Why does my hot water smell like rotten eggs?

You turn on the hot tap and the bathroom smells like a struck match, or a drainage ditch in August. Cold water from the same faucet smells fine. That pattern is so specific that it points straight at the water heater, and it is one of the most common water quality complaints in Lee and Collier County, especially on well water.

The smell is hydrogen sulfide, the gas that gives rotten eggs their name. At the levels found in a home water heater it is a nuisance, not a hazard, but it makes a shower unpleasant and it does not go away on its own.

Why only the hot water smells

Inside every tank water heater is a sacrificial anode rod, a long magnesium or aluminum bar screwed into the top of the tank, there to corrode instead of the steel. Water everywhere carries small amounts of sulfate and a class of harmless bacteria that feed on it. Those bacteria like warm, still water, which is what a tank provides, and when they meet the hydrogen given off by a corroding magnesium anode, the reaction makes hydrogen sulfide gas. It dissolves into the hot water and comes out at the tap.

The cold side never touches the anode, so it does not smell. That is the whole diagnosis. If the cold water smells the same, skip to the cold water section below; that is a different problem.

The smell often shows up after the house has sat unused. Seasonal residents notice it the first week back in the fall; a heater turned down for the summer gives the bacteria months of warm, still water to multiply in.

Well water and city water are different cases

• City water in Fort Myers, Cape Coral, Bonita Springs, Naples, and the districts between them carries chlorine or chloramine, which keeps the bacteria count low. The smell still happens, usually with an old magnesium anode, a low temperature setting, or a house that sat empty, but it is milder and easier to clear. A whole-home carbon filter that removes the chlorine can, as a side effect, let the bacteria back in.
• Well water in Lehigh Acres, Alva, Buckingham, eastern Estero, and rural Collier County has no disinfectant, often more sulfate and iron, and sometimes hydrogen sulfide straight from the aquifer. Heaters on well water get the smell more often, and it comes back faster after a fix. A softener makes it worse: softened water is more conductive and eats a magnesium anode faster, which feeds the reaction.

The lasting fix: change the anode rod

Flushing alone brings the smell back in weeks. Removing the magnesium removes the fuel for the reaction. Two options:

• Aluminum-zinc anode. Mostly aluminum with a little zinc. It still protects the tank but does not give off hydrogen the way magnesium does, so the bacteria have nothing to react with. The standard first fix on city water.
• Powered anode. A titanium rod with a small plug-in power supply. It protects the tank electrically, never wears out, and produces no reaction at all. It is the reliable answer on well water and softened water, where a metal anode disappears in a year or two.

Changing an anode means shutting off the water and power, relieving pressure, and breaking loose a fitting on top of the tank that has not moved in years. In an open garage with room overhead, a handy homeowner with a breaker bar and a socket can do it. In a closet with no clearance, the rod cannot come out straight, and a segmented rod or a plumber is needed. If the tank is past eight years old, this is also the moment to ask whether it is worth another anode.

Flush and disinfect the tank

A new anode stops the reaction, but the bacteria and the sulfur already in the tank need to go. The steps, in order:

1. Turn off the breaker (electric) or set the gas control to Vacation or Pilot.
2. Close the cold inlet valve on top of the tank.
3. Connect a hose to the drain valve at the bottom, run it outside or to a floor drain, open a hot tap somewhere in the house, and drain the tank.
4. Open the cold inlet in short bursts to stir the sediment, and drain again until the water runs clear.
5. Add the disinfectant: hydrogen peroxide, roughly a pint or two of the 3 percent drugstore kind per 40 gallons, poured in through the anode port or the hot outlet. A weak bleach solution also works but must be flushed out thoroughly.
6. Refill the tank with the hot tap still open until water runs from it with no air, then close the tap.
7. Let the disinfectant sit for an hour or two, then run hot water at every tap until the smell of peroxide or chlorine is gone.
8. Restore power, or turn the gas control back to On.

Never power an electric heater with the tank partly empty; the upper element burns out in seconds. And a drain valve that has not been opened in years may not reseal, so have a cap or a new valve on hand.

When the cold water smells too

If the smell is on both hot and cold, the heater is not the source. On city water, a sulfur smell from the cold tap is unusual and worth a call to the utility, then a look at any whole-home filter or softener that may have gone bad. On a well, it usually means hydrogen sulfide in the aquifer or sulfur bacteria in the well and pressure tank. The fix is at the well: shock chlorination of the well and plumbing, and if the smell returns, aeration, an oxidizing filter, or chlorine injection sized to your water. A lab test tells you what you are treating before you buy equipment.

Where the DIY line is

Flushing a tank and adding peroxide is within reach of most homeowners comfortable with a hose and a breaker panel. Pulling an anode in a tight closet, choosing the right rod for your water, or treating a well that smells on both sides needs the right tools, a water test, and a plan.

Tell us whether the smell is hot only or both, whether you are on a well or a utility, and how old the heater is. With those three facts we can usually say what the fix is and whether the tank is worth it.