Why Only the Hot Water Smells of Rotten Eggs

First: run the cold tap

This is the whole diagnosis, and it takes a minute. Run cold water and smell it. Then run hot and smell that.

Hot only — the heater is the source, and the anode is the usual reason. Cold as well — the water is arriving that way, which on a well means sulfur bacteria or sulfide in the groundwater itself, and needs treatment rather than a heater repair [1][2].

Penn State's extension publication puts the hot-water case plainly: when the smell is only in the hot water, "the magnesium rod can chemically reduce sulfates to form hydrogen sulfide" [1].

• Hot only → water heater (anode rod, bacteria in the tank)
• Cold and hot → the supply itself → treatment
• Only at one fixture → drain or trap, not the water

Why a rod that protects the tank makes the smell

A steel tank is lined with glass, and the lining is never perfect. The anode rod is a sacrificial metal — usually magnesium — that corrodes instead of the steel. That is why the tank lasts.

The same reaction has a side effect. Minnesota's health department describes the mechanism: sulfur bacteria in the water produce hydrogen sulfide, and a water heater's magnesium rod can supply the electrons that drive the sulfate conversion — which is why the odour can be strong in hot water and absent in cold [2].

So you are not choosing between a smell and a working heater. You are choosing which metal does the protecting.

What actually fixes it

Change the anode. Penn State recommends replacing the magnesium rod with an aluminium one: the odour goes and the corrosion protection stays [1]. This is a plumbing job — the rod is often seized and the tank has to be drained down.

Disinfect the tank. Bradford White, a heater manufacturer, publishes a chlorination procedure for hydrogen-sulfide odour in its own units [3]. It kills what is living in the tank; if the water keeps feeding the bacteria, the smell can return.

Treat the water, if the cold smells too. Penn State lists the options by concentration: chlorination with filtration, oxidising filters, potassium permanganate systems, activated carbon and aeration [1].

What does not fix it: turning the heater up and hoping, or removing the anode entirely. Pull the rod out and the tank starts rusting through from the inside — you have traded a smell for a leak.

How little it takes to smell it

Hydrogen sulfide is detectable by smell far below the level at which it would be a drinking-water problem. Penn State notes that most people can detect it "well below 0.5 mg/L", and that it has no drinking-water standard precisely because water becomes undrinkable by odour long before it becomes harmful [1].

That is worth knowing before anyone panics: what you have is usually an unpleasant nuisance with a mechanical cause, not a health emergency.

What this means in Tampa Bay

Well water is common through Pasco, Hernando and the eastern edge of Hillsborough, and that is where we meet this most: the cold smells faintly, the hot smells strongly, and the heater is getting the blame for water that arrives that way.

The order we work in is: smell both taps, then look at the anode and the age of the heater, then test the water if the cold is involved. We change anodes, we disinfect tanks, and we install treatment when the supply is the problem.

FAQ

Sources

We review and cite. We do not reproduce a paper or a manual — follow the links for the originals.

1. [1]Hydrogen Sulfide (Rotten Egg Odor) in Water Wells — Bryan Swistock · Penn State Extension · updated 2024University extension · magnesium rod reducing sulfate; aluminium rod fix; detection below 0.5 mg/L; treatment options
2. [2]Hydrogen Sulfide and Sulfur Bacteria in Well Water — Minnesota Department of HealthGovernment or standards body · sulfur bacteria, where the gas comes from, hot-water case
3. [3]Hydrogen Sulfide Odor and Chlorinating Water Heaters — Bradford WhiteManufacturer's manual · manufacturer's chlorination procedure for odour in a tank