Why does my hot water smell like rotten eggs?

Hot only, or hot and cold?

The Minnesota Department of Health suggests a simple test: after you've been away for a few hours, smell the hot and cold water at each faucet. If only the hot water smells, the water heater is the likely source. If cold water smells too, the gas is coming from the well or the water supply.

Why the water heater makes the smell

Many water heaters have a magnesium anode, a rod that protects the tank from corrosion. In some water, sulfur bacteria and that magnesium work together to produce hydrogen sulfide gas. It's most common in homes on well water.

Fixes that work

• Replace the anode. The Minnesota Department of Health suggests asking about a replacement anode made of a different material, such as aluminum, which can protect the tank without feeding the smell. It warns that removing the anode entirely may significantly shorten the heater's life.
• Disinfect the tank. Flushing the heater with a chlorine bleach solution can kill the bacteria, though the smell may return within a few weeks if some survive.
• Heat treatment. Raising the water to 160°F for several hours kills the bacteria. This is dangerous without the right precautions: the relief valve must work, and the thermostat must go back down afterward to prevent scalding. Have a plumber do it.

The health department recommends having a plumber or water system professional do this work unless you're very familiar with water heaters.

If the cold water smells too

Penn State Extension covers treatment options for rotten egg odor in well water, from aeration to filtration and chlorination. A water test tells you what you're dealing with before you buy equipment.

In towns where some homes still run on private wells, like parts of Eagle, Idaho, it's worth testing the well.

Rotten eggs or natural gas?

Gas utilities add a sulfur-smelling odorant to natural gas so leaks can be noticed. If the smell is in the air near the heater rather than in the water, treat it as a possible gas leak: leave the house and call 911 and your gas company from outside.

Local pages mentioned

• Eagle, ID