A rotten-egg smell from hot water only — cold water smells fine — points almost exclusively at the anode rod reacting with naturally occurring bacteria inside the tank, not a problem with your water supply itself. This is more common with well water and with certain anode rod materials, and it is fixable through flushing, disinfection, or an anode rod change, roughly in that order of effort.
Why it happens
Most tank water heaters contain a sacrificial anode rod, usually magnesium or aluminium, whose entire purpose is to corrode instead of the tank’s steel lining — it is designed to be consumed over the years so the tank itself is not. Under the right conditions, naturally occurring sulphate-reducing bacteria in the water react with that corroding rod and produce hydrogen sulphide gas, which is exactly what produces the rotten-egg smell. This is a chemical reaction happening inside your specific tank, not a sign that water arriving at your house is unsafe.
Confirm it is the water heater, not the supply
Run cold water from the same tap and smell it. If only the hot water smells, the cause is almost certainly inside the water heater. If cold water smells too, the cause is elsewhere in your plumbing or supply, and this guide will not solve it — that is worth investigating separately, particularly on well water.
Common causes
| Cause | How likely | Typical sign |
|---|---|---|
| Bacteria reacting with a magnesium anode rod | Very common | Especially common on well water systems |
| Tank left unused for an extended period | Common | Smell appeared after the house was empty for a while |
| Water heater set to a lower temperature | Common | Lower temperatures favour the bacteria responsible |
| Aluminium or zinc-alloy anode reacting differently | Occasional | Varies by original equipment and any anode replacement done |
| Sediment build-up harbouring bacteria | Occasional | Tank has not been flushed in a long time |
| Contamination in the actual water supply | Uncommon | Cold water also smells — not a water heater issue |
How to fix it
- Flush the tank
Draining several gallons through the drain valve at the base clears sediment that can harbour bacteria and refreshes the water in contact with the anode. This alone resolves the smell for some households, at least temporarily.
- Raise the thermostat temporarily
Running the tank at a higher temperature (around 60°C / 140°F) for several hours kills the bacteria responsible, then return it to a normal, safer setting afterward. Be aware that water above about 49°C (120°F) can scald, so this is a temporary measure with care taken around anyone using hot water during it, not a permanent setting change.
- Disinfect the tank
A more thorough approach involves draining the tank and flushing it with a diluted hydrogen peroxide or chlorine solution, left to sit before a full flush and refill — this is more involved than a routine flush and worth following a model-specific procedure or having a plumber carry out if you are not comfortable with it.
- Consider an anode rod change
Replacing a magnesium anode with an aluminium or zinc-alloy version often stops the reaction that produces the smell, since the bacteria respond differently to different anode materials. This is a straightforward job for a plumber, and worth mentioning specifically if the smell keeps returning after flushing and disinfection.
Removing the anode rod entirely to stop the smell also removes the protection it provides the tank, and the tank will then corrode and fail considerably sooner. Change the anode material, do not eliminate it.
When to call a plumber
Call a plumber if flushing and a temporary temperature increase do not resolve the smell, if you want an anode rod changed to a different material, or if cold water is also affected — which points at your supply rather than the water heater and needs a different kind of investigation entirely.
Stop and call a qualified appliance technician or electrician if you find scorched or melted wiring, smell burning, see water that has reached electrical components, or if the fault returns immediately after a repair. Never bypass a safety device — thermal fuses, door interlocks and flood switches fail for a reason, and defeating one converts a broken appliance into a fire or flood risk.
