Hot Water Smells Like Rotten Eggs: Causes, Fixes, and How to Tell It From Gas

A sulfur or rotten-egg smell from the hot tap is hydrogen sulfide gas dissolved in the water. If only the hot water smells, look at the water heater: A. O. Smith traces the smell to bacteria in the tank reacting with the anode rod, the metal rod that corrodes so the tank doesn't. The fixes are to disinfect the tank, change the anode, and, in stubborn cases, have a plumber run the tank hotter for a short time with anti-scald mixing valves in place.

Rule out gas first, then check whether the cold water smells too. If the heater also rumbles, see water heater popping noise; if there is water at its base, see water heater leaking.

First, Check: Air or Water, Hot or Cold

Air or water? Gas smells in the air around the appliance or meter whether or not a tap is running, and Bradford White and State warn that propane is heavier than air and collects at floor level, so a smell that is strongest low down is gas too. The water heater odor comes with the water: Bradford White's manual places it "at the hot water faucet." Run hot water into a glass, carry it to another room, and smell it. If the glass smells, it is the water. If the utility room smells with no water running, or you can't tell, treat it as gas. Bradford White warns that the odorant can fade; if in doubt, call the gas supplier from a neighbor's phone.

Hot only, or cold too? Run the cold tap for a minute and smell it, then the hot. Hot only points to the water heater, causes one and two below. Hot and cold both point to the water coming into the house, causes three and four. If one sink smells but a glass of its water does not, the smell is probably coming from the drain.

Most Likely Causes

1. Bacteria reacting with the anode rod (hot only). Most tank water heaters have at least one anode rod, often magnesium. A. O. Smith's manuals say the smell develops only when four things are present at once: sulfate in the supply water, little or no dissolved oxygen, sulfate-reducing bacteria in the heater, and extra active hydrogen from the anode's corrosion protection. The bacteria turn sulfate into hydrogen sulfide. A. O. Smith calls them harmless to humans and says they thrive below 138°F, which includes a normal 120°F setting.
2. A softener, a well, or a heater that sat unused (hot only). A. O. Smith lists water softeners, well water, and long periods with no water moving through the system as conditions that raise the odds of these bacteria, and says they often enter through construction or a break in underground piping.
3. Hydrogen sulfide in a private well (hot and cold). The smell is in the water before it reaches the heater. EPA says to test right away when the odor changes, and lists hydrogen sulfide among the things to test for when water tastes or smells objectionable. Use a state-certified lab; CDC says your health department can help you find one. Both advise testing every year for total coliform bacteria, nitrates, total dissolved solids, and pH. Treating the supply itself is a job for a water treatment professional.
4. A problem on the city side (hot and cold, municipal water). Call the water utility and ask whether others have reported it; EPA says the annual water quality report your utility must send lists its phone number. CDC says a change in smell is not necessarily a health concern but is sometimes a sign of a problem, and your health department is the other call.

What You Can Fix Yourself

These are the makers' fixes. Several mean pulling the anode, often a plumber's job, and chlorination and running the tank hotter belong with the plumber too; the next section says when.

1. Flush the tank, then have it disinfected. A plain flush, as in our flush guide, clears sediment; for odor, Bradford White says to flush "with appropriate dissolvers to eliminate any bacteria." The disinfection A. O. Smith spells out is shock chlorination (Technical Bulletin 23), and it is a job for a plumber or the heater's service agency, which is where A. O. Smith's manual sends you for it. It means turning off the gas or power, closing the cold supply, draining the tank, and pulling the anode, the step that often needs a plumber (see below); then household bleach goes in through the anode opening, one gallon per 25 gallons of tank capacity, every hot fixture, dishwasher and washer included, runs until it smells of chlorine, and the solution sits for an hour or more before the tank and lines are flushed clean. A. O. Smith warns that an untreated line can reinfect the rest and that heavy infections may need repeat treatments. If you do it yourself anyway, follow the bulletin exactly, let the heater cool completely first, as A. O. Smith advises before touching components, and follow the CDC's bleach rules: never mix bleach with ammonia or any other cleaner, including a descaler or dissolver from an earlier flush; wear gloves and eye protection; and open windows for fresh air. Keep everyone off the hot water until the chlorine is flushed out, and restart the heater by its label; Rheem says the tank must be full before an electric heater is turned on.
2. Swap the magnesium anode for a less active one. Rheem says the smell "can be solved by switching to an aluminum anode or powered anode"; A. O. Smith suggests an aluminum/zinc composite anode and says a less active anode, followed by chlorinating the tank and hot lines, may eliminate or reduce the smell in some models. Bradford White says an alternative anode from your installer "will minimize the odor while protecting the tank." A powered anode is a titanium rod that runs on a small current instead of wearing away; State says it is less likely to contribute to smelly water. Check your manual and warranty first, since A. O. Smith and Bradford White point you to the heater's supplier. Most tanks have a dedicated anode port on top; most Bradford White models put the anode in the hot water outlet.

