If the hot tap in your O'Fallon home smells like rotten eggs while the cold tap smells normal, the water heater is the first place to look. The source water may still be fine. The odor is often created inside the tank, where it can go unnoticed until it becomes a daily annoyance.
How a water heater can create the smell
Most storage water heaters contain a sacrificial anode rod, a metal rod hung inside the tank that corrodes so the steel shell stays protected. In water that contains sulfate, certain bacteria can work on the rod and tank surfaces and release hydrogen sulfide, the gas behind the rotten-egg smell. Warm water speeds that activity, which is why the odor usually shows up mainly at the hot tap. Once bacteria settle into the tank, a simple flush may give only short relief.
Compare the hot and cold sides first
Run the cold tap at the kitchen sink for a minute, then smell it. Run the hot tap at the same sink and smell that. Repeat at a second location, such as a bathroom sink, so you know whether the problem is limited to one fixture.
- If only the hot side smells, focus on the heater, its tank, and its anode rod.
- If both sides smell, look upstream. The well, pressure tank, or supply line may be involved, and the heater is less likely to be the main cause.
- If the smell is strongest after water has sat in the lines overnight, the pipes themselves may be part of the picture.
Simple checks you can make
- Check the thermostat dial on the heater. A high setting speeds the reaction, so lowering it may reduce the odor, though it will not remove the source.
- Find the heater's age and model label. Older tanks with their original rods are more likely to have an ongoing odor.
- Drain some water from the valve near the bottom of the tank until it runs clear. Turn off the power or gas supply first and follow the owner's manual.
- Watch whether the smell returns within a few days. A quick return suggests bacteria or residue inside the tank rather than a one-time sediment problem.
What a repair usually involves
Common next steps include replacing the anode rod with a different rod material, or disinfecting the tank and connected lines. Both are service tasks that can involve draining the tank, handling gas or electric controls, and checking the pressure relief valve. If the work is unfamiliar, hire a licensed plumber rather than improvising. Ask which rod material is being installed and why it was chosen for your water.
When the cold water is involved too
If both taps smell, or if the smell comes with cloudiness or a change in taste, collect a sample for bacterial testing before assuming the heater is the cause. The Missouri State Public Health Laboratory offers testing for coliform bacteria and E. coli, and iron bacteria testing is also available. Follow the lab's instructions for collection and hold times. For a private well in St. Charles County, testing after a repair, or when the odor first appears, gives you a clear baseline to compare against later.
If the odor is strong, avoid drinking or cooking with the hot water until the cause is clear. Use cold water for those purposes only if it smells normal and the lab results are satisfactory.
When to stop troubleshooting
If the odor persists after a flush and a lower thermostat setting, or if the tank is leaking, rusty, or noisy, stop troubleshooting and schedule service. The goal is a heater that delivers hot water without a sulfur smell, along with a clear answer about whether the odor is coming from the tank or from the source water.