissolved in the water.

That last point matters for your own supply. Sulfate in the water is one of the four, and we publish no sulfate figure for Lancaster-area water, because the utility serving an address, or a private well, determines it. If you want to know what your supply contains, that is a question for your water utility’s published data or for a water test.

Is The Hot Water Odor Dangerous?

Penn State Extension says hydrogen sulfide is poisonous and flammable, and that the human nose can detect it well before it causes health concerns. The State manual describes the bacteria involved as harmless to humans. Neither source says the smell is something to ignore indefinitely, and neither addresses a particular household’s exposure, so if the smell is strong, persistent or accompanied by symptoms, have the water and the heater looked at rather than guessing.

What The Sources Say Can Be Done

Both sources point to the anode rod, and both warn against simply taking it out. The State manual says smelly water “may be eliminated or reduced in some water heater models” by replacing the anode or anodes with one of less active material and then chlorinating the water heater tank and all hot water lines. Penn State Extension says replacing the magnesium rod with an aluminum rod “should eliminate the rotten egg odor while maintaining corrosion protection.” The manual adds that if the smell persists after the anode replacement and chlorination treatment, continuous chlorination and filtering and conditioning equipment could be considered.

The warning is the same in both. The manual says not to remove the anode and leave the tank unprotected, and that doing so voids all warranty on the tank. Penn State says removing the magnesium rod could cause increased corrosion and reduced life of the heater, and will likely void the manufacturer’s warranty. For what the rod does, see our anode rod guide; the replacement and the chlorination involve a pressurised tank, hot water and chemicals, so they are a technician’s job and this page gives no procedure.

What This Page Will Not Tell You

It will not tell you that the smell means your anode rod is spent, how long a rod lasts, or that raising the thermostat will cure it. The manual says the smell needs sulfate, bacteria and active hydrogen together, which is not the same as a worn rod. Online advice to turn the water temperature up circulates widely; we found no equivalent instruction in the manufacturer documents we read, and the same manual warns that hotter water can scald. For general repair help, see water heater repair in Lancaster, PA, and for the other problems a heater can show, no hot water.

Rotten Egg Smell FAQ