How an electric cylinder works
An element heats the water and an insulated tank keeps it hot until you turn on a tap. A thermostat switches the element on and off to hold the water at its setting.
Both makers we checked sell them in low and mains pressure versions. Rheem's electric range includes a low pressure copper cylinder made for a wetback, and Rinnai also sells coiled mains pressure cylinders that can take heat from solar or another closed-loop source. It's one of several cylinder types on our hot water cylinders page.
Choosing an electric cylinder
| What to think about | |
|---|---|
| Pressure | Low pressure, mains pressure, or a model that can do either. Your pipes decide. See mains pressure cylinders. |
| Size | Enough for the household at its busiest time of day. See cylinder sizes. |
| Place | A cupboard indoors, or a model made for outside. See outdoor cylinders. |
| Extra heat sources | A wetback or solar needs a cylinder designed to take it. |
| A heat pump instead | Still electric, but it moves warmth from the air into the water. See heat pump hot water. |
Insulation and heat loss
A cylinder loses some heat through its walls and pipes all day, whether or not anyone uses hot water. A New Zealand Standard, NZS 4305, sets minimum energy efficiency requirements for heat loss from home hot water installations, and the current Acceptable Solution for clause G12 cites it.
Beyond that standard, compare makers' figures with care. We don't repeat savings percentages: the ones in advertising are brand claims. Practical things that help are a sensible location, fixing leaks promptly, and a thermostat setting chosen by an authorised person. Rheem recommends the lowest setting that meets your needs, set by someone authorised, never by you.
Signs your electric cylinder needs a tradie
- No hot water while the circuit shows as on.
- Water around the base or from a valve.
- Hot water that runs short sooner than it used to.
- Rust-coloured water from the hot taps.
- A breaker that trips when the cylinder heats. See thermostats and elements.
What the plumber and electrician will do
A plumber will size and place the cylinder, fit its valves, safe tray and restraints as the maker requires, and connect the water. An electrician connects it to its circuit.
On a replacement, they should confirm the job fits the Building Act's water heater exemption (Schedule 1 clause 38) before they start, because that exemption only applies to work by an authorised person and in the conditions it sets. Either way, the finished cylinder must meet the Building Code.
What's yours to do
You can
- Check the hot water switch and circuit breaker if the water goes cold.
- Keep the space around the cylinder clear so it can be reached.
- Note the details on the label.
- Ask about the thermostat setting, but leave the dial alone.
Leave to an authorised tradie
- Water connections, valves and restraints.
- Wiring and the electrical connection (an electrician).
- Elements and thermostats.
- Any temperature change.
Questions before you buy
- Low, mains or all-pressure: which suits our pipes?
- What size do you recommend for our household, and why?
- How well insulated is this model?
- Who will connect it to the power?
- What warranty comes with this model, and from what date does it run?
What we would do
Pick the pressure and size first, then the model. An electric cylinder that suits your pipes and your household will serve you better than a bigger or flashier one. Before you buy, get an electric cylinder specialist to look at the space, the pipework and the circuit.