Appliance Spares
Quick answer: When an electric storage hot water system — Rheem, Vulcan, Dux — goes lukewarm, swings scalding-then-cold, or starts tripping the switchboard, the usual culprit is the 60–80°C control thermostat, an ST12-80-style part typically $25–40, or the heating element behind it. The tank is hard-wired 240V and plumbed at mains pressure, so in every Australian state and territory the repair itself is a job for a licensed tradie. Your job: match the model number on the model sticker, buy the right part, and hand it over.
Last updated August 2026
Half a hot shower, then a cold negotiation
It is the last week of August, the mornings are still single digits, and the shower that used to run hot for twenty minutes now manages five lukewarm ones. The electric storage tank is the most ignored appliance in the house — it hums away in a cupboard or against the back wall for a decade, then picks the coldest week it can find to sulk.
The classic signs the tank itself is struggling:
- Water is lukewarm at every tap, every hour of the day.
- The temperature swings — scalding one minute, cool the next.
- Hot water runs out far faster than it used to.
- The safety switch or breaker trips whenever the tank tries to heat.
- The tank is silent at times of day it used to quietly tick and creak.
Most of those trails end at two cheap parts under the tank’s covers: the control thermostat and the element it switches. This article gets you to the exact part — then hands the spanner to a licensed tradie, because a storage tank is 240V and mains plumbing in one steel cylinder. Know the part, then call the right person.
Match the symptom to the part
Start here. Each part link opens the live listing with photos and the current price.
| Symptom | What it usually means | The part to know |
|---|---|---|
 Lukewarm at every tap, all day | The control thermostat has drifted low with age, or one element is down | Electric hot water thermostat ST 60–80°C — Rheem, Dux |
 Scalding, then cold, then scalding | The thermostat is sticking instead of cycling cleanly | Genuine electric hot water heater thermostat |
 Safety switch or breaker trips when the tank heats | An element leaking to earth, or a failed thermostat or cut-out — the tradie tests both on site | Vulcan, Rheem & Dux thermostat ST12-80 (60–80°C) |
| Lukewarm mornings only, on an off-peak tariff | Tariff timing, not a fault — the tank only heats overnight | No part. Read the free checks below first. |
| Pipe off the tank is hot, but the taps run lukewarm | The tempering valve is past its service life | A plumbing part — note it down for the plumber. |
Different badge on the cylinder, or nothing above lines up? Browse the full hot water parts range, or run the part finder linked at the top of this page.
Three free checks before anyone gets paid
Two of the most common “lukewarm” complaints are not a broken part at all. Rule these out first — none of them involves opening anything electrical.
1. The off-peak tariff
If your tank sits on a controlled-load or off-peak circuit, it only heats overnight. A house full of guests and two long evening showers can empty it before bed, and the meter will not let it recover until the small hours. Lukewarm mornings after heavy evenings on off-peak is the system working as designed — check your bill for a controlled-load line before blaming the hardware.
2. The tempering valve’s age
Australian bathrooms are fed through a tempering valve that blends hot water down to a safe temperature at the tap. The valves last roughly five to eight years, and they fail towards lukewarm. The tell: the copper pipe leaving the tank gets properly hot, but the shower never does. That is a plumber’s valve — a new thermostat will not fix it — and its tag usually carries the install date.
3. The reset button — with the power off
Most tanks carry an energy cut-out with a reset button. If yours sits behind a simple user-accessible cover, switch the hot water circuit off at the switchboard first, press it, close up, restore power. Comes good once? Note it and move on. Trips again? Stop resetting — a cut-out that keeps firing is protecting you from a real fault, usually the stat or element. And if reaching it means unscrewing a panel that exposes wiring, leave the panel alone. That is the tradie’s territory.
What the control thermostat actually does
The thermostat is a surface-mounted switch clamped against the cylinder wall under the insulation, beside the element it controls. It senses tank temperature through the steel and cycles the element to hold the dial setting — an adjustable 60–80°C band on Australian tanks. The ST12-80-style stat is the standard fitment across Rheem, Vulcan and Dux cylinders, which is why one part number covers so many tanks.