3. Running the tank hotter is a plumber's step, not a dial turn. For persistent odor, A. O. Smith says to raise the water to 140°F for a short period to kill the bacteria, then return to a safer temperature, and to install thermostatic mixing valves at each point of use to prevent scalding. Don't raise the setting without those valves in place: the CPSC urges a 120°F setting and says 140°F water causes third-degree burns in six seconds, and most tap water scald injuries and deaths involve children under five and older adults. Installing mixing valves is plumber work, so have the plumber handle the temperature step too and set the heater back to 120°F afterward.
4. Never take the anode out and leave it out. A. O. Smith: "Do not remove the anode leaving the tank unprotected. By doing so, all warranty on the water heater tank is voided." Rheem says running without it "will greatly shorten the life of the glass lined tank and will exclude warranty coverage," and Bradford White says removing any anode except for inspection or replacement "will nullify the warranty."

When to Call a Pro

Call a licensed plumber for the anode swap unless you have the tools, the clearance, and the confidence. Rheem says replacing an anode "often requires tools and safety precautions best handled by a qualified plumber." The job starts with the gas or power off, the cold supply closed, and some water drained. A factory anode can be stuck tight: Corro-Protec's removal tips call for a breaker bar or an impact wrench, with the heater braced so it does not turn. A full-length rod may need more headroom than you have, which can mean cutting it into sections on the way out without dropping pieces into the tank. On most Bradford White heaters the anode is part of the hot water outlet fitting, a plumbing connection. Call as well for tank chlorination, for running the tank at 140°F and the mixing valves that requires, for any gas valve or burner work, and for a smell that returns after a new anode and chlorination, which A. O. Smith says means treating the water supply. Rheem says odors, like leaks, should be inspected by a licensed technician.

Prevent It

Have the anode checked every year. Rheem says to replace it when more than 6 inches of core wire is exposed, and Bradford White says softened water uses it up faster, so check more often with a softener. Flush the tank yearly, keep the heater at 120°F, and test a well every year. Put a leak alarm at the base of the tank, too: A. O. Smith says that once the anode is mostly gone, the tank starts rusting from the inside.

Frequently Asked Questions

Is it safe to shower in water that smells like rotten eggs?

When only the hot water smells, A. O. Smith describes the bacteria as nontoxic to humans and says hydrogen sulfide is toxic only in extremely high concentrations, and can be smelled long before it reaches harmful levels. If the cold water smells too, test a well or call the utility; CDC says a change in smell is sometimes a sign of a problem.

Can I just remove the anode rod to stop the smell?

No. A. O. Smith, Rheem, and Bradford White all say running without the anode voids the tank warranty, and Rheem says it greatly shortens the tank's life. Replace it with an aluminum/zinc or powered anode instead, and chlorinate the tank.

Why does my hot water smell like rotten eggs after a vacation?

A. O. Smith lists long periods without water movement among the conditions that encourage the bacteria. After two weeks or more away, Rheem and A. O. Smith also warn that hydrogen gas can collect in the hot water system, and both call it extremely flammable. Before using the dishwasher, washer, or any other electrical appliance on the hot water, open a hot faucet for several minutes (A. O. Smith says the one farthest from the heater; Rheem says the kitchen sink), with no smoking or open flame nearby. A sound like air escaping from the pipe as the water starts means gas is coming out.

The Bottom Line

Gas first: a rotten-egg smell in the air near a gas appliance means leave and call from outside. If only the hot water smells, bacteria are reacting with the heater's anode, and the makers' fixes are chlorination, an aluminum/zinc or powered anode, and, for stubborn cases, a plumber running the tank at 140°F for a short time with mixing valves in place. If the cold water smells too, test the well or call the utility. Never run the tank without an anode, and hand the swap to a plumber when it is stuck, short on headroom, or built into the hot outlet.

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