The hot water thermostat: what it controls and how it fails
Stored temperature and delivered temperature are two different things, set by two different parts. Almost every piece of bad advice about hot water starts by confusing them.
A hot water thermostat is the control that decides how hot the water in the cylinder gets and when the system stops heating. When it drifts or fails, the water is either never hot enough, too hot, or gone altogether — and the fix is never to turn it up, because on almost every Australian installation the stored temperature is not the temperature that comes out of the tap.
What the thermostat is actually controlling
There are two different temperatures in a hot water system, and confusing them is behind most of the bad advice on this subject.
Stored temperature is how hot the water in the cylinder is held. It has to stay high enough that bacteria cannot establish in the tank, which is why storage systems run hotter than anyone would want at a shower rose. The thermostat is what holds that.
Delivered temperature is what reaches the outlet. It has to be low enough not to scald, particularly at bathroom outlets. That is not the thermostat's job at all — it is done further along by a tempering valve, which blends the stored water down on its way to the bathroom.
Once that split is clear, the single most common piece of DIY advice on hot water — "just turn the thermostat up" — is visibly the wrong move. Raising stored temperature to compensate for a bathroom that runs lukewarm does not restore the tempering valve that has drifted; it raises the temperature of the water sitting behind a scald-protection device that is no longer protecting properly.
What a failing thermostat looks like from outside the cabinet
Every one of these is observable without opening anything, which matters because opening it is not an owner task.
| What you see | What it usually points at |
|---|---|
| Hot water that ran out sooner and sooner over weeks, then stopped | An element or thermostat that has failed progressively on an electric system |
| No hot water at all, suddenly, with no warning | Power, a tripped circuit, or the energy cut-out having operated — not usually thermostat drift |
| Warm but never properly hot, at every tap including the kitchen | A thermostat set low or drifting, or a failed lower element |
| Lukewarm in the bathroom while the kitchen scalds | Not the thermostat. This is the tempering valve, which is why turning the thermostat up is the wrong response |
| Water hotter than it used to be, or the relief valve discharging very hot water | A thermostat or control that has failed high. The system is overheating and this one is urgent |
| An error code on the display of a heat pump or continuous flow unit | A controller and sensor fault, not a mechanical thermostat. The code is the diagnosis — write it down |
If the symptom is no hot water at all rather than the wrong temperature, work through the checks in order first — the thermostat is well down that list, and several causes above it are things you can establish yourself.
The thermostat is a different part on every system type
- Electric storage. A thermostat and an energy cut-out sit against the cylinder behind a cover plate, inside the electrical enclosure, alongside the element. Everything behind that plate is live equipment.
- Gas storage. The temperature control is part of the gas control valve assembly — a single component that also controls the burner. It is not a separate part that can be considered on its own, and nothing on it is owner-serviceable.
- Heat pump and continuous flow. There is no mechanical thermostat in the old sense. A controller reads sensors and decides, and when it is unhappy it reports an error code rather than simply running cold.
- Solar thermal. There are usually two controls — the solar circuit and the electric or gas booster — which is why a solar system can produce perfect water for months and then fail only on cloudy weeks, when the booster it has quietly been depending on turns out to be the faulty part.
Why do thermostats fail?
They are wear parts in a hostile place. The common mechanisms are contacts that eventually stop switching reliably after years of cycling; calibration that drifts, so the setting on the dial and the temperature in the tank no longer agree; and scale building up around the probe pocket, insulating the sensor from the water so that it reads cooler than the tank really is and holds the system hotter than intended.
The energy cut-out is worth understanding separately, because it is often blamed for a thermostat fault. It is a safety device that shuts the system down when the water goes past a limit. If it has operated, the useful question is not how to reset it but what made it necessary — a cut-out that has tripped is reporting that something upstream allowed the water to overheat, and resetting it without finding that cause simply restages the fault.
What is the right thermostat setting?
It is not an owner setting, and there is no number worth publishing here. The stored temperature required and the maximum delivered temperature permitted at different outlets are both set by the plumbing rules that apply to the installation, and they are achieved with the thermostat and the tempering valve working together as a pair. A licensed plumber measures delivered temperature at the outlet rather than reading the dial, because after a few years the dial is the least reliable thing in the system.
What is genuinely yours to do
- Establish whether it is every tap or one tap. Compare the kitchen with the bathroom, because bathroom outlets are tempered and kitchen sinks generally are not. Both lukewarm points at the system; bathroom only points at the tempering valve.
- Establish whether it got worse gradually or stopped suddenly. Gradual decline and sudden failure have different causes and it is the single most useful thing you can report.
- Write down any error code exactly as displayed, before it clears.
- Check whether the switchboard circuit has tripped, and whether the system is on an off-peak or controlled-load tariff that only heats at certain hours.
- Note the age of the system and its last service. A thermostat failing on a system near the end of its expected life changes the conversation from repair to replacement.
Frequently asked questions
What does a hot water thermostat do?
It controls how hot the water in the cylinder is held and tells the system when to stop heating. It does not control the temperature that comes out of a bathroom tap: on a typical Australian installation that is done further along by a tempering valve, which blends the stored water down before it reaches the outlet.
Should I turn my hot water thermostat up if the shower is not hot enough?
No, and it is usually the wrong diagnosis as well as the wrong action. If the kitchen tap is properly hot while the bathroom is only warm, the system is heating correctly and the fault is in the tempering valve between the tank and the bathroom. Raising stored temperature does not restore that valve, and it is not an owner adjustment in any case.
How do I know if it is the thermostat or the element?
From outside the cabinet you often cannot tell them apart, and both sit inside the electrical enclosure where an owner should not be. What is worth reporting is the pattern: hot water that ran out sooner and sooner over weeks before stopping, water that is warm but never properly hot at every tap, or a sudden total loss with no warning. Those patterns point a plumber at different parts.
What does it mean if the energy cut-out has tripped?
That the water went past a safety limit and the system shut itself down. The useful question is not how to reset it but what allowed the overheating in the first place, because resetting a cut-out without finding the cause simply restages the fault. It is a safety device reporting a problem, not the problem itself.
What temperature should a hot water system be set to?
It is not an owner setting. The stored temperature required and the maximum delivered temperature permitted at different outlets are set by the plumbing rules that apply to the installation, and they are achieved by the thermostat and the tempering valve together. A licensed plumber measures the temperature delivered at the outlet rather than reading the dial, which drifts out of agreement with the tank over the years.
Can I replace a hot water thermostat myself?
No. On an electric system it sits inside the electrical enclosure with the element and the cut-out, against a pressurised cylinder of very hot water. On a gas system the temperature control is part of the gas control valve and is the work of a licensed gasfitter. The work is licensed in every Australian state and territory, and this site publishes no instructions for doing it.
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
- Australian Building Codes Board. National Construction Code — Volume Three (Plumbing Code of Australia)