If Your Rotten-Egg Smell Is Only in the Hot Water, Do Not Buy a Filter
A two-minute check that saves some households thousands of dollars.
This is the shortest useful article on this website, and it may be the most valuable one.
Do this before anything else
- Run the cold tap at the kitchen sink for a minute. Smell it.
- Run the hot tap. Smell that.
- Fill a glass, carry it to another room, and smell it there.
What each result means
Hot water only. The source is almost certainly inside your water heater, and no filtration system will fix it. Most tank water heaters contain a magnesium sacrificial anode rod, fitted to protect the tank from corrosion. In the presence of sulfate-reducing bacteria, that rod can drive a reaction producing hydrogen sulfide — the rotten-egg gas. The result is a smell in hot water and nowhere else.
The remedy is a water heater service. That typically means flushing the tank and discussing the anode rod with a plumber, since a different anode material can resolve it. It is a fraction of the cost of a treatment system.
Both hot and cold. The hydrogen sulfide is in your source water. On a private well this is a recognised possibility in Ozark groundwater, and it is worth testing and treating. See our page on sulfur and rotten-egg odour treatment.
The smell vanishes when the glass leaves the room. It was the drain, not the water. A drain trap harbouring bacteria produces a convincing rotten-egg smell that seems to come from the tap because that is where you are standing. Cleaning the drain is the fix.
Why this matters
A whole-house sulfur treatment system is a significant purchase. A water heater service is not. Selling the first when the second is needed is one of the more expensive errors in this industry, and it happens because nobody asked the customer to smell the cold tap.
If a company quotes you for sulfur treatment without establishing whether the smell is in the cold water, that tells you how carefully they are working.
A related check worth doing
If the smell is in both hot and cold, note whether it is constant or intermittent. Intermittent odour often points at bacterial activity, which varies with temperature and water usage, or at a seasonal change in the well. Testing while the smell is present gives a far more useful result than testing on a good day — and hydrogen sulfide dissipates from a sample quickly, so it is measured on site rather than posted to a laboratory.