Hot Water Smells Like Rotten Eggs? Water Heater Anode Rod vs. a Well Problem
If your hot water smells like rotten eggs but the cold water is fine, the problem is almost always inside your water heater, not your well. Most tank water heaters hold a magnesium anode rod that protects the steel tank from rust. In water that contains sulfate, harmless sulfate-reducing bacteria living in the warm, low-oxygen tank use the hydrogen given off by that rod to make hydrogen sulfide gas, the rotten-egg smell. The usual fix is to flush the heater, disinfect it, and have a plumber swap the magnesium rod for an aluminum-zinc or powered anode. If the cold water smells too, the sulfur is coming from your well and needs treatment at the source. A Clear Alternative tests South Jersey well water for free; call 1-888-577-8088.
The Two-Faucet Test
Before you buy anything, figure out where the smell starts. It takes five minutes:
- Run the cold water at a sink for a minute and smell it in a clean glass.
- Run the hot water at the same sink and do the same.
- If you have a softener or filter, try an outside spigot, which is often plumbed before treatment, or the pressure tank drain.
- Hot only: the water heater is producing the gas. Keep reading.
- Hot and cold, everywhere: hydrogen sulfide is in the well water itself. See our guide to why well water smells like rotten eggs.
- Treated taps only, not the outside spigot: look at the softener or filter. Bacteria can grow in a softener that sits unused or is overdue for service.
- One sink only: the drain may be the source, not the water. Smell the water in a glass away from the sink.
Hot water usually smells stronger even when the sulfur is in the well, because heat drives the gas out of the water at the tap. The difference with a water heater problem is that the cold water has little or no odor at all.
Why a Water Heater Makes Rotten-Egg Gas
Three things have to come together inside the tank:
- Sulfate dissolved in the water. Sulfate itself has no smell and is common in groundwater.
- Sulfate-reducing bacteria. These occur naturally, are not a health threat, and like warm, dark places with little oxygen, which describes a water heater tank perfectly.
- A source of hydrogen. As the magnesium anode rod slowly corrodes to protect the tank, it releases hydrogen. The bacteria use it as fuel to turn sulfate into hydrogen sulfide.
The University of Georgia Extension fact sheet on hydrogen sulfide describes exactly this: when the odor comes only from the hot faucet, the magnesium rod can react with sulfate to form hydrogen sulfide, and replacing it with an aluminum or zinc rod can eliminate or reduce the problem.
Why It Often Starts Suddenly
Homeowners are often surprised when the smell appears with a brand-new heater, after a vacation, or after a softener was installed. A new magnesium rod is at its most active. Water sitting unused in the tank for a week gives the bacteria time to build up. Softened water can make a magnesium anode work harder, which releases more hydrogen. Turning the thermostat down to save energy can also let bacteria thrive.
How to Fix Hot Water That Smells Like Sulfur
1. Flush the Tank
Turn off the power or gas, close the cold supply, attach a hose to the drain valve and drain the tank to a safe spot. Flushing removes sediment where bacteria hide and often reduces the smell for a while on its own.
2. Disinfect the Heater
A plumber can disinfect the tank with a chlorine or hydrogen peroxide treatment, then flush it thoroughly. Another approach, described in state drinking water guidance on sulfur bacteria, is to raise the heater to about 160°F for several hours to kill the bacteria and then drain it. That temperature is a scald hazard, so only do this with every hot tap closed, warn everyone in the house, and turn it back down afterward. Either method is temporary if the cause remains.
3. Replace the Anode Rod
This is the lasting fix in most homes. A plumber removes the magnesium rod and installs one of these:
- Aluminum-zinc anode: produces much less hydrogen and is the standard choice for sulfur odor.
- Powered (impressed-current) anode: protects the tank with a small electric current and does not dissolve, so it gives the bacteria nothing to feed on. A good choice when odor returns even with an aluminum-zinc rod.
Do not simply pull the rod out and leave it out. Without an anode the tank can rust through, and many manufacturers will not honor the warranty. Check your heater’s manual or ask the manufacturer which replacement anode is approved for your model.
4. Keep the Temperature Reasonable
Very low settings can let bacteria grow, and very high settings are a burn risk. Keep the heater at the setting recommended by the manufacturer and your plumber, and use the hot water regularly rather than letting the tank sit for long periods.
When the Smell Is Coming From Your Well
If the cold water smells too, or the odor comes back after the anode is changed, the hydrogen sulfide is in the groundwater, and that needs a different approach. Options include oxidizing filters that convert sulfur to particles and filter them out, chlorination or hydrogen peroxide injection followed by filtration, and aeration systems. The right choice depends on the sulfur level, the iron and manganese in the water, and the pH. Our articles on how to test for sulfur in water and iron and sulfur filters for South Jersey well water walk through the choices.
Sulfur and iron often appear together in South Jersey wells, and both can feed odor and slime problems in softeners and water heaters. That is why we test for the whole picture before recommending anything.
Is Rotten-Egg Water Safe?
At the levels found in household water, hydrogen sulfide is mainly a nuisance: it smells, it can taste bad, and it tarnishes silverware and darkens fixtures. The bacteria that make it are not the kind that cause illness. That said, a sudden new odor in a well is a good reason to test for coliform bacteria too, especially after flooding or heavy rain. Our guide to well water after flooding covers that situation.
Serving South Jersey Homes
We help homeowners sort out sulfur and water heater odor across Burlington, Camden, Gloucester, Atlantic, Salem, Cape May, Mercer and Monmouth County. See all of our options on the water treatment page.
When to Call A Clear Alternative
Call us if the cold water smells as well as the hot, if the odor came back after an anode change, if you also see orange or black staining, or if your softener or filter seems to be the source. A Clear Alternative is a family-run company that has been treating New Jersey well water since 1991, working from 9230 Collins Ave in Pennsauken. We will test your water in your home at no charge and recommend only what it actually needs. Call 1-888-577-8088 to book your free in-home water analysis.
Frequently Asked Questions
Why does only my hot water smell like rotten eggs?
Usually because bacteria in the water heater use hydrogen released by the magnesium anode rod to turn sulfate in the water into hydrogen sulfide gas. The cold water never passes through the tank, so it does not smell.
Will replacing the anode rod fix the smell?
In most cases, yes. Swapping the magnesium rod for an aluminum-zinc rod, or a powered anode, removes the hydrogen source the bacteria need. Flush and disinfect the tank at the same time.
Can I just remove the anode rod?
No. Without an anode the steel tank can corrode quickly, and many manufacturers will not honor the warranty. Use an approved replacement anode instead.
Why did my new water heater start smelling?
A new magnesium anode is at its most active, so it releases more hydrogen. If your water has sulfate and sulfate-reducing bacteria, that can start the odor within weeks.
Is hot water that smells like sulfur dangerous?
At household levels it is mainly a nuisance. The bacteria are not disease-causing, but a new odor in a well is a good reason to test for coliform bacteria too.
What if my cold water smells too?
Then the sulfur is in your well water, and you need treatment at the source, such as an oxidizing filter. Call 1-888-577-8088 for a free in-home water analysis.