Why does my hot water smell like rotten eggs?

By Dana Whitlock. Updated October 2026. Repair or replace?

First, rule out gas. Natural gas and propane carry an added odor that also smells like rotten eggs. If the smell hangs in the air around the heater or the house, leave now and call the gas utility from outside. A smell that only shows up when you run hot water is a different story. The usual cause is harmless bacteria in the tank producing hydrogen sulfide, often helped along by the anode rod.

Likely causes

Sulfate-reducing bacteria in the tank Safe to check

Some tap water, especially well water, carries bacteria that thrive in a warm tank. A. O. Smith's manual says these bacteria can produce a sulfur odor and that the condition is treatable. It shows up most in heaters that sit at low temperatures or go unused for a while.

The anode rod reacting with the water Pro job

The anode rod protects the tank from rust. Magnesium rods can react with certain water to feed the odor. The manual says a special zinc anode may help in some cases, and that an anode must be installed at all times or the warranty is void. Swapping it is a plumber's job.

A gas leak Pro job

An odor hanging in the air, with no water running, may be fuel gas. That is an emergency for the gas utility. The plumber comes later.

The water supply itself Safe to check

Cold water that smells too means the well or the supply is the source. A water test will point you to treatment options.

Safe things to check first

• Run cold water from the same faucet. If only hot smells, the heater is the source.
• Smell the air near the heater with no water running. Any gas odor means leave the house now.
• Check the temperature setting. A heater set very low gives bacteria an easier home.
• If you are on a well, have the water tested for sulfur and bacteria.
• Ask a plumber about the anode rod type before buying one, since a missing rod voids many warranties.

When to stop and call

A smell of gas in the air is the one case where you call the gas utility first, from outside, after getting everyone out. Do not use switches or appliances on the way. For a water-only odor, a licensed plumber can chlorinate and flush the tank, raise the temperature temporarily, or fit a different anode. Do not raise the setting yourself without an anti-scald mixing valve, since the manual warns higher settings increase burn risk.

Repair or replace?

Start with its age. Type the serial from the label:

An odor problem rarely justifies replacement by itself. It is worth thinking about only if the heater is also old or failing. Date it with the age decoder above. DOE's analysis puts gas tank heaters at about 14.5 years on average and electric tanks at about 15.1. If yours is past those figures and the odor comes with rusty water or noise, put the money toward the replacement range above rather than a new anode.