If the rotten egg smell shows up only when you run the hot tap in your Wentzville home, the cold side is probably not the first place to look. Before you call anyone or buy anything, sort out where the smell is coming from. A hot-only odor is worth logging, because it points you toward different checks than a smell that appears everywhere.
Why hot-only matters
A smell that is present on both hot and cold lines usually points to the water supply itself, such as hydrogen sulfide dissolved in a private well or groundwater source. A smell that appears only on the hot side narrows the search. Water heaters can produce sulfur odors when certain bacteria in the tank react with sulfate in the water and with the metal anode rod inside the tank. The gas involved is hydrogen sulfide, which gives the smell its character. Heat makes the gas easier to detect, so the same water can smell stronger from the hot tap than from the cold one.
Run a side-by-side check
- Pick one sink close to the water heater and one sink farthest from it.
- Run only the cold tap for a few minutes, then smell the water in a clean glass.
- Run only the hot tap for a few minutes, then smell it the same way.
- Repeat at an outdoor spigot or laundry tub if you have one, since it may sit on a different line.
- Write down which taps smell, how strong the smell is, and whether it changes after the water runs longer.
What the pattern suggests
- Smell on both hot and cold: test the source water, especially if you are on a private well in or near Wentzville.
- Smell only on hot, and strongest at the heater: the water heater tank or its anode is a likely suspect.
- Smell only at one fixture: check that drain, trap, and faucet before assuming the supply is the cause.
- Smell that fades after the tap runs: stagnant water sitting in the lines or tank may be the source, which is different from a continuous supply problem.
Testing the hot side
Home test kits may not capture hydrogen sulfide reliably, so a lab test is often the more useful step. Ask the lab for a test that measures sulfide directly, and ask whether it can also run a bacteria screen on both the hot and cold samples. Collect each sample from the tap you want to test, after running it as described above, and keep the container clean. Label each sample with the tap name, date, and time. Use the same method for both samples so you can compare them fairly.
If the lab finds sulfide in the cold water too, the supply itself is involved, and a Wentzville private-well owner should look at treatment options for the source. If the cold water is clear and only the hot side is affected, the water heater is where your next steps belong.
Check the water heater before changing the water
Before you treat the whole house, look at the water heater. Note its age, whether it has a visible anode rod, and whether the smell is stronger right after the tank refills. A licensed plumber can inspect the anode and flush the tank. Do not treat this as a quick fix, since a sulfur smell from the heater can return if the underlying conditions stay the same.
What to keep in your log
Write a short note each time you notice the smell. Include the date, the tap, whether it was hot or cold, recent weather, any recent work on the water heater or well, and whether anyone had just run the dishwasher or washer. Over a few weeks, these notes often show a pattern that a single smell check cannot. Bring the log with you if you call a tester or contractor in the Wentzville area.
The goal is simple: find out whether the smell starts at the source or at the heater, then test the right sample. Once you know which side of the house is involved, the next decision gets much easier.