Compare cold and hot water at the same faucet. Hot-only odor points toward the heater; odor in both hot and cold water points back toward the incoming supply, where well-water sulfur treatment may be relevant.
Why the heater can create sulfur odor
Water-heater conditions can support reactions that generate hydrogen sulfide, especially in systems with certain anode-rod chemistry and sulfate-reducing bacteria. The result is a rotten-egg smell that may be much stronger in hot water than cold.
The simplest diagnostic test
Run cold water into one clean glass and hot water into another from the same faucet. Smell both after allowing obvious chlorine odor to dissipate. If only hot water smells, do not start by shopping for a whole-house sulfur filter.
If both hot and cold smell
Then the incoming water source becomes more likely. Private wells can contain hydrogen sulfide or sulfur bacteria, and treatment depends on concentration, pH, iron, manganese and flow. Start with the well-water sulfur guide.
Where to go next
Practical next step for Rotten-egg smell only in hot water? Check the water heater first.
Turn the information on this page into a specific decision by writing down the source water, the problem you are trying to solve and the result you need to verify. For municipal water, start with the utility water-quality report and identify the disinfectant plus any contaminant that matters to the purchase. For a private well, use recent laboratory results instead of relying on color, taste or odor alone. Then match the treatment method to the measured condition and check whether the system can handle the household's realistic peak flow or daily volume. This is especially important for iron, sulfur, hardness, low pH, sediment, membranes and UV systems, where the wrong sizing or pretreatment can reduce performance even when the equipment itself is sound.
Before spending money, confirm the exact model or replacement part, current operating limits, installation requirements, consumable schedule and the evidence behind the claim you care about. Compare ownership over several years rather than only the purchase price. Filters, membranes, media, lamps, salt, water sent to drain, electricity and proprietary parts can materially change long-term value. After installation, establish a baseline for pressure, flow, taste or test results and keep service dates with the system. That makes it easier to verify whether treatment is working and to diagnose future changes without guessing.
Use the broader learn guides to compare adjacent options before committing to a larger or more complex system than the water problem actually requires.