Rotten-egg smell from the hot water
How to tell hydrogen sulfide at the hot tap from a gas leak, and which anode change addresses sulfate-reducing bacteria.
A rotten-egg odor that shows up only when you run hot water, and not when you run cold, is almost always hydrogen sulfide produced in the tank. Sulfate-reducing bacteria use the tank as a warm, oxygen-poor home, and the sacrificial anode — especially a magnesium rod — supplies a reaction path that makes the smell obvious.
The cold water skips that tank, so the kitchen cold tap smells normal. This is unpleasant and it is not, by itself, a gas leak. A gas leak does not wait for you to open the hot tap, and it does not spare the cold tap. The distinction matters because the wrong response to each odor is expensive in a different way.
Emergency. If you smell sulfur in the room before any tap is open, if the odor is strong near the heater or the furnace rather than at the faucet, or if you are not sure, leave the house and call the gas utility from outside.
Do not use switches. You can sort bacteria from odorant after a technician or the utility has said the atmosphere is safe.
The venting guide is the carbon monoxide companion. Natural gas and propane odorant are added so you will leave. Do not talk yourself into staying.
Prove it is the hot side
Run the cold tap for a full minute into a glass and smell it. Run the hot tap into another glass. If only the hot glass smells, the reaction is downstream of the cold supply, which means the heater or, less often, a device that only sees hot water.
If both smell, the sulfide is in the well, the municipal supply, or a softener, and replacing the anode will disappoint you.
If a single hot fixture smells and the others do not, clean that aerator and that supply tube before you blame the tank. Bacteria colonize aerators. Pink or gray slime at the screen is a fixture problem, also mentioned in discolored water.
Check whether a softener was installed or regenerated just before the smell began. Softened water is more aggressive toward magnesium anodes, and the combination of a softener plus a magnesium rod is a classic sulfide complaint. The hardness side of that story is in hard water and scale. Note the date the smell started.
A smell that began the week the new heater was installed is consistent with a fresh magnesium anode in water that already had the bacteria. A smell that began after ten quiet years is consistent with a changing well or a failing rod that has exposed more reaction surface as it corrodes.
What the anode has to do with it
The anode exists to corrode instead of the steel tank. Magnesium is very effective at that job and, in some waters, very effective at making hydrogen sulfide noticeable. Aluminum anodes, and aluminum anodes alloyed with zinc, are the usual substitute when the complaint is smell.
A powered anode, which uses a small electrical current and a titanium electrode, does not donate the same sacrificial metal and is another way out of the odor, at a higher part cost and with a need for a power connection. The mechanical job of changing a rod — clearance, socket, when to stop pulling — is anode rods. This page only chooses the type.
Do not remove the anode and plug the port as a permanent fix. The smell may fade and the tank will rust without its volunteer. Manufacturers treat a missing anode as a warranty problem for a reason.
If a plumber suggests removal “just for the weekend to test,” ask what goes back in on Monday. The test is legitimate. The empty port is not a destination.
Chlorination can knock the bacterial population down, and it is easy to do badly. Do not pour bleach into the heater unless that model’s manual gives the dose, the contact time, and the flush. An improvised slug can burn skin at a tap, damage a softener or a septic field, and leave disinfectant in the shower.
If the manual is silent, hire a plumber or a water-treatment technician and ask them to follow a written procedure: unscented household bleach only, a stated concentration, every hot tap flushed until the chlorine is gone, and filters or treatment equipment bypassed. This page is not that dose chart.
After chlorination, change the anode to a type less prone to the reaction or the bacteria will have the same metal to work with when they return. Wells with a persistent sulfate problem may need ongoing treatment, which is a water-quality design, not a water-heater part.
What else smells, and is not this
A metallic or fishy smell when an electric element first fails can be insulation or a cooked component.
Shut the breaker off if the smell is electrical, and do not reset a breaker that tripped.
That path is the breaker article. A burning smell the first hour a new gas heater fires can be manufacturing oil on the burner. Watch it, ventilate, and stay present. A burning smell that continues, or any smell of scorched wire, is a shutdown.
A sulfur smell that is actually a drain, not the water, will be there when the taps are closed and will get worse when a dry trap in a floor drain lets sewer gas in. Pour water in the floor drain. If the “hot water smell” vanishes, the trap was empty.
Rotten eggs plus brown or black water means the corrosion story is further along. Read the discoloration guide and plan on an anode inspection soon.
Rotten eggs plus a dripping relief valve are two problems. Fixing the smell does not fix a closed system. Why the relief valve drips still applies.
Checklist: smell only, after the house is safe
- If the room smells with the taps closed, treat it as gas until the utility says otherwise.
- Compare a cold glass and a hot glass. Hot only: stay on this page. Both: test the supply, not just the heater.
- One fixture only: clean the aerator and retest.
- Note a softener, a new heater, or a recent well change.
- Replace a magnesium anode with an aluminum-zinc or powered anode suited to the water, per the heater manufacturer’s guidance for that model.
- Do not leave the tank without an anode.
- Chlorinate only from a written procedure, then flush every tap.
- If the smell survives a new anode and a proper disinfection, get the well or the supply tested. The heater is no longer the prime suspect.
Health and filters, briefly
Hydrogen sulfide at odor levels in hot water is a nuisance and a corrosion issue. This site does not set a drinking-water limit or tell you the water is safe to drink.
If the cold supply also smells, or if a lab has already flagged the well, follow that result and your local health department. Point-of-use carbon filters can strip some odor and then become a place for bacteria to grow if they are not changed.
They are not a substitute for an anode strategy inside the tank. A whole-house filter that was installed to solve the smell and then forgotten will eventually make the water worse. Put a date on it.
Temperature plays a small role. Very low set points give bacteria an easier tank. The Consumer Product Safety Commission still wants the tap near 120°F because of scalding.
Do not “boil the smell out” at 150°F without a mixing valve. The tradeoff is in temperature, scalds, and Legionella. Legionella and sulfate-reducing bacteria are not the same organism and they do not call for the same home experiment.
Sources
Manufacturer technical service bulletins and owner’s manuals on anode selection and sulfur odor (A. O. Smith, hotwater.com; Bradford White, bradfordwhite.com; Rheem, rheem.com). U.S. Consumer Product Safety Commission, “Avoiding Tap Water Scalds,” for why the remedy is not an unbounded temperature increase.
Gas-utility instructions, universally, for odorant response: leave the building and call from outside. Centers for Disease Control and Prevention Legionella temperature context is a different organism; do not mix the remedies. Local health departments and certified water labs are the right next step when the cold water smells too.