Why Your Hot Water Smells Like Rotten Eggs (And How to Fix It)

TL;DR: A rotten egg smell in hot water almost always comes from sulfate-reducing bacteria reacting with the magnesium anode rod inside your water heater, producing hydrogen sulfide gas. If only hot water smells, the fix is usually a $20-40 aluminum or zinc-alloy anode rod swap and a tank flush, not a plumber visit. If both hot and cold water smell, the sulfur is coming from your well or municipal supply and needs a water test.

_Last reviewed: July 2026 · 6 min read_

You turn on the shower and it smells like a science experiment gone wrong. It's not dangerous in most cases, but it's unpleasant, it can stain fixtures over time, and it usually means something specific is happening inside your water heater tank right now.

Okoniq Property Hub logs your water heater's age, last flush date, and anode rod replacement so you're not guessing when the smell started or what maintenance is overdue.

What actually causes the rotten egg smell?

The smell is hydrogen sulfide gas, produced when sulfate-reducing bacteria feed on the magnesium in your water heater's anode rod. Every tank-style water heater has a sacrificial anode rod that corrodes on purpose, protecting the steel tank from rusting out. In most homes that reaction is odorless. But if naturally occurring sulfate bacteria get into the tank, usually through the water supply, that magnesium reaction turns into hydrogen sulfide gas instead.

The bacteria thrive in warm, low-oxygen environments, which is exactly what sits inside a water heater tank between uses. Higher tank temperatures, generally above 140°F, tend to make the smell worse because bacteria grow faster in that range. Homes on well water see this more often since well water tends to carry more of the sulfate bacteria and minerals that feed the reaction, but it happens on municipal supply too, especially in homes with softened water.

Why does only the hot water smell, not the cold?

If only hot water smells, the problem is inside the water heater tank, not your plumbing or supply line. Cold water never touches the anode rod or sits in the tank, so it bypasses the reaction entirely. This is the single fastest diagnostic test you can run at home: fill a glass from the cold tap and one from the hot tap, and compare.

If both taps smell the same, the sulfur is coming from your water source itself, whether that's a private well or, less commonly, the municipal system. That calls for a water test rather than a heater repair, since replacing an anode rod won't fix source water. Homes that sit vacant for stretches, like seasonal properties closed up for months, often notice the smell worse when they reopen the house, because stagnant water gives bacteria more time to build up in the tank.

How do you actually fix a rotten egg smell in hot water?

Swap the magnesium anode rod for an aluminum or zinc-alloy rod, and flush the tank. The magnesium rod is what feeds the bacterial reaction, so replacing it with a different alloy usually stops the smell within a few days. The rod itself runs $20-40 and sits on top of the tank under a hex-head plug, though it can take some torque to break loose if it's never been changed.

A full tank flush clears out sediment and bacteria that have built up over months or years. Shut off the cold water supply, connect a hose to the drain valve, and run the tank empty into a floor drain or outside. For persistent cases, a chlorine bleach shock treatment, roughly a cup of unscented bleach through the tank followed by a full flush, kills off the bacterial colony more aggressively. If you're not comfortable draining a tank under pressure or handling the anode rod yourself, this is a reasonable job to hand to a plumber, similar to deciding when a clogged drain needs a snake versus a plumber rather than a DIY attempt.

| DIY Anode Rod Swap | Full Chlorine Shock Treatment | |---|---| | $20-40 in parts | $0-15 in bleach | | 1-2 hours | 3-4 hours including flush | | Fixes root cause long-term | Kills existing bacteria, doesn't prevent regrowth | | Best for mild, recurring smell | Best for strong, persistent smell |

When should you worry it's more than a nuisance smell?

Rotten egg smell from a water heater is almost never a health hazard at typical household concentrations, but persistent smell alongside rust-colored water or a tank older than 10-12 years is a sign the tank itself may be failing. Hydrogen sulfide becomes a genuine safety concern only at much higher concentrations than a home water heater produces, so this is a comfort and maintenance issue, not an emergency, in the vast majority of cases.

That said, an aging tank corroding internally can leak, and a tank past its expected 10-15 year lifespan is worth inspecting regardless of the smell. Annual maintenance, similar to the checklist for boiler maintenance basics, catches sediment buildup and rod wear before they turn into bigger repairs or a full tank replacement, which typically runs $1,200-2,500 installed.

FAQ

Is it safe to shower in water that smells like rotten eggs?

Yes, in nearly all residential cases, the concentration of hydrogen sulfide from a water heater is far below any level considered a health risk, though it's unpleasant and worth fixing.

Will lowering my water heater temperature stop the smell?

Sometimes. Dropping the thermostat from 140°F to around 120°F can slow bacterial growth and reduce the smell, though it usually won't eliminate it without also changing the anode rod.

How often should I replace my water heater's anode rod?

Check it every 2-3 years and replace it when more than 6 inches of the core wire is exposed, or sooner if you already have a bacterial odor problem.

Does a tankless water heater get this problem?

Rarely, since tankless units heat water on demand and don't hold standing water in a tank with an anode rod, removing the main condition that causes the smell.

Can water softeners make the smell worse?

Yes, in some cases. Softened water can be more corrosive to the anode rod and speeds up the reaction that produces hydrogen sulfide, which is why some plumbers recommend a powered anode rod in homes with softeners.

This is educational information, not plumbing advice. Call a licensed plumber if the smell persists after an anode rod swap and flush, or if you notice rust-colored water or visible tank leaks.