TL;DR
- The way your hot water fails tells you a lot. A sudden total loss usually points to power, gas, or pilot light. Slowly getting worse points to a dying element, sediment, or thermostat.
- Lukewarm-but-never-hot is nearly always a thermostat or, on electric units, one failed element. Hot that runs out fast is usually sediment stealing tank space or a leak.
- Safe things you can check: switchboard, the isolation switch, whether other gas appliances work, and the pilot light on older gas units.
- Leave elements, thermostats, relief valves, thermocouples, and anything gas to a licensed plumber or gas fitter. In NSW that work is licensed for a reason.
- If you smell gas, stop, get outside, and call the 24-hour gas emergency line on 1800 427 532.
Most people troubleshoot a hot water fault by system type: is it gas, is it electric, and so on. That works, but it skips the fastest clue you’ve got. The pattern of the failure often tells you more than the system does. Water that vanished overnight is a different problem to water that’s been getting weaker for a month.
So before you go poking around the unit, ask one question: what exactly changed, and how fast? This guide reads the symptoms with you and points you to a safe next step. If you’d rather work through it by system type instead, we’ve covered that in why is my hot water system not working. This one is the symptom-first companion.
Symptom 1: no hot water at all, and it happened suddenly
Sudden total loss almost always means the unit lost its power source, not that it wore out. Something cut the supply.
Electric systems. Check the switchboard for a tripped breaker or blown fuse, often labelled “hot water” or “off-peak”. Then find the isolation switch near the unit or in the meter box and make sure it’s on. If the breaker trips again the moment you reset it, stop. That’s usually a failed element or a wiring fault, and repeatedly resetting it is not safe. Leave it and call a plumber.
Gas systems. First check whether your gas is actually flowing. Try the cooktop or another gas appliance. No gas anywhere often means a supply interruption, not a broken unit. Check your meter or letterbox for a network notice. If gas is fine elsewhere, an instantaneous (continuous flow) unit that won’t fire could be a power issue, since most of them need electricity for ignition.
Off-peak timing. On some older electric setups the water only heats overnight. If it ran out during a big laundry day and hasn’t recovered, that’s the tariff working as designed, not a fault.
Symptom 2: the pilot light keeps going out
This is a gas-only problem, and only on older units with a standing pilot light. Newer continuous flow systems use electronic ignition and won’t have one.
If the pilot has gone out, you can follow the relighting steps printed on the panel of the unit. Do that only if you cannot smell any gas. If you can, don’t touch it, get outside, and call the gas emergency number.
If the pilot lights but won’t stay lit, that’s the classic sign of a failing thermocouple, the little safety sensor that shuts off the gas when the flame drops. Replacing it is a licensed gas fitter’s job in NSW. Don’t try to clean or bend it yourself.
Symptom 3: the water’s warm but never properly hot
Lukewarm and stuck there is a temperature fault, not a supply one. The unit is running, it’s just not heating to the right point.
On electric storage units, a common cause is one of two heating elements failing. The remaining element still warms the water, just never to full temperature, and it takes longer. A faulty thermostat can do the same thing, on electric, gas, solar, and heat pump systems alike.
None of that is a DIY fix. Testing and replacing an element or thermostat means working with electrical components and the tank. That’s licensed work. What you can do is note when it started and whether it’s every tap or just one, then hand that detail to the plumber so the diagnosis is faster.
Symptom 4: hot water runs out much faster than it used to
If a shower that used to last ten minutes now goes cold in three, the tank isn’t holding as much usable hot water as it once did. Two usual suspects.
Sediment buildup. Over years, minerals settle at the bottom of the tank and take up space that used to hold water. Sydney’s water is fairly soft, so this is slower here than in hard-water regions, but on a tank over eight or ten years old it still adds up. You may also hear rumbling or popping as the unit heats.
A leak somewhere. A slow leak on the tank, the fittings, or a run of pipe means heated water is escaping before you use it. If you’ve noticed a damp patch, a rising water bill, or the unit seems to run constantly, get it looked at. Hidden leaks are worth catching early, which is exactly what our leak detection service is for.
