If the hot tap in your St. Louis home smells like rotten eggs but the cold tap does not, the problem is most often on the hot side of the house. That split is the most useful clue you have, and you can check it yourself before you call anyone.

What the smell usually is

The rotten egg odor comes from hydrogen sulfide, a gas that can form when certain bacteria work in water that has little oxygen and some sulfur compounds present. Water heaters are a common place for this to happen because the warm tank gives those bacteria a more favorable environment. The sacrificial anode rod inside many tanks, usually made of magnesium or aluminum, can also react with the water and add to the smell. Hydrogen sulfide can show up in well water too, but homes on a municipal supply are less often affected at the source, so the water heater is usually the first place to look.

Run each side on its own

  1. Fill a clean glass from the cold kitchen tap and smell it right away.
  2. Let the cold tap run for a while, then smell a fresh sample.
  3. Do the same at the hot tap, using a clean glass or container.
  4. Check a bathroom sink or shower on the hot side as well, since a second fixture helps confirm the pattern.
  5. Write down which sides smell and which do not, and note whether the smell is faint or strong.

What the pattern tells you

  • Smell on hot only: check the water heater and its anode rod, and look at the hot line near the heater.
  • Smell on both sides: the cause is more likely the supply, a well, or a plumbing issue upstream of the heater.
  • Smell only after the home has sat unused: stagnant water in the pipes is a common cause, and running every tap fully often clears it.
  • Smell that resembles natural gas: this is a different situation. Leave the area, avoid flames, switches, and anything that could spark, and call your gas utility from outside the home.

Testing the water more closely

A smell test cannot tell you how much hydrogen sulfide is present, and it cannot tell you exactly where it enters the system. If the pattern points to the water heater, a plumber or a certified lab can run a sulfide test on a sample from the hot side. Ask the lab which method it uses so you can compare results later. Home test strips and drop kits exist for a rough check, but read the label closely and follow the directions exactly, because not every kit is designed to detect sulfide.

What to do next

  1. Confirm the pattern with the steps above and write it down.
  2. Check the water heater's age and whether the anode rod has ever been inspected.
  3. If the smell is only on the hot side, flush the water heater following the manufacturer's instructions for your model.
  4. If the smell is on both sides, ask your water provider for its recent water quality information, or have a lab test a supply sample.
  5. Recheck both sides after any work is done before deciding the problem is solved.

Once you know which side the smell comes from, the next step becomes much clearer. A short check in your own kitchen and bathroom gives you a better starting point than guessing at the cause, and it works the same way in older St. Louis homes and newer ones.