Turn on the kitchen tap and the cold water runs clean, but the hot side has a faint rotten egg smell. That split is one of the most useful clues you can get. It suggests the problem sits downstream of the main supply, most often in the water heater, rather than in the source water or in your filter.
Why the smell starts at the heater
Most sulfur odor in household water comes from hydrogen sulfide gas. Certain bacteria produce it when they find warm, low-oxygen conditions, and a water heater offers both. Water sits in the tank for hours, sediment settles on the bottom, and the tank's anode rod, a metal rod that slowly corrodes to protect the steel, can become a food source for these bacteria. Heat speeds the whole process up, which is why the hot line usually smells first and strongest.
Check the pattern before you act
Start with a simple test. Run the cold tap for a minute, then smell the water. Run the hot tap for the same length of time and smell again. Then check other hot taps in the house, including the shower and the laundry sink. If the smell appears only at one hot tap, the heater is the likely suspect. If it shows up at every hot tap but never on the cold side, the heater is still the focus.
If the cold water smells too, the issue likely sits upstream, in the well, the municipal line, or a plumbing section that carries both lines. That calls for a different conversation, starting with your utility or a certified lab. Until you know the cause, use cold water for drinking and cooking if the cold side is clean, and avoid drinking from the hot line.
Testing with a lab, not a guess
If you are on a private well, a lab test moves you past guesswork. Ask for testing for hydrogen sulfide and total coliform bacteria, and ask the lab which reporting limit it uses. The limit determines what the report can and cannot show. The same idea applies to other contaminants. A recent Circle of Blue report on PFAS in drinking water, by Alissa de Chassey, makes the point that "When PFAS concentrations in drinking water fall below laboratory reporting limits, they are" hard to read with confidence. Whatever the contaminant, a result is only as clear as the limit behind it, so ask the lab what it could and could not detect.
When to call a plumber
If the cold water is clean and the smell is limited to the hot side, call a licensed plumber to inspect the heater. Ask them to check the anode rod, look at the sediment in the tank, and flush the tank if needed. Ask whether the rod should be replaced and whether a different rod type suits your tank. Do not add bleach to the tank yourself unless a professional tells you how and how much to use.
Ask for the work in writing, and ask what they will test afterward. A smell that fades for a few days may only be masked, so a follow-up check at the same tap is the real proof that the problem is gone.
Read the smell as a map
A smell at the hot tap only points to the heater. A smell at both taps points to the source or the main supply. A clean result after service, confirmed at the same tap where you first noticed the problem, is the signal that the repair worked. Until then, keep the cold line for drinking and cooking, and let the pattern guide your next call.