Age gets them two ways. The switch contacts drift, so the stat cuts out early and the tank runs cooler and cooler — that is your slow slide into lukewarm. Or the contacts stick, so the tank overshoots, the cut-out slams the power off, and you get the scalding-then-cold lurch. Larger twin-element tanks carry two stats — off-peak low, boost up top — and losing either shows up as lukewarm water or a tank that empties fast.
Because the stat and the element live behind the same cover, a good repairer will test both while the tank is open. The stat is the cheap first suspect; ask for an element insulation test in the same visit.
Know the part: matching your tank
Find the model sticker on the side of the cylinder — it lists the model number, capacity in litres and element rating. Photograph it; tanks live in dark cupboards and stickers fade.
Then match the stat. The ST12-80 covers Vulcan, Rheem and Dux cylinders, the ST 60–80°C control stat is listed for Rheem and Dux, and the genuine electric hot water thermostat covers tanks that specify the genuine part. All three sit in the $25–40 band — against a tank replacement, that is a rounding error. Not sure which one your cylinder takes? Send us the model number — a photo of the sticker is perfect — and we will check it against the listings before you order.
The safety part, said plainly
An electric storage tank is the one appliance in the house that is both hard-wired to a 240V circuit and holding a couple of hundred litres of scalding water at mains pressure. There is no plug to pull, and no version of this repair that is legally yours: in every Australian state and territory, electrical work on a hard-wired appliance must be done by a licensed person, and everything on the water side — valves, tempering, the cylinder itself — is licensed plumbing work. Many hot water repairers hold both tickets, which makes them the right first call. It is the same framing we use for hard-wired oven elements: know the right part, then decide who fits it — except here, “who” is always licensed.
The maths still lands on your side. A call-out and diagnosis typically runs $100–165 before parts. Turn up with the symptom described, the sticker photographed and the correct $25–40 thermostat in hand, and two visits collapse into one with no parts mark-up. While the tradie is booked, run the end-of-winter half-hour check over the rest of the laundry.
Frequently asked questions
Why is my electric hot water lukewarm but not cold?
Genuinely cold usually means no power to the tank. Lukewarm means something is half-working: a control thermostat drifted low, one failed element in a twin-element tank, a tired tempering valve, or an off-peak tank that ran out overnight. Run the free checks — tariff, tempering valve, reset — then have a licensed tradie test the stat and element.
Can I replace a hot water thermostat myself in Australia?
No. A storage tank is hard-wired 240V, and in every Australian state and territory electrical work on a hard-wired appliance must be done by a licensed person; the water side is licensed plumbing work. What you can legally do is worth real money anyway: confirm the symptom, match the ST12-80-style stat to your model number, and supply the part so the job is one visit instead of two.
Which thermostat fits Rheem, Vulcan and Dux electric hot water systems?
The surface-mounted 60–80°C control stat in the ST12-80 style is the standard fitment across Rheem, Vulcan and Dux electric storage tanks, with an ST 60–80 version for Rheem and Dux and a genuine stat for tanks that specify it. The model sticker on the side of the cylinder decides — send the model number through and we will confirm before you order.
Why does my hot water system keep tripping the safety switch?
Usually a heating element leaking current to earth through broken-down insulation, or a failed thermostat or cut-out holding the element on. Leave the circuit off at the switchboard and book a licensed tradie — the safety switch is doing its job, and repeatedly resetting it is the one move that makes this dangerous.
How much does it cost to fix lukewarm electric hot water?
The control thermostat itself sits typically $25–40, and a repairer’s call-out and diagnosis typically runs $100–165 before parts. Supplying the correct stat yourself keeps the total in the low hundreds — a fraction of replacing a cylinder with years left in it.
Ready to sort it? Photograph the model sticker on the side of the cylinder and search the model number — or the part number off the old stat if a tradie has already had the cover off. Not sure? Send us the model number and we will match the part before you order. Get it to us by 12pm Sydney time and your part is dispatched from Sydney’s Northern Beaches the day you order — so the tradie’s one visit fixes it.
Disclaimer: this article is general information only and is provided without warranty of any kind. Appliance designs vary between models and production runs — always follow the manufacturer’s instructions, switch off and unplug the appliance before starting any work, and confirm parts against the model number on your appliance’s model sticker. Any repair is undertaken at your own risk and Oz Appliance Spares accepts no liability for injury, loss or damage arising from work performed after reading this guide, to the maximum extent permitted by law. If in doubt, use a qualified technician.
