The smell of rotten eggs from a faucet or in the shower is one of the most alarming things a homeowner can notice. It is distinctive, it is unpleasant, and it raises immediate questions about whether the water is safe to use. The source of that odor is almost always hydrogen sulfide gas, and in most residential situations it is coming from the water heater rather than the water supply itself.
This is a common call NWA C&S Plumbing receives from homeowners throughout Springdale, Fayetteville, Bentonville, and Rogers. The good news is that it is almost always diagnosable and fixable. This guide walks through what causes the odor, how to identify the source in your specific situation, and what the solution looks like.
What Causes the Rotten Egg Smell
Hydrogen sulfide gas is produced when sulfur-reducing bacteria interact with the sulfate compounds naturally present in water. The bacteria are not harmful in themselves at the concentrations found in residential plumbing, but the gas they produce is the source of the odor.
The bacteria thrive in environments with warm water, low oxygen, and a food source. The inside of a water heater tank meets all three conditions, particularly the sacrificial anode rod that sits inside the tank. The anode rod is made from magnesium or aluminum and is designed to corrode instead of the tank. As it corrodes, it releases electrons that attract sulfur-reducing bacteria, which then produce hydrogen sulfide as a byproduct.
This is why the rotten egg smell is almost always associated with hot water rather than cold. Running only the cold tap usually produces odor-free water. Running the hot tap produces the smell. That distinction is the first diagnostic step.
Diagnosing Where the Smell is Coming From
Run only the cold water tap at a sink for a minute and smell it. Then run only the hot water tap and smell it. If the smell is present only on the hot side, the water heater is the source. If it is present on both hot and cold, the source is either the water supply itself or a water softener that is serving both.
If the home is on a well, the well water supply may contain natural hydrogen sulfide from the aquifer. This is a geological condition that requires water treatment at the point of entry rather than at the water heater.
If the home has a water softener and the smell is present on both hot and cold, the softener’s resin tank or brine tank may be harboring the same sulfur-reducing bacteria. Disinfecting the softener system addresses this.
Homes on municipal supply that experience the smell only on hot water have almost certainly identified the water heater as the source.
The Anode Rod and Why It Matters
The solution in most cases of hot-side-only rotten egg odor is replacing the anode rod. A magnesium anode rod in particular produces more hydrogen sulfide than an aluminum rod because magnesium reacts more strongly with sulfate compounds in the water.
Replacing the magnesium rod with an aluminum or zinc-aluminum alloy rod reduces the chemical reaction that produces the odor. This is a standard repair that a licensed plumber handles during a water heater service call.
If the anode rod is severely depleted, which is common in water heaters that have not been serviced in several years, the bacteria have been operating unchecked for an extended period. In this case, the plumber may recommend flushing and disinfecting the tank with a diluted bleach solution in addition to replacing the rod.
When the Water Heater Temperature is Set Too Low
Water heater thermostats set below 120 degrees Fahrenheit allow the tank water to stay at a temperature warm enough for bacteria to thrive but not warm enough to suppress their growth. Temporarily raising the water heater temperature to 140 degrees for a day can kill off the bacteria population inside the tank, reducing the odor. The temperature should be returned to a safe setting afterward.
This is a short-term measure to confirm that bacteria in the tank are the source of the smell, not a permanent solution. Addressing the anode rod and flushing the tank is the complete fix.
How to Address the Smell in a Water Softener
If the rotten egg smell is coming from both hot and cold water after going through a water softener, the softener system requires disinfection. Sodium hypochlorite solution, which is household bleach in a diluted form, is introduced into the brine tank and cycled through the softener’s resin tank. This kills the sulfur-reducing bacteria without damaging the resin.
The process needs to be repeated if the bacteria recolonize. Homes with persistent odor issues in softened water may benefit from a UV disinfection unit installed before the softener, which prevents bacteria from establishing in the first place.
Well Water and Natural Hydrogen Sulfide
Homes on private wells in Northwest Arkansas occasionally draw from aquifers with naturally elevated hydrogen sulfide concentrations. This produces odor in both hot and cold water regardless of the water heater condition.
Addressing hydrogen sulfide in well water requires treatment at the point of entry. Options include aeration systems that allow the gas to escape before the water enters the home, catalytic carbon filtration, or chemical injection systems. A water quality test identifies the concentration and the appropriate treatment approach.
Frequently Asked Questions
Is hydrogen sulfide in my water dangerous?
At the concentrations typically found in residential plumbing, hydrogen sulfide is not considered dangerous. The odor is detectable at very low concentrations, which is why it is noticeable long before it reaches levels of concern. If the smell is extremely strong or persists in the air even when no water is running, additional investigation is warranted.
Will the smell go away on its own?
It is unlikely. The bacteria producing the odor will continue to thrive as long as the conditions that support them remain unchanged. The anode rod continues to deplete. The tank temperature stays within the bacteria-friendly range. Addressing the source is the only reliable fix.
Can I just remove the anode rod?
Removing the anode rod eliminates the food source for the bacteria and will reduce or eliminate the odor. However, it also removes the only protection the tank steel has against corrosion. Without the sacrificial rod, tank corrosion accelerates significantly, shortening the life of the water heater. Replacing with the right type of rod is the better solution.
How do I know if my anode rod needs replacing?
A plumber can check the anode rod condition during a water heater service call. Rods that are more than 50 percent depleted should be replaced. In hard water areas like Northwest Arkansas, anode rods deplete faster and may need replacement every three to five years.
Will a tankless water heater eliminate the rotten egg smell?
Tankless water heaters do not have an anode rod and do not store standing water, which eliminates the environment in which sulfur-reducing bacteria thrive in tank heaters. Many homeowners who switch to tankless units find that the odor issue resolves as a result.
Conclusion
Water that smells like rotten eggs is almost always a water heater problem with a known cause and a straightforward solution. Replacing the anode rod, adjusting the water heater temperature, and flushing the tank eliminates the bacteria producing the odor in the vast majority of residential cases. If the smell persists after addressing the water heater, or if it is present in cold water as well, the source may be the water softener or the supply itself, both of which have their own solutions. NWA C&S Plumbing handles water heater service, anode rod replacement, and water quality assessments throughout Springdale, Fayetteville, Bentonville, Rogers, and surrounding Northwest Arkansas communities. Call us if your hot water smells like rotten eggs and you want the actual cause identified and fixed.