If only your hot tap smells, the water coming into the house is usually not the first suspect. St. Charles tap water is chlorinated and hard, so a plain cold water sample gives you a baseline. A quick side-by-side check can narrow the cause before you spend money on anything.
What a hot-only smell usually points to
Hot water passes through the water heater, which adds heat, metal parts, and sediment. In many homes, a rotten-egg or sulfur smell that shows up only on the hot side comes from hydrogen sulfide produced inside the tank. Bacteria that live in low-oxygen spots, such as the bottom of a tank or around the anode rod, can produce it. A magnesium anode rod is a common contributor because it reacts with the water. Sediment in the tank can also hold odor-causing material.
Run the side-by-side check
- Pick a bathroom sink or kitchen tap that is easy to reach.
- Run the cold side for a minute or two, then fill a clean clear glass and smell it close to the rim. Note whether there is a smell.
- Run the hot side the same way, filling a second clean glass.
- Compare the two glasses side by side, smelling each one from the rim rather than from across the room.
- Repeat at a second tap, ideally one farther from the water heater, such as a laundry sink on the far side of the house.
Reading the results
- Smell in both cold and hot: the source water is the likely cause, so treat it as a whole-house question.
- Smell only on hot: look at the water heater first.
- Smell only at one tap: check that fixture, its aerator, and its drain trap.
- A sharp, chlorine-like smell instead of a sulfur smell is a different problem, and it often relates to the disinfectant used in the city supply.
Testing for hydrogen sulfide
A smell check is quick, but it is not a measurement. Home test strips and drops can indicate whether hydrogen sulfide is present, but they vary in sensitivity, so read the labels carefully. A lab report gives a clearer answer, especially for a private well. Note when you ran the water, whether the smell is from hot water only, and what you already checked. That detail helps a lab and any plumber you call.
Private wells in and around St. Charles
Private wells in the area draw from the Missouri River alluvial aquifer. Hydrogen sulfide is one of the classic groundwater problems, so a well owner with a rotten-egg smell on both cold and hot should not assume the water heater is responsible. Test the cold line at the well or the first accessible tap, and keep the results with your well records. Iron and manganese cause staining more than smell, but a lab panel can check for them alongside hydrogen sulfide.
Water heater checks you can do
Look at the tank's age, any leaks around the top fittings, and whether the heater has an anode rod. Some tanks use aluminum or zinc rods instead of magnesium. A plumber can inspect the rod and advise on replacement. If sediment has built up, draining the tank may help. Follow the label instructions for any draining work, and stop if you are not comfortable doing it.
Next steps
- Record which taps smell and which do not.
- Repeat the check after running the water for a few minutes, so you know the smell is consistent.
- If the smell is only on hot water, inspect the heater and ask a licensed plumber about the anode rod and tank condition.
- If the smell is on cold water too, arrange a lab test of the source water before you choose any treatment.