Why does only my hot water smell like rotten eggs?

Hot-only odor means the hydrogen sulfide is being produced inside the water heater, where bacteria react with the anode rod. Source-water sulfur shows up on both hot and cold.

Is hydrogen sulfide in water dangerous?

At the concentrations household water carries, it is a nuisance: foul smelling and corrosive to plumbing over time, but not an acute health hazard. Its main damage is to metals, appliances, and patience.

Why does the smell come and go with the seasons?

Well sulfur often tracks aquifer conditions and usage: heavy pumping, drought, and stagnation change the sulfide load. A heater-source smell tends to be steadier and tracks hot water use instead.

Can a water filter remove sulfur smell?

Ordinary carbon pitchers, no. Whole-house treatment matched to the concentration, aeration, oxidizing media, or chlorination followed by filtration, removes source-water sulfide effectively.

Sulfur Smell: Well Water or Water Heater? Telling Them Apart

Page last reviewed: August 2026 · Reviewed by Harold R. Bennett, retired home inspector

The rotten egg smell in water is hydrogen sulfide, and before anything can be fixed, one question must be answered: is it coming out of the ground with your water, or being manufactured inside your water heater? The sulfur smell in water diagnostic takes three glasses and five minutes, and getting it right saves you from treating the wrong machine. Here is the decision tree.

The three-glass test

Run each test at a sink far from the bathroom drains you might otherwise be smelling.

Glass 1: cold only. Let the cold run 30 seconds, fill, step away from the sink, sniff.

Glass 2: hot only. Same procedure on full hot.

Glass 3: cold after resting. First draw in the morning, before any water has run.

Now read the results. Hot smells, cold does not: the water heater is manufacturing the sulfide; skip to the heater section. Both smell, worse on first morning draw: source water, typically a well; skip to the well section. Neither glass smells away from the sink, but the sink area stinks: the drain, not the water; bacteria in the overflow or P-trap produce identical odor, and the sewer gas diagnostic covers that branch.

One refinement worth the extra minute: if both hot and cold smell but hot is dramatically worse, you may have both a modest source load and a heater amplifying it, which is common on wells and simply means working both fixes.

Verdict: the water heater

Hot-only odor means sulfate-reducing bacteria in the tank are converting harmless sulfate into hydrogen sulfide, fueled by the electrochemical activity of the anode rod, with magnesium rods the most generous hosts. The full mechanism and fix ladder live at WHY DOES MY HOT WATER SMELL LIKE ROTTEN EGGS, and the short version is a two-step: shock the tank to kill the colony, typically with hydrogen peroxide, then change what feeds it, swapping to an aluminum-zinc rod per the material comparison or removing sacrificial metal entirely with a powered anode, the maintenance-supplier ecosystem, Chromex included, stocking both the treatments and the rods. Shock alone is a reset; shock plus anode change is the fix.

Softened water deserves a note here: softening accelerates anode activity, which is why sulfur smell so often appears weeks after a softener install. The softener is not malfunctioning; it changed the tank’s chemistry, and the anode answer above is still the fix.

Verdict: the well

Both-taps odor means sulfide is arriving in the supply, produced by bacteria in the aquifer or the well itself, often worse after stagnation and in certain seasons. The treatment ladder runs by concentration, which a lab test pins down as part of a proper well panel: shock chlorination of the well knocks down sulfur and iron bacteria together and sometimes settles light cases for months; continuous treatment for real loads means aeration systems that strip the gas, oxidizing media filters (the same AIO and catalytic beds that handle iron, convenient since the two problems share geology), or chlorine injection followed by carbon filtration for heavy concentrations. Point-of-use carbon alone disappoints; sulfide wants whole-house oxidation or stripping.

A well-source smell also quietly taxes equipment: sulfide is corrosive to copper and steel, participates in the staining economy, and gives the water heater a head start on its own odor chemistry, one more entry in the case for treating at the wellhead.

The bottom line

Hot-only: the heater made it, and the anode is the story. Both taps: the well sent it, and treatment starts at the source. Just the sink: it was the drain all along. Three glasses, one honest sniff each, and the rest is executing the right fix instead of financing the wrong one.