If the cold tap at the kitchen sink smells fine but the hot tap does not, the cause is often not the water coming into the house. The water heater is the first place to check, and you can separate the two possibilities with a few observations before you pay anyone to look.
Run each tap separately
Fill a glass from the cold tap and smell it. Then run the hot tap at the same sink for a moment and fill a second glass. Note whether the odor is the same, weaker, or only in the hot water. Repeat the test at a second fixture, such as a bathroom sink or tub. If the smell appears at every hot tap and at no cold tap, the source is most likely inside the water heater or the hot water piping, not the supply.
Check whether both sides smell the same
If hot and cold water smell alike at every fixture, the issue is more likely in the supply or in a treatment device upstream of the heater. Write down which taps you tested, what you noticed, and when the smell started, so you can describe it accurately to whoever comes out.
What points to the water heater
A smell that is limited to hot water usually comes from something happening inside the tank. Many tanks use a sacrificial anode rod to protect the metal lining, and under some conditions that rod or the water sitting in the tank can produce an odor. Microorganisms that live in standing water can also contribute, especially in a tank that sits unused for long stretches. You cannot confirm either cause by smell alone, which is why the next step is a closer look at the unit.
What to check on the heater itself
Find the tank's label and note its age. Find out whether the anode rod has ever been inspected or replaced, and whether any service records exist. Ask the technician whether the tank has a drain valve suitable for flushing. Do not adjust the gas valve, electrical controls, or pressure relief valve yourself. Those parts belong to a licensed plumber or technician.
Questions to ask before you approve work
Ask the technician to test the water at the hot tap and the cold tap, not only at the heater. Ask what they found, what they propose to replace or flush, and what the work will cost before it starts. Ask how they will confirm the odor is gone, and whether they will check the expansion tank and the anode rod at the same visit. If the tank is old and needs replacement, compare the quote against the unit's age and the installer's warranty terms on both parts and labor.
When a treatment device may be the cause
If the smell shows up in both hot and cold water, look at whether it began after a filter change, a softener service, or another visit to equipment in the line. A device problem can affect the whole house, so the water heater check may not solve it. For help comparing companies that test water and service equipment, see the St. Louis Water Reviews ratings, and read the review methodology and editorial criteria before you rely on them.