If only the hot tap in your O'Fallon home runs rusty, cloudy, or smells like rotten eggs, the cold supply is probably fine. The problem is most likely inside the water heater or in the hot-side piping. Checking in the right order saves time and keeps you from treating water that does not need it.
Start with a side-by-side check
Run the cold tap at the kitchen sink for a minute, then run the hot tap at the same sink. Compare color, clarity, and smell in a clear glass. Repeat at a bathroom sink and at the tub. Note which fixtures show the problem.
- Cold and hot both affected: look at the supply, whether that is a private well or the municipal line, and any whole-house treatment ahead of the water heater.
- Hot only: the water heater, its anode rod, or the hot-side piping is the likely source.
- Only one fixture affected: check that fixture's aerator and valves first.
Why the hot side behaves differently
Heat speeds up reactions inside the tank. Local water reports describe tap water in O'Fallon as hard, and that hardness can settle as scale on the heating element and the bottom of the tank. Sediment from the supply and rust from the tank walls collect there too. Water that sits in a tank can show a problem that the cold side hides.
Rusty or gritty hot water
Flakes and grit at the hot tap usually point to sediment or a corroding tank. Fill a clean bucket from the hot tap and let it stand. Sediment that settles to the bottom suggests the tank needs a flush. Rust-colored water that does not clear after flushing may mean the tank lining is failing, which is a question for a licensed plumber.
Rotten-egg smell on the hot side only
A sulfur smell that appears only on the hot side is a recognized pattern. Sulfur-reducing bacteria can grow in a heater tank, and the anode rod can play a role in that chemistry. Hydrogen sulfide is the gas behind the smell. Smell the cold water too. If the cold water is clear and odorless, the heater is the place to focus. If the cold water smells the same, your supply needs its own check.
Inspect the anode rod
The anode rod is a sacrificial metal rod that screws into the top of many tanks. It corrodes so the tank does not have to. A rod that is heavily worn, coated, or gone may explain corrosion or odor problems. Shut off power or gas to the heater and let the water cool before you work on it. Unscrew the rod from its fitting and look at its condition. A replacement rod is a common part, and a plumber can handle the swap if you would rather not.
Flush the tank safely
Flushing removes loose sediment from the bottom of the tank. Turn off power or gas, close the cold supply valve to the heater, and attach a hose to the drain valve near the bottom. Open a hot tap elsewhere in the house so air can enter. Drain until the water runs clear, then close the drain, reopen the supply, and let the tank refill before restoring heat. Hot water can scald, so let it cool before draining.
Settings that affect the hot side
- A thermostat set very high speeds scale formation and can wear the tank faster. A moderate setting that still meets your household needs is the usual choice.
- A tank that has never been flushed often shows gritty water first.
- A water softener, if you have one, reduces the hardness that reaches the heater, but it does not remove sediment or rust that comes from the tank itself.
When to test the water
If the cold water is fine, you usually do not need a full water test to chase a hot-only problem. If you run a private well, though, the cold side is your supply and deserves its own check. Coliform, E. coli, and iron bacteria testing is available through the Missouri State Public Health Lab at free or low cost, which is a practical starting point for well owners in and around O'Fallon.
When to call a plumber
Call a licensed plumber if the tank leaks, the drain valve will not close, the temperature and pressure relief valve drips steadily, or the water stays rusty after flushing. Those are repair decisions. Keep a short record of when the problem started and which taps show it, because that information helps whoever comes to the house.