Hot Water Smells Like Rotten Eggs: The Water Heater Fix

If the rotten egg smell comes out of the hot tap and the cold tap is clean, your water heater is making the gas, and no amount of well treatment will fix it. Here is why heaters do this, the three fixes that actually work, and the one case where the well is the real source after all.

Why does only the hot water smell?

A water heater is a warm, still, sealed tank, which is a comfortable home for sulfur-reducing bacteria that arrive with the groundwater. Most tanks also carry a magnesium anode rod, a sacrificial metal bar that protects the tank from rust. The rod's slow corrosion feeds the chemistry those bacteria use to turn sulfate in the water into hydrogen sulfide gas, and the gas comes out with the hot water. The cold side never sits in that tank, so it never picks up the smell.

Prove it with one glass from each side. Fill a cold glass at a tap the water reaches before any treatment equipment, fill a hot glass at a sink, and smell both at arm's length. Hot only means the heater. Both, or cold only, means the well, and the well guide takes over from here.

The three fixes that work

First, swap the anode. An aluminum-zinc rod protects the tank without feeding the reaction, and a powered anode does the same job with no sacrificial metal at all. It is an hour of a plumber's time, or a careful afternoon with a breaker bar if the old rod is not seized. Second, flush and disinfect the tank the same day, so the colony that built up leaves with the old water instead of restarting on the new rod. Third, if the smell keeps returning after both, the well is seeding the tank with fresh bacteria on every fill, and the well itself needs a cleaning.

If the cold water smells too, the well needs a cleaning.

Answer a few questions and we pull your well's dimensions from your state's official well record, then pack a chlorine dioxide kit to those exact numbers from $399. A contractor charges $1,500 to $3,000 for the same cleaning.

Find my well See what's in the kit

When the smell is really the well

Sulfur bacteria live inside biofilm, the same slime that iron bacteria build on the casing, the pressure tank, and the pipes. A well that smells on the cold side, or keeps re-seeding a fixed heater, needs a shock treatment that reaches all of that. Our kit runs it as chlorine dioxide at the manufacturer's published 20 to 50 ppm shock range, sized to your well's gallons from its official record, and the residual test at the end tells you the dose held. Chlorine dioxide matters for this problem in particular: per the manufacturer's published data it penetrates biofilm, which is where the sulfur bacteria actually live. A smell that only shows up at the first tap of the morning is its own case.

Straight answers

Is rotten egg smell in hot water dangerous?

At the levels that make a shower smell, hydrogen sulfide is a nuisance rather than an emergency: it offends the nose, tarnishes fixtures, and corrodes plumbing over years. Fix it for those reasons, ventilate the bathroom meanwhile, and treat any new smell as a reason to test the water.

Can I just turn the water heater up to kill the smell?

Raising the tank to a high setting for a few hours can knock the bacteria back, and the smell usually returns once the temperature comes down, because the anode reaction and the bacteria are both still there. It also creates a scald risk at every tap. Treat it as a stopgap, not the fix.

Will a new water heater fix the rotten egg smell?

Only until the new heater's magnesium anode starts the same reaction with the same bacteria. If you are replacing the heater anyway, order it with an aluminum-zinc or powered anode, and clean the well if the cold side smells too.

Why did the smell start after we installed a water softener?

Softened water speeds up the anode reaction, so a heater that was quiet for years often starts smelling within weeks of a softener going in. The anode swap is the usual answer, and the softener can stay.

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