Fixit: A look at what causes rotten-egg odor in hot tap water. Plus tips on clearing the stink.

Q: Why does the new water heater make the water smell of sulfur? We had it installed and then left for vacation only to return to stinky hot water.

A: The rotten-egg smell comes from hydrogen sulfide gas dissolved in the water. Concentrations of as little as one part per million can result in this odor.

For odor to develop in the water heater, there must be:

• A high concentration of sulfate in the water source.

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• Little or no dissolved oxygen in the water.

• Sulfate-reducing bacteria within the water heater. (This bacteria is nontoxic to humans.)

• An excess of active hydrogen in the tank. This is provided by the cathodic action of the anode, a metal rod inside the tank that protects against corrosion.

With these factors present, the hydrogen and sulfur combine to form the hydrogen sulfide gas that gives the rotten-egg odor to the water. Active use of the water heater will reduce the problem, but idle water heaters (during vacations) will allow the accumulation of this hydrogen sulfide gas and aggravate the odor problem.

Smelly water can most easily be eliminated or reduced by replacing the anode with one of less active material (magnesium to aluminum) and then chlorinating the water heating system.

Do not remove the anode; that would leave the tank unprotected. Although it will not affect safer heater operations, it does shorten its life. This is because the anode acts as a sacrificial rod, thus slowing the corrosion process. By removing the anode you will void any warranty on the water heater.

Fixit by Karen Youso is an occasional feature. Send questions to Sorry, no personal replies.