Six causes in the order they actually happen, split by system type, and the three things worth checking yourself before a callout that you pay for either way.
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When the hot water stops
The symptom tells you a lot. No hot water anywhere is a different fault to no hot water in one shower, and neither of them means you need a new system.
Read the symptom before you read the quote
Hot water faults are unusually easy to narrow down, because a hot water system only does a few things: it fills, it heats, it holds temperature, and it delivers under pressure. Each of those failing has its own signature.
No hot water at all usually means the heat source stopped — a tripped circuit or failed element on electric, a pilot or ignition fault on gas, a controller fault on a heat pump. Hot water that runs out fast usually means the tank is fine but undersized, or the lower element has failed and only the top of the tank is heating. Hot water that is warm but never hot often points at the thermostat, or at a tempering valve that has drifted. Low hot water pressure while cold is fine is almost always a blockage or a valve on the hot side, not the system itself.
Where this site stops. Diagnosis is yours. The repair is very often not: in Australia, work on the water side of a hot water system is plumbing work and gas work is gasfitting, and both are licensed trades. We will tell you what has failed and what the part is called, so you are not buying a story — and then we will tell you to call someone with a licence. What you can legally do yourself covers where that line sits.
Two things we will never ask you to do: touch anything on the gas side of a gas unit, including relighting or adjusting a pilot; and open an electric unit's element or thermostat cover. The first is a licensed gasfitter's job, the second sits behind mains voltage in a wet enclosure.
No hot water guides
Water at the relief valve, at the base, at the fittings or from the tank itself mean completely different things. One is normal, one means the tank is finished.
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