A sizing mismatch matters too. If your household grew, added a bathroom, or the kids are now teenagers, the tank might simply be too small for the demand. That’s not a fault, it’s a capacity problem, and it’s a good moment to think about the right unit for how the house actually runs now.
Symptom 5: the hot water comes back briefly, then dies again
Water that works for a few minutes then cuts out, over and over, often points to a control or safety device tripping. On electric units it can be a safety cutout (a manual reset button that trips when the thermostat overheats). On gas units it can be the thermocouple or a control fault.
You can look for a reset button on an electric unit and press it once. If it trips straight back, that’s a fault telling you something’s wrong. Don’t keep resetting it. Repeatedly forcing a safety device is how a small problem becomes a dangerous one.
Symptom 6: discoloured water, a rotten-egg smell, or noises
These aren’t strictly a “no hot water” problem, but they show up in the same phone calls, so they’re worth naming.
- Rusty or brown hot water that clears from the cold tap usually means corrosion inside the tank or a spent sacrificial anode. The tank is ageing.
- Rotten-egg smell from the hot tap is often a reaction inside the tank, again pointing to the anode.
- Rumbling or banging is usually sediment, as above.
All of these are diagnosis-and-service jobs. Photograph anything on a digital display and note when it started, it saves time.
Quick symptom-to-cause guide
| What you’re seeing | Most likely cause | Safe to check yourself? |
|---|---|---|
| Sudden, total loss of hot water | Tripped breaker, isolation switch off, gas supply interruption, pilot out | Yes, the basics |
| Pilot light won’t stay lit (gas) | Faulty thermocouple | Relight only, then call a gas fitter |
| Warm but never hot | Failed element or thermostat | No, licensed work |
| Runs out too fast | Sediment, a leak, or undersized tank | Look for leaks only |
| Works briefly, then cuts out | Tripping safety cutout or control fault | One reset attempt, no more |
| Discoloured water, smell, noises | Corrosion, spent anode, sediment | No, needs service |
What’s safe to check, and what to leave alone
Here’s the honest line between the two.
Fine for you to do: check the switchboard and isolation switch, test whether other gas appliances work, look for a network outage notice, relight a standing pilot light (only if there’s no gas smell), press an electric reset button once, and check for visible leaks and damp patches.
Leave to a licensed plumber or gas fitter: heating elements, thermostats, the temperature and pressure relief valve, thermocouples, any gas pipe or fitting, solar controllers, heat pump refrigerant work, and anything involving the tank body. In NSW, this work is licensed because it involves electricity, gas, and pressurised hot water. Doing it yourself can void your warranty and insurance, and it can be genuinely dangerous.
If you ever smell gas, don’t switch anything on or off, don’t use a phone inside the house, get everyone outside, and call the gas emergency line on 1800 427 532 or 000.
When it’s a fix, and when it’s time to replace
A single failed part on a unit under about eight years old is usually worth repairing. A tank that’s ten-plus years old, leaking from the body, and running out fast is often telling you it’s near the end. If you’re weighing that up, it’s worth reading our take on the real benefits of upgrading your hot water system before you commit either way.
The trap most people fall into is waiting until the unit dies completely on a cold morning, then taking whatever’s available in a panic. If yours is throwing symptoms now, that’s your warning shot. Sort it on your terms.
Get it diagnosed properly
If you’ve read the symptom that matches yours and it’s on the “call a plumber” side of the line, that’s your next step. We’re a local, family-run team covering the St George area and the Shire, from Kogarah and Hurstville through to Cronulla, and we send licensed plumbers, never apprentices working alone. Tell us what changed and how fast when you call, and we’ll give you an honest read on whether it’s a repair or a replacement.
Book through our hot water systems page, or if you’ve got no hot water and it can’t wait, our emergency plumber line is on call day and night. If the fault turns out to be on the gas side, we hold a NSW licence with gas fitting endorsement, so our licensed gas fitters can handle it in the same visit. You can also get in touch here for a fixed quote before any work starts.