Rotten Egg Smell From Your Tap: Tell Sulfur Apart From Other Odors

A rotten egg smell from the tap is usually caused by hydrogen sulfide gas. It can come from the water supply itself, from bacteria that live in a water heater, or from the drain side of the plumbing. Where the smell shows up tells you a lot about where it starts, so check that before you assume the worst or buy equipment.

Check the hot and cold taps separately

Run the cold tap for a minute, then smell the water. Then run the hot tap and smell it again. If the smell is only in the hot water, the source is most often the water heater, not the supply. If both taps smell, the cause is more likely in the water coming into the house, which points to a water test rather than a heater check.

Check the drains

If the smell comes from the sink or shower drain rather than the water coming out of the tap, the cause may be a dry trap, a clogged drain, or sewer gas. Run water in the fixture and check whether the smell is strongest near the drain opening. This is a plumbing question, not a water quality one.

Know when to stop and call

If the smell is strong, sudden, or resembles natural gas rather than sulfur, leave the area, do not use any flame or switches, and call your gas utility from outside the house. Do not treat a gas smell as a water question.

Confirm the source with a test

A smell alone does not tell you what is in the water or how much. A certified lab test of your water can measure hydrogen sulfide and other contaminants, and it gives a result you can compare across providers. Ask whether the test was taken from the cold tap at the point of entry or at a fixture, since the result depends on that.

What to compare before you buy

If the test points to the water supply, compare treatment methods that address sulfur, and ask each provider how they would confirm the smell is gone after installation. If the test is clean and the smell is in the hot water only, ask for a water heater check before you commit to any whole-house equipment. For how we assess providers and the evidence we ask for, see Review Methodology and Editorial Criteria.