A rotten egg smell in water is one of the most common reasons homeowners start looking at treatment. The smell is usually hydrogen sulfide, and where it appears tells you a good deal about where it starts. Before you buy anything, find out which tap carries it.
Run each tap separately
Run the cold tap for a minute or two in the kitchen, then smell the water. Then run the hot tap at the same sink and smell that. Do the same at a bathroom sink. Note which taps smell, which do not, and whether the smell is strong at the start of the run or only after the water has been sitting. Write this down, because a provider or plumber will ask.
If both hot and cold smell
A smell in both lines usually points to the source. On a private well, hydrogen sulfide can be dissolved in the groundwater itself, and sulfur-related bacteria can grow in the well, pipes or pressure tank. Ask whether the water has been tested for hydrogen sulfide and for bacteria, and ask what the test report says about each. Do not rely on a smell test alone, since the smell is only a sign that something is present.
If only the hot water smells
A smell that appears only on hot water often starts inside the water heater rather than the supply. A common cause is a reaction involving the anode rod in a tank heater, which can produce the smell in the hot line only. Ask a plumber whether the anode rod has been checked, and whether the heater has been drained and flushed recently. Be wary of any quote that goes straight to a treatment system without first asking about the heater.
If the smell only appears at a drain
If the odor is strongest at a drain and fades when the tap runs, the trap or the drain line may be the source, not the water. Ask whether the smell is present at the tap at all before you assume the supply is involved.
What to bring to the next conversation
Bring your tap-by-tap notes, the date and results of any water test you have, and a short description of how the smell changes over time. Ask each provider which test they recommend first, what the test measures, and whether the proposed treatment addresses the cause you found. A treatment that targets the wrong line will not fix the smell, and the Methodology page explains how we set up our comparisons if you want to see how we check that fit: Methodology: How We Match Systems and Compare Providers.