Brown hot water: is it your cylinder or the mains?
Rusty or brown water is not a plumbing mystery — it is an iron problem, and the only real question is where the iron is coming from. Run the cold tap first and you have narrowed it to one of two answers.
Start here, before anything else: run a cold tap on its own, at a fixture as close as possible to where the water enters the house, and let it run for a minute or two.
If the cold runs brown as well, the discolouration is arriving from outside and your hot water system is a bystander. If the cold runs clear and only the hot is discoloured, the iron is being picked up somewhere between the cold inlet of your hot water system and the tap. That single test splits the problem cleanly in two, and almost everything else follows from it.
| What you see | Where it is coming from | Who deals with it |
|---|---|---|
| Brown at hot and cold, whole house | The mains supply, or a shared service pipe | Your water utility in the first instance |
| Brown at hot only | The cylinder, its anode, or hot-side pipework | A licensed plumber |
| Brown for a few seconds, then clear | Settled sediment being disturbed on first draw | Worth monitoring; not urgent on its own |
| Brown after works in the street or a supply interruption | Disturbed mains sediment | Usually clears itself; utility if it does not |
| Brown after the system was off or the house empty | Water that has sat still in the tank | Monitor once normal use resumes |
Why hot water goes brown and cold water does not
A storage hot water system is, for most Australian homes, a steel cylinder full of hot water. Hot water is considerably more corrosive to steel than cold, which is why the cylinder is protected in two ways: a glass or vitreous enamel lining, and a sacrificial anode — a rod of a more reactive metal that corrodes in place of the tank.
The anode is a consumable. It is designed to be eaten away, and when it is gone the protection it provided is gone with it. From that point the water is working directly on the steel, and what you eventually see at the tap is the result: iron oxide, in the water, from your own tank.
That is why discolouration on the hot side alone is worth taking seriously even when it looks minor. It is rarely a fault that appeared overnight. It is usually the visible end of a process that has been running quietly for years.
The other things it can be
Disturbed sediment
Sediment accumulates at the bottom of any storage cylinder over its life. Normally it sits there undisturbed. Anything that stirs it — a pressure change, a supply interruption, a service, a system that has been drained and refilled — can put it into suspension for a while.
The signature is that it is temporary and improving. If the water is clearing a little more each day after an identifiable event, this is very likely what you are looking at.
Galvanised pipework
Older houses may still have galvanised steel pipe in part of the plumbing. The zinc coating that protects it does not last forever, and once it goes the steel underneath corrodes from the inside. Discoloured water is one symptom; the other, arriving at the same time, is falling flow as the internal diameter narrows with corrosion.
If the brown water has crept up alongside gradually worsening pressure, the pipe is worth considering rather than the tank — see low hot water pressure for how to tell those apart.
Water that has simply sat still
A holiday house, a system switched off while travelling, or a rarely used outlet will all produce discoloured first-draw water without anything being wrong. Water that stands still in contact with metal picks up what it is standing in. This one resolves itself with normal use.
What the colour is telling you
- Orange or rust brown: iron. Either corroding steel, or iron already present in the supply.
- Brown with visible grit or flakes: sediment or scale in suspension, rather than dissolved iron.
- Dark specks or a greyish cast: often anode debris or scale carried out of the cylinder.
- Cloudy white that clears from the bottom up: not discolouration at all. That is dissolved air, and it is harmless.
A rotten-egg smell is a separate problem with a separate cause, and it is worth not conflating the two: smell without colour is usually a reaction involving the anode and bacteria in the tank, not corrosion of the tank itself.
The checks that are yours to make
- Run the cold on its own and watch it. This is the whole diagnosis in one step.
- Check more than one tap, at both ends of the house. A single discoloured outlet is a different problem to a house-wide one.
- Time how long it takes to clear. Seconds means first-draw sediment. Never means something ongoing.
- Ask the neighbours. If the street has it too, it is the supply, and your water utility will usually know about it already.
- Check whether it followed an event — works in the street, a supply interruption, a service, a period with the system off.
- Note the age of the system, if you know it. Discolouration on an ageing cylinder is a different conversation to the same symptom on a recent one — see how long hot water systems last.
What to tell the plumber
- Hot only, or hot and cold. Say this first; it determines whether they are looking at your system at all.
- Constant, or only on the first draw after the water has been sitting.
- Whether flow has dropped too, which points at pipework rather than the tank.
- The age of the system, and whether the anode has ever been checked.
That last one matters more than it sounds. An anode that has never been inspected across the life of a cylinder is the most common reason a tank reaches this point, and it changes the conversation from a repair to an honest assessment of what the cylinder has left.
How far can you take this yourself?
Running taps, comparing outlets, timing how long the discolouration lasts and asking your utility whether there is a supply issue are all yours, and 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 any work on hot-side pipework is licensed work in every Australian state and territory, and the plumbing regulator for your state or territory is who licenses and inspects it. Where that line falls is set out in full on the licensed work pillar.
Frequently asked questions
Why is only my hot water brown?
Because the iron is being picked up after the cold inlet of your hot water system. Hot water corrodes steel far faster than cold, so a cylinder whose sacrificial anode has been consumed will start to shed iron oxide into the water. Disturbed sediment in the tank and corroding galvanised hot-side pipework produce the same symptom. If the cold runs clear, the supply is not the cause.
Is brown water from the hot tap dangerous?
Nobody can answer that from a description, and this site will not try. What you can do is narrow down the source: run the cold on its own, and if it is discoloured too, contact your water utility, which is responsible for the water it delivers. If your supply is a rainwater tank or a bore, testing is a job for a NATA-accredited laboratory. In the meantime, do not assume discoloured water is fine to drink or cook with.
Does brown hot water mean I need a new hot water system?
Not automatically, but it is a warning worth acting on. If the discolouration is constant, hot-side only, and the cylinder is well into its service life, it usually means the anode is spent and the steel is corroding — which is a replacement conversation rather than a repair. If it followed an identifiable event and is clearing day by day, it is far more likely to be disturbed sediment.
Why did my water go brown after work in the street?
Mains work, a burst repair or a supply interruption changes the flow in the pipes and lifts settled sediment into suspension. It normally clears on its own once normal flow resumes, and it shows up at both hot and cold taps. If it persists for more than a day or two, tell your water utility.
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