Short answer
Check first: does only the hot water smell? If cold water smells too, the problem is the water supply (common with wells), not the heater. If only hot water smells, the usual cause is sulfate reducing bacteria reacting with a magnesium anode rod. The usual fixes, in order: flush the tank, swap to an aluminum zinc anode rod, or fit a powered anode rod, which does not feed the reaction.
Hot only, or hot and cold?
- Only hot water smells: the heater is the cause. Read on.
- Hot and cold both smell: the supply water contains hydrogen sulfide, typically from a well. Treat the supply (a water test tells you what kind of filter).
- The smell is in the air, near a gas appliance: that may be gas, not water. Leave and call your gas utility.
Why the heater makes the smell
Sources describe it as a reaction between the anode metal and sulfate reducing bacteria in the water, which produces hydrogen sulfide gas. Warm water in the tank suits the bacteria, and a magnesium anode gives the reaction what it needs. The anode rod is there to protect the tank, so the fix is to change its metal, not to leave it out.
Fixes, in order
Flush the tank
Drain and flush to clear sediment and the bacteria living in it. See how to flush a water heater.
Swap to an aluminum zinc anode
This is the common fix. Makers sell universal aluminum zinc rods as replacements; zinc helps suppress the odor.
Or fit a powered anode
A powered titanium rod protects the tank with a small current instead of a sacrificial metal, so there is nothing for the reaction to use. It needs an outlet nearby.
If it returns: disinfect
Some makers describe a hydrogen peroxide or chlorine treatment of the tank. Follow your heater’s manual exactly, then flush thoroughly.
The step by step for the swap is in how to replace an anode rod, and the flush in how to flush a water heater.
What you need
Universal aluminum zinc rods are a standard maker part, for example A. O. Smith’s universal aluminum zinc anode.
Questions people ask
Is rotten egg smelling hot water dangerous?
At the levels that cause an odor in household water it is usually a nuisance rather than a health risk, but have well water tested if the smell is strong or in the cold water too. A rotten egg smell near a gas appliance in the air, not the water, can be a gas leak: leave and call the gas company.
Will removing the anode rod stop the smell?
It can, but without an anode the tank corrodes and fails early, and many makers say removing it can affect the warranty. An aluminum zinc or powered rod fixes the smell and still protects the tank.
Does turning up the temperature kill the bacteria?
Raising the tank temperature for a while can reduce the bacteria, but it raises the scald risk and the smell often returns. Follow your manual and use a mixing valve at taps if you store water hot.
Why did the smell start after installing a water softener?
Softened water often speeds up anode wear and the reaction that causes the odor. An aluminum zinc or powered anode is the common fix.
Sources
Last reviewed October 29, 2026. Models, codes and listings change; check your heater’s manual and your local code, and follow the maker’s instructions.