Hot water that smells like rotten eggs
The smell is hydrogen sulphide, and in a storage system it almost always comes from the tank rather than the water supply. One test tells you which one you have.
If the rotten-egg smell is on the hot tap only, the cause is almost certainly inside your hot water system: a reaction between the sacrificial anode, the water, and sulphate-reducing bacteria living in the warm tank. If both hot and cold smell, the water is arriving that way and the system is a bystander.
That is the whole diagnosis, and it takes a minute. Run a cold tap on its own, well away from the hot water system, and smell the water in a glass rather than at the tap. Then do the same with the hot. The difference between those two glasses decides everything that follows.
| Where the smell is | What it points to | Who deals with it |
|---|---|---|
| Hot only, every hot outlet | Reaction in the storage tank involving the anode | A licensed plumber |
| Hot and cold, whole house | The supply itself, or a tank or bore source | Your water utility, or a NATA-accredited lab if you are not on mains |
| One outlet only, hot and cold | The drain or trap at that fixture, not the water | Worth checking the waste before the water |
| Only after the house has been empty | Water standing still long enough for bacteria to build up | Often clears with normal use; monitor it |
Why does a hot water tank make a sulphur smell?
A storage cylinder is a steel vessel, and steel in hot water corrodes. It is protected by a sacrificial anode: a rod of a more reactive metal, usually magnesium or aluminium, that is designed to corrode instead of the tank wall. That is its entire job, and it works.
The side effect is that as the anode does its work it releases hydrogen. Most water also carries a small amount of naturally occurring sulphate, which is harmless and has no smell. Put those two together in a warm tank that also contains sulphate-reducing bacteria, and the bacteria convert the sulphate into hydrogen sulphide gas. Hydrogen sulphide is what you can smell, and the human nose detects it at concentrations far below anything that would show up in an ordinary water test.
Three things have to line up for this to happen: a reactive anode, sulphate in the water, and bacteria in the tank. Change any one of them and the smell usually goes. That is why the same house can have the problem after replacing a system and not before, and why a neighbour on the same street can have no smell at all.
Why magnesium anodes smell more
Magnesium is the more reactive of the two common anode metals, which makes it better at protecting the tank and also more likely to drive the reaction that produces the smell. Aluminium anodes are less reactive, which is why a change of anode type is the usual answer when a tank smells but is otherwise sound.
The choice is not free, though. Anode selection depends on water chemistry, and an anode that suits your water is what keeps the tank alive � see how long hot water systems last for why this one part decides so much. Swapping to a different anode to chase a smell, without regard to what the water is like, can trade an odour problem for a corrosion problem. This is a conversation to have with the plumber, not a part to choose from a shelf.
What makes it worse
- A tank that sits unused. Holiday houses, spare bathrooms and systems switched off while travelling all give bacteria time to multiply. The smell is often strongest on the first draw after a quiet period.
- A system running cooler than it should. Storage temperature is set for a reason, and part of that reason is bacterial control. A thermostat that has drifted low, or a system struggling to reach temperature, creates conditions bacteria prefer. If the water is also not getting properly hot, read hot water that is not hot enough first � that fault and this smell often share one cause.
- Tank, bore or untreated water. A supply that is not chlorinated carries more of the organisms involved. If your water comes from a rainwater tank or a bore, see is tank water safe to drink.
- Warm ambient conditions around a cylinder that is already borderline.
Smell without colour, and colour without smell
These are two different faults and it is worth not treating them as one. A smell with clear water is the anode-and-bacteria reaction described above, and it says nothing in itself about the condition of the tank. Discoloured water, on the other hand, means iron is being picked up somewhere � which is a corrosion question, and a more serious one. If your water is brown or rusty as well, start with brown hot water instead, because that symptom takes priority.
A third possibility catches people out: if the smell appears at one fixture on both hot and cold, and there is nothing wrong anywhere else in the house, the water is probably fine and the smell is coming from the waste side � a dry trap or a fouled overflow. Fill the trap by running the fixture and see whether it goes.
The checks that are yours to make
- Compare hot and cold in two glasses, away from the fixture, so you are smelling the water and not the drain.
- Check every hot outlet, not just the one you noticed. Hot-only and house-wide is the classic tank signature.
- Note when it is strongest � first draw of the day, after a period away, or constantly.
- Ask a neighbour on the same supply. If theirs smells too, it is not your tank.
- Find out the age of the system and whether the anode has ever been inspected or replaced.
- Note any recent change: a new system, a change of supply, a thermostat adjustment, a long absence.
What to tell the plumber
- Hot only, or hot and cold. Say this first � it decides whether they are looking at your system at all.
- Every outlet, or just one. One outlet suggests the waste, not the water.
- Constant, or worst after the house has been quiet.
- The age of the system, and whether the anode has ever been checked.
- Whether the water is also discoloured, or the hot is not getting properly hot.
With those five answers a plumber can usually tell you before attending whether this is an anode conversation, a temperature conversation or a water-supply conversation � and that is the difference between one visit and three.
How far can you take this yourself?
Smelling the water, comparing outlets, timing when it is worst and asking your utility whether there is a supply issue are all yours, and between them they resolve a good share of these cases without anyone attending. Everything inside the system is not: draining a cylinder, inspecting or replacing an anode and adjusting stored water temperature are licensed plumbing work in every Australian state and territory, and the plumbing regulator for your state or territory is who licenses and inspects that work. Where that line falls is set out in full on the licensed work pillar.
Frequently asked questions
Why does only my hot water smell like rotten eggs?
Because the smell is produced inside the storage tank, not in the supply. The sacrificial anode releases hydrogen as it corrodes, sulphate-reducing bacteria in the warm tank convert naturally occurring sulphate into hydrogen sulphide, and that gas is what you smell. Cold water never sits in the tank, so it never picks it up. If your cold water smells too, the cause is the supply rather than the system.
Is water that smells like rotten eggs safe to use?
Nobody can answer that from a description, and this site will not try. The smell itself is detectable at concentrations far below anything a water test would flag, so it is not evidence of danger on its own — but it is not evidence of safety either. If hot and cold both smell and you are on mains water, contact your water utility. If your supply is a rainwater tank or a bore, testing is a job for a NATA-accredited laboratory.
Will replacing the anode stop the smell?
Often, yes — changing to a less reactive anode is the usual fix when a tank is otherwise sound. But anode selection depends on your water chemistry, because the anode is also what keeps the tank from corroding. Choosing one to chase a smell without regard to the water can shorten the life of the cylinder. It is licensed plumbing work and a decision to make with the plumber.
The smell only appears when we come back from holiday. Why?
Because water that sits still in a warm tank gives the bacteria involved time to multiply. The first draw after a quiet period is when concentrations are highest, and the smell usually fades once the system is being used normally again. If it does not fade, or it is there during ordinary use, treat it as an ongoing fault rather than a stale-water effect.
Sources
What this page explains is based on these official sources. If you check one and it does not match what you read here, send it to us and the article gets corrected.
- Australian Government, energy.gov.au. Hot water systems — types, installation and energy performance
- NSW Fair Trading. Plumbers, drainers and gasfitters — licensing