If the hot tap in your O'Fallon home smells like rotten eggs or has a metallic odor while the cold tap smells fine, the water coming into the house is probably not the main suspect. The water heater is. Water that sits warm in a tank for long periods gives some microbes and chemical reactions time to work, and the tank itself contains metal parts that can react with the water.
Why only the hot side changes
Cold water travels through the same pipes, so if it smells normal, the source water and most of the plumbing are less likely to be the cause. Hot water differs in three ways inside the heater. It is warmer, it moves less often through the tank, and it contacts the steel tank lining and a sacrificial anode rod that slows corrosion of the tank.
Two common sources of odor are worth knowing. Hydrogen sulfide is a gas that gives water a rotten egg smell. Sulfate-reducing bacteria can produce it in warm, low-oxygen spots, including the bottom of a tank. The second source is the anode rod itself. Magnesium rods are known to contribute to odor in some water, and the smell is often strongest on the hot side.
A simple side-by-side check
- Run the cold tap at the kitchen sink for a minute, then smell the water in a clean glass.
- Run the hot tap the same way and smell it.
- Note whether the odor appears only on the hot side, only after the water has sat, or on both sides.
- Write down whether the smell started after the water heater was last serviced, replaced, or left unused for a trip.
What to check at the water heater
Before opening anything on the heater, turn off its power or gas supply and follow the manufacturer's shutoff instructions. If you are not comfortable doing this, a licensed plumber can do the same checks for you.
- Find the anode rod's material on its label or top fitting if your heater has an accessible one.
- Check the temperature setting. Higher settings can change how quickly odors develop in the tank.
- Look at the drain valve and any sediment that comes out when you drain a small amount of water. Dark or black material can point toward bacterial activity or corrosion.
Test before you change anything
Guessing at the cause can waste time and money, so test first. Collect a hot sample and a cold sample on the same day, using clean containers. The MO State Public Health Lab offers free or low-cost coliform, E. coli, and iron-bacteria testing, which can help you rule out some microbial causes. For hydrogen sulfide, ask any lab you use whether it tests for sulfide specifically, because not every standard panel includes it. Your O'Fallon water report describes the public supply, which can help you confirm that the cold water source is typical for the area.
When to call for help
Get a qualified technician involved if the odor is strong, the hot water looks discolored, or you find black slime in the drain. Do not mix cleaning chemicals into a water heater on your own, since some combinations are hazardous. A technician can flush the tank, inspect or replace the anode rod, and advise whether the heater itself needs attention.
What to do next
Once you know whether the odor is tied to the tank, the rod, or the water itself, the next step becomes clearer. Keep notes on when the smell appears, how long the water has been sitting, and what the test results show. That record makes it easier for a technician or lab to narrow the cause, and it helps you decide whether the issue is simply a maintenance matter or something that needs a longer-term fix.