What each part does
The element is an electric heater that sits inside the cylinder, in the water. The thermostat senses the water temperature, switches the element on until the water reaches its setting, then switches it off again.
Elements turn up in more than plain electric cylinders. Rheem says its heat pump water heaters carry an electric back-up element for very cold weather, and its solar water heaters come with a gas or electric boost. A wetback does the opposite: while the fire is going, it heats the water so the element doesn't have to run.
Signs of an element or thermostat fault
- No hot water, even though the circuit is on and nothing is leaking.
- Water that gets warm but never properly hot.
- Hot water that runs out far sooner than it used to.
- The hot water breaker trips when the cylinder starts heating.
- Water that is suddenly much hotter than normal.
Why the setting isn't a DIY dial
The thermostat setting is doing two jobs. Clause G12 of the Building Code says the installation must be capable of being controlled to prevent Legionella bacteria growing. It also says hot water at basins, baths and showers must avoid the likelihood of scalding. The second job is what a tempering valve is for, so the stored water doesn't need turning down.
Rheem's owner's guide says an authorised person must make any temperature adjustment on its mains pressure electric cylinders, and recommends the lowest setting that meets your needs. Don't turn the dial down yourself to save power: ask the tradie.
What a tradie will do
Confirm the supply
They check the circuit and the hot water switch first, so you don't pay for a part you didn't need.
Test both parts
Testing shows whether the element, the thermostat or both have failed. If both are old, they may suggest doing them together.
Look over the cylinder
Rust, weeping or a valve fault changes the advice, sometimes toward a replacement.
Replace and set
The failed part is swapped, often with the cylinder drained first, and the thermostat set by an authorised person.
Prove it works
They confirm the water heats and the breaker holds before they leave.
Who should do the work
An element or thermostat sits on a fixed-wired circuit inside a cylinder full of water. Rheem says repairs that need covers or casing removed with tools should only be done by a suitably qualified plumber, gasfitter or electrician.
New Zealand's electricity law does leave an owner-occupier some narrow room for work on their own home, with conditions and testing rules attached. A live cylinder is not where we'd use it. Have an electrician do the electrical side, and an authorised plumber anything that disturbs the water connections.
Repairing a water heater with comparable parts falls under the clause 36 consent exemption when an authorised person does it. More on cylinders in general on our hot water cylinders page.
What you can check first
You can
- Look at the hot water switch and the circuit breaker at the switchboard.
- Reset a tripped breaker once. If it trips again, leave it off and book a tradie.
- Note whether the water gets warm at all, and how long it takes.
- Read the model off the cylinder label.
Leave to an authorised tradie
- Taking off the element cover or casing.
- Testing and replacing elements and thermostats.
- Any change to the temperature setting.
- Draining and refilling the cylinder.
Worth asking
- Is it the element, the thermostat, or both?
- Is the tank itself still sound?
- Who will do the electrical work, and what licence do they hold?
- What setting will you leave the thermostat on, and why?
- Are the replacement parts the maker's own, and does that matter for the warranty?
What we would do
Check the switchboard before anything else. If the breaker is on and the water is still cold, book a test of the element and thermostat rather than shopping for a new cylinder. An element and thermostat specialist who carries common parts may sort it in one visit, and will tell you if the tank is the real problem.