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.
Weak hot water with normal cold water rules out your mains supply straight away. Six causes that hit the hot side alone, from a gravity-fed system to a tempering valve on the way out.
Hot at the kitchen and cold at the rose means the system is working. Six things that fail at a single outlet, from a worn cartridge to a limit stop put back in the wrong place.
Some discharge while the system heats is the valve doing its job. A constant dribble is not — and the discharge pipe must never be capped, whatever it is doing.
Warm from the first second is a thermostat or a heat source. Hot then warm is capacity. Warm in the bathroom while the kitchen stays hot is the tempering valve.
Run the cold tap first. Discoloured cold water points outside the house; clear cold and brown hot points at the cylinder, its anode or the hot-side pipework.
A sulphur smell on the hot tap only is produced inside the cylinder, not in the supply. One comparison tells you which of the two you have.
Shared plant or your own unit? Ask the neighbours first, then run a few checks that need no tools and find out who is responsible.
Seven symptoms, the fault each one usually means, what you can check safely and what to tell the plumber.
Four faults cover most dead electric systems. Check the switchboard and the schedule, then stop at the cover.
Hot runs at other taps but not this one? Five faults that hit a single outlet, the one-minute test and where DIY stops.
One glass and two minutes tell you if it is air or something worse. Scale, sediment and where DIY stops.
Both out at once? Find the shared link: switchboard, gas supply or a combined unit, and where DIY stops.
Repair or replace, which type, what size and what a licensed plumber must supply before you accept a quote.
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