No Hot Water? How Elements and Thermostats Fail, and How to Tell Which One Did

An electric storage hot water system is one of the simplest appliances in an Australian house. Water sits in an insulated steel cylinder, an immersion element heats it, and a thermostat clamped against the tank wall switches that element on and off to hold a set temperature. Behind the thermostat sits a second, dumber device: an over-temperature cut-out that kills power outright if the water gets dangerously hot. That is the whole heating story, which is good news, because it means the symptom you are seeing usually narrows the fault to one of about four things.

The bad news is that all of it lives behind a cover plate on a pressurised tank full of scalding water, wired to a circuit carrying serious current. Diagnosing from the outside is fair game. Opening it up is not a homeowner job in Australia, and the rest of this assumes the work goes to a licensed trade.

Rule out the boring explanations first

Two things routinely look like a failure and are not.

The first is the tariff. Many Australian electric tanks run on a controlled load or off-peak tariff, meaning the element is only energised overnight. Those tanks are deliberately oversized because they get one bite at heating per day. If your hot water runs out at 8pm and comes back by morning without anyone touching anything, nothing is broken. The fixes are a bigger tank, a continuous tariff, a boost switch or a different technology, not a repair.

The second is the switchboard. Find the breaker or fuse for the hot water circuit and check it has not tripped, and check the off-peak timer or contactor if the system has one. A tank cold for exactly as long as the power has been off is not a mystery.

What each symptom points at

No hot water at all, tank stone cold. Either no power is reaching the element, or the element is open circuit, or the over-temperature cut-out has tripped. A cut-out has a manual reset button and the temptation is to press it and walk away. Resist that. The cut-out is a consequence, not a cause. It tripped because something let the water run past its safe limit, and the usual reason is a thermostat stuck closed that has stopped switching the element off. Reset it without fixing the thermostat and you have re-armed the same event.

Lukewarm water only, never properly hot. Often a thermostat that has drifted out of calibration or lost contact with the tank wall, so it switches off far too early. It can also be a thermostat somebody turned down. Storage tanks are held hot enough to suppress bacterial growth, typically around 60C in the cylinder, with the water blended down at the outlets. Winding that down to save energy trades the tank's safety margin for a very small gain.

Hot water that runs out embarrassingly fast. The classic, and usually a dead element in a twin-element tank. Larger tanks often carry two elements at different heights, one low for the bulk of the cylinder and one high as a fast-recovery boost. If the lower element dies, only the top portion ever gets heated. The system appears to work, delivers hot water for a few minutes, then hands you cold. A tank running on its boost element alone behaves exactly like a tank far too small for the house, which is why people replace perfectly good systems over a fault worth a fraction of a new install. Before going down that road, the comparison of continuous flow against storage sizing is worth a read.

Water far too hot, or scalding at the tap. A thermostat stuck closed drives the tank up until the cut-out intervenes. Separately, if delivery temperature at a bathroom outlet is dangerous, the tempering valve that limits bathroom outlets to 50C may have failed or may never have been fitted. That is a scald risk and needs attention promptly.

Tripping the breaker or RCD. An element failed to earth, usually because the sheath has corroded through, will trip protection every time the circuit is energised. That is a genuine electrical fault and the system should stay isolated until an electrician has been through it.

Why elements die, and what replaces them

Elements fail from two directions. Scale is the big one. In hard water areas calcium plates onto the element sheath, insulates it, and forces it to run hotter than designed until it cooks itself. The other is corrosion of the sheath, accelerated once the tank's sacrificial anode has been consumed and nothing is left protecting the steel inside the cylinder.

That is why element material matters. Copper elements are cheap and transfer heat well but cope poorly with aggressive or hard water. Incoloy and similar stainless alloy sheaths tolerate scale and corrosive water far better. Low watt density elements spread the same power over a longer heating surface, so the sheath runs cooler and scales more slowly. If your tank eats elements every couple of years, the answer is usually a different element rather than a different plumber.

Elements are either screw-in or flange-mounted with a bolt pattern and gasket, and the two are not interchangeable. Either way the tank must be isolated electrically, the water isolated, and the cylinder drained before anything comes out. The job is electrical work and plumbing work at once, on a pressure vessel, which in Australia means a licensed electrician and licensed plumber, not a weekend.

Spares turn up regularly, and it is worth checking element listings and thermostats and controls before ordering new, particularly for older tanks where the retail part is discontinued. There is a broader parts and spares category for the odds and ends, and if you have not got a trade already, the plumber directory finds someone who works on tanks rather than someone who only fits them.

When to stop repairing

Elements and thermostats are consumables. Tanks are not. If a cylinder is beyond about a decade old, has never had its anode checked, and is now on its second element, you are spending money on the cheap half of an appliance whose expensive half is quietly rusting. A tank weeping from the shell is finished regardless of how good the element is, and the question becomes which system replaces it. The comparison of electric, gas, heat pump and solar beats defaulting to whatever is already on the wall, and if the house is staying all-electric it is worth reading what a heat pump costs to run before buying another resistive tank.