A rotten egg smell from the tap is one of the few water problems you can notice without a test kit. It usually means hydrogen sulfide gas is dissolved in the water. The gas is common in well water, and it can also develop inside a water heater or a treatment unit that holds water for long periods. The smell does not tell you the source by itself, so your job is to narrow it down before you call anyone.
Start with the pattern, not the smell
Before you change anything, note where the smell shows up and when. A smell at the cold tap only points to the supply or the treatment equipment ahead of that tap. A smell at the hot tap only, with the cold tap clean, points toward the water heater. A smell at both suggests the source is upstream of the heater. A smell that appears only after the house has sat unused, such as after an absence, often means stagnant water in the lines or in a tank. A smell that is present every day is a different situation, and it deserves a direct test.
Check whether the smell is in every fixture or just one. A single sink, tub, or floor drain that smells like rot or sewage is more likely a dry or failed trap, a drain problem, or sewer gas, and not dissolved gas in the water. Run the water and smell the faucet, then smell the drain with the water off. If the drain is the source, the water chemistry is not the question, and a plumber should look at the drain line and trap.
Compare the taps before and after the equipment
If the smell is in the water itself, compare a tap before the treatment equipment with a tap after it. Your installer or service company should be able to show you where those taps are. If the smell appears only after the unit, the equipment may be holding water and growing bacteria, or the media inside may be the problem. If the smell is already present before the unit, the treatment equipment is not the first suspect, and the source is the supply itself.
Look at the surfaces in the house. Hydrogen sulfide tends to darken silver, so silverware or silver jewelry that tarnishes quickly, especially alongside the smell, is a useful clue. It points toward sulfide gas rather than iron or copper, which leave different colors on fixtures. Write down what you see, because the difference matters when someone tests the water.
Hot water only: a separate check
A smell that shows up only at hot taps often comes from the water heater. Some heaters create conditions where sulfur-producing bacteria can thrive, and the sacrificial anode rod is a common part of that chemistry. Checking the rod is its own job, and you can decide whether to do it yourself or hire a plumber. For this problem, the simpler point is this: if the cold tap is clean and the hot tap smells, the water heater is where to look next, and a plumber is the right person to inspect it.
A gas smell is a different emergency
Natural gas is supposed to smell like rotten eggs, because a warning odorant is added to it. If the smell is strongest near a gas appliance, a gas line, or a furnace, or if you hear hissing, leave the building and call your gas utility or emergency services from outside. Do not treat that as a water problem to test or troubleshoot.
What to ask before you pay for a fix
Ask whether the service includes testing for sulfide and for the bacteria that produce it, and whether samples are taken before and after the equipment. Ask what the company will do if the result points to the well, the water heater, or the house plumbing, and whether that work is within their scope or a separate referral. Ask how they plan to treat the water if disinfection is needed, and whether they will explain the plan in writing before they start.
Ask for the results in writing, with the sample location listed on the report. A sample taken from the wrong tap will not tell you what you need to know, so make sure the location matches the pattern you recorded.
What to keep track of
Write down when you first noticed the smell, which taps it appears at, whether it follows a period of non-use, and any tarnish or staining on fixtures. Keep the note short and update it when the smell changes. A simple log helps whoever tests the water narrow the cause faster, and it shows you whether a fix is actually working.