A rotten egg smell that appears only on the hot taps is a water heater problem rather than a water supply problem, and knowing that saves buying whole house treatment for something happening inside one appliance. The cause is a reaction between the tank's sacrificial anode and sulfate in the water, assisted by bacteria that thrive in the warmth.
Confirming it is the heater
Run the cold tap and smell it, then the hot. If only the hot smells, the heater is the source and nothing upstream needs treating. If both smell, the supply carries hydrogen sulfide and the answer is oxidation and filtration on the incoming water instead. If the smell appears only after the house has stood unused, that also points at the heater, since the reaction proceeds fastest in stagnant warm water. This one test decides where the money goes.
Why the anode causes it
Every glass lined tank has a sacrificial magnesium or aluminum anode that corrodes in place of the steel and is the reason the tank lasts. Magnesium is highly reactive, and in water containing sulfate it drives a reaction that produces hydrogen sulfide gas, which is the smell. Sulfate reducing bacteria in the tank accelerate it considerably. So the smell is a side effect of the tank protecting itself, which is why removing the anode entirely fixes the smell and shortens the heater's life.
The fixes, in order of durability
Flushing and shock chlorinating the tank clears the bacteria and the smell, and it usually returns within weeks or months, so treat it as diagnosis rather than repair. Raising the thermostat temporarily suppresses the bacteria and carries a scald risk that must be managed at the taps. The durable fix is changing the anode to an aluminum zinc alloy type, which resists the reaction while still protecting the tank. A powered impressed current anode eliminates it entirely and costs more.
What not to do
Do not simply remove the anode. The smell stops and the tank starts corroding, which usually voids the warranty and shortens its life substantially. Do not fit whole house treatment for a smell that appears on the hot side only, since it is aimed at the wrong place. And if the tank is old and the smell has appeared alongside discolored hot water, consider whether the heater is near the end of its life, in which case replacement is the sensible spend. Our tankless softener guide covers water quality upstream.
Questions people ask about water heater odor killer
Why does only my hot water smell of rotten eggs?
The heater's sacrificial anode reacts with sulfate in the water to produce hydrogen sulfide, helped along by bacteria in the warm tank.
Does flushing the tank fix it?
Only temporarily. Flushing and shock chlorinating clears the bacteria, but the smell usually returns within weeks or months.
What is the lasting fix?
Changing the anode to an aluminum zinc alloy type, or fitting a powered impressed current anode, which eliminates the reaction.
Can I just remove the anode?
No. The smell stops and the tank begins corroding, which usually voids the warranty and substantially shortens the heater's life.