You turn on the hot tap and immediately notice a strong sulphur smell. It is often described as rotten eggs, and once you smell it, it is impossible to ignore.
The strange part is that the cold water smells perfectly normal. Only the hot water seems affected.
For many homeowners, this can be alarming. Is the water contaminated? Is something wrong with the tank? Is it safe to use?
In most cases, the answer is simpler than people expect.
If your hot water smells like rotten eggs, the cause is usually a natural chemical reaction occurring inside the hot water system. While the smell is unpleasant, it is rarely a health concern and can usually be fixed without replacing the entire unit.
What Causes Hot Water to Smell Like Rotten Eggs?
The smell is caused by a gas called hydrogen sulfide.
Hydrogen sulfide develops inside some hot water systems when three conditions exist:
1. Sulfates in the Water
Small amounts of sulfate occur naturally in many Australian water supplies. These minerals are harmless and commonly found in both mains and bore water.
2. Sulfate-Reducing Bacteria
Certain bacteria thrive in warm, low-oxygen environments, making the inside of a hot water tank an ideal place to live. These bacteria consume sulfate and produce hydrogen sulfide gas as a by-product.
3. The Sacrificial Anode
Most storage hot water systems contain a sacrificial anode, typically made from magnesium. This rod is designed to corrode over time, protecting the steel tank from rust.
As the anode breaks down, it releases hydrogen into the water. This hydrogen provides a food source for the bacteria, helping them generate more hydrogen sulfide gas.
When these three factors combine, the result is the distinctive rotten egg smell coming from your hot water taps.
Why Does the Smell Only Come From the Hot Water?
The cold water supply does not pass through the heated storage tank and therefore does not experience the same chemical reactions.
Without the warm environment, sacrificial anode and bacterial activity, hydrogen sulfide gas is unlikely to develop.
If only the hot water smells, the problem is almost always inside the hot water system itself.
Is Water That Smells Like Rotten Eggs Dangerous?
In most residential situations, no.
Hydrogen sulfide has a very strong odour even at extremely low concentrations. In fact, people can detect the smell long before it reaches harmful levels.
The water is generally safe for:
- Showering
- Washing dishes
- Laundry
- General household cleaning
The issue is primarily one of comfort and convenience rather than safety.
That said, nobody wants their bathroom, kitchen or laundry smelling like sulphur every time they use hot water, so it is worth addressing promptly.
Why Some Homes Experience the Problem More Than Others
Not every hot water system develops this issue. Several factors influence whether the smell occurs.
Water Chemistry
Some areas have naturally higher sulfate levels than others. Bore water and rural supplies are often more susceptible.
Type of Anode
Magnesium anodes are more likely to contribute to hydrogen sulfide production than aluminium anodes.
Water Temperature
Bacteria thrive in temperatures between approximately 40°C and 55°C. Systems operating below 60°C can create favourable conditions for bacterial growth.
Usage Patterns
Holiday homes, vacant properties and households with low hot water usage often experience stronger odours because water remains stagnant in the tank for longer periods.
Tank Age
New hot water systems occasionally develop the smell shortly after installation because fresh magnesium anodes are particularly reactive.
How to Fix Rotten Egg Smells in Hot Water
The good news is that several proven solutions are available.
Flush the Tank
Draining and refilling the tank can temporarily remove dissolved hydrogen sulfide and reduce the smell.
While this often provides short-term relief, the odour may return if the underlying cause is not addressed.
Increase the Water Temperature
Raising the thermostat to at least 60°C can help eliminate the bacteria responsible for producing hydrogen sulfide.
After the system reheats, running the hot taps helps flush older water from the pipework.
Replace the Magnesium Anode
One of the most effective long-term solutions is replacing the magnesium anode with an aluminium anode.
Aluminium corrodes more slowly and produces less hydrogen, making it much harder for bacteria to generate hydrogen sulfide gas.
Install a Powered Anode
A powered (impressed current) anode provides ongoing corrosion protection without sacrificing metal into the water.
Because it does not corrode like a traditional anode, it removes one of the key contributors to the odour problem.
Although more expensive initially, powered anodes are often considered the most permanent solution.
Chlorinate the Tank
Professional chlorination can kill the bacteria responsible for producing the smell.
While effective as an immediate treatment, bacteria may eventually return if the anode remains unchanged.
What If Both the Hot and Cold Water Smell?
If the rotten egg smell is present in both hot and cold water, the issue is unlikely to be the hot water system.
Instead, hydrogen sulfide may already be present in the incoming water supply.
This is more common in properties using bore water or untreated groundwater sources.
In these cases, water treatment systems such as activated carbon filtration or aeration may be required.
Common Mistakes Homeowners Make
Ignoring the Problem
The smell rarely disappears on its own. In many cases, it gradually becomes stronger over time.
Removing the Anode Completely
Some people remove the sacrificial anode to eliminate the smell. While this may reduce the odour, it leaves the tank vulnerable to corrosion and can significantly shorten its lifespan.
An anode should always be replaced, not removed.
DIY Chemical Treatments
Adding bleach or other chemicals without proper guidance can damage the tank lining, create unwanted reactions or introduce additional issues.
Professional treatment is always the safer option.
Assuming the Water Is Unsafe
The smell can be unpleasant enough to make people think the water is dangerous. In reality, the odour is usually far more offensive than the actual risk.
When to Call a Hot Water Specialist
If your hot water smells like rotten eggs, a licensed plumber can quickly identify the cause.
In most cases, resolving the issue involves:
- Inspecting the sacrificial anode
- Checking thermostat settings
- Flushing the tank
- Replacing the anode if required
- Recommending a powered anode where appropriate
The solution is often straightforward and can usually be completed in a single service visit.
Final Thoughts
A rotten egg smell coming from your hot water is unpleasant, but it is rarely a sign of a serious problem.
In most cases, the smell is caused by a reaction between naturally occurring bacteria, sulfate minerals and the sacrificial anode inside the tank. Once the source is identified, the issue can usually be resolved quickly and effectively.
If your hot water smells like rotten eggs, don’t put up with it. A professional inspection can identify the cause and restore fresh, odour-free hot water throughout your home.
About Central Coast Hot Water Systems
At Central Coast Hot Water Systems, we provide fast, reliable hot water repairs, replacements, servicing and installations across the Central Coast. With more than 30 years of industry experience, our licensed team works on electric, gas, solar, storage tank and continuous flow hot water systems.
Whether your hot water smells like rotten eggs, is leaking, not heating properly or has stopped working altogether, we can identify the cause and recommend the most cost-effective solution for your home.