Why does my hot water smell like rotten eggs?

Updated August 2026

That rotten-egg odor coming out of only the hot taps is hydrogen sulfide gas, made when a magnesium anode rod reacts with sulfate-reducing bacteria living in the tank. The rod is doing its job protecting the steel, but this smell is its byproduct. The two-part fix is to swap the magnesium rod for an aluminum-zinc one and to chlorine-shock the tank so the bacteria die off. If the cold water smells too, the source is your well or supply, not the heater.

Safety first: the hydrogen sulfide at household levels is a nuisance smell rather than a poisoning risk, but the chlorine shock is where people get hurt. Never mix bleach with any other cleaner, especially anything with ammonia, because the fumes are toxic. Work with a window open, cut the power or gas to the heater while it sits full of bleach, and flush every hot line until the chlorine smell is gone before anyone drinks or bathes.

Causes, most likely first

What makes a furnace do this, most to least common (Updated August 2026)
Cause	How likely	The fix	Typical cost
Magnesium anode reacting with tank bacteria	Most common	Replace the magnesium anode rod with an aluminum-zinc rod, which produces far less sulfide.	$30–60 part
Established bacteria colony in the tank	Common	Chlorine-shock the tank: add bleach, fill, let it sit a few hours, then flush thoroughly.	Free (a jug of bleach)
Water sitting too long or set too cool	Occasional	Raise the tank to 130–140°F for a few hours to pasteurize, then return to 120°F. Bacteria thrive in lukewarm tanks.	Free
Sulfur bacteria in the well or supply	Occasional	If the cold side smells too, shock-chlorinate the well or add treatment, not just the heater.	$100–1,500
Sacrificial anode fully consumed	Rare	A rod eaten down to the wire lets the tank corrode; a fresh one also resets the smell chemistry.	$30–60 part

Check these first

Smell the cold tap and the hot tap separately. If only the hot side stinks, the heater is the source; if both do, treat the well or supply.
Run an outside hose bib on cold. A clean-smelling yard hydrant plus a smelly hot tap confirms it is the tank, not the incoming water.
Check when it is worst. A smell that spikes after the water sits overnight or after a vacation points squarely at bacteria in a warm tank.
Look up the anode type. A magnesium rod, the factory default on most tanks, is the usual culprit and the first thing to change.
Note the setpoint. A tank kept below about 120°F grows bacteria faster, so a low dial is often part of the story.

Prevent it next time

Keep the smell from coming back by pairing an aluminum-zinc rod with regular tank care. Check the anode rod every 3–5 years and swap it before it is fully consumed, and hold the setpoint around 120°F, warm enough to discourage bacteria but below the 140°F scald threshold. An annual flush clears the sediment that gives bacteria a place to hide. Done together, these keep a tank odor-free across its 8–12 year life.

When to call a pro

Bring in a plumber if the smell survives a rod swap and a chlorine shock, if the old anode is seized and will not break loose (they can torque past 200 foot-pounds), or if the cold water also smells and you need well treatment. A rod replacement runs $150–300 installed, a professional tank sanitize $150–250, and whole-house sulfur filtration or aeration $1,000–3,000. Any of those beats living with the odor or re-shocking a tank that keeps regrowing bacteria.

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Frequently asked questions

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

At the levels a home water heater produces, the hydrogen sulfide is more offensive than dangerous, and a short shower is not a health emergency. The bigger concern is what the smell signals: bacteria in the tank, and a supply worth testing if the cold side smells too. Over time it can also corrode plumbing. Fix the source rather than masking it, and if the odor is strong on the cold, get a water test before drinking it regularly.

Will a whole-house water filter get rid of the sulfur smell?

It depends on where the sulfur comes from. If only the hot water smells, the problem is inside the heater, and no filter upstream of it will help; you need the anode swap and a chlorine shock. If the cold water smells too, the sulfur is in your supply, and a carbon filter, an oxidizing filter, or an aeration system on the main line is the right tool. Confirm which side smells before spending $1,000 or more on filtration.

Why does the smell come back a few weeks after I flush the tank?

Because a flush alone does not remove the two things that make the gas: the reactive magnesium anode and the surviving bacteria. Sediment washes out, the smell fades, then the same rod feeds the same bacteria and it returns. The lasting fix is to change the rod to aluminum-zinc and chlorine-shock the whole tank so the colony is actually killed, not just diluted. Skip either step and you will be flushing again by next month.