Why a chart would mislead you
Two households with the same headcount can need quite different cylinders. Four showers back to back before school ask far more of a cylinder than the same four spread across the day.
The word cylinder also covers a huge range. NZS 4606.1, the New Zealand standard for most storage water heaters, covers units from 6.5 L to 630 L. A gas continuous flow heater, often called a califont, has no tank at all, so its sizing question is about flow, not litres.
What to count and measure
- Your busiest hour. How many showers run one after another, and does a bath get filled in the same window?
- Taps running together. A shower plus the kitchen tap, or two showers at once, is a bigger ask than one outlet at a time.
- The rating label on the cylinder you have. Take a photo. Rheem uses that label for the date of manufacture and serial number, and it helps a plumber identify exactly what you've got.
- Whether it keeps up now. If it does, that's a useful starting point. If it doesn't, note when it runs out and who was using water.
- The space. Measure the cupboard or outdoor spot, including the height, and how a new cylinder would get in.
- Pressure and heat sources. Low pressure or mains pressure? A wetback, solar or gas? Each one changes the options.
Running out sooner than it used to?
If nobody new has moved in and the hot water now runs out early, suspect a fault before you pay for a bigger cylinder. Rheem points to a faulty tempering valve as one reason water isn't hot enough, and it warns that a small leak wastes more hot water than you'd think. Our guide to a cylinder not heating covers what to check.
What changes the answer for each type
Electric cylinder. It stores heated water, so the size is about how many litres your busiest hour uses and how quickly the cylinder heats back up. Ask the plumber about both.
Heat pump. Rheem describes its heat pump water heaters as having a storage tank much like an electric cylinder, with an electric back-up element for very cold weather. They need outdoor space too, which can mean moving the cylinder, so measure outside as well.
Gas continuous flow. With no tank, ask whether one unit can keep up with your two busiest outlets at once. Rheem says continuous flow needs a larger gas pipe from the meter than most storage heaters, which a gasfitter checks.
Gas storage. Like an electric cylinder, it heats water and keeps it ready, so tank size matters.
Wetback or solar. These feed heat into a cylinder, and Rheem's solar systems carry a gas or electric boost for cloudy days. The cylinder has to suit the heat source as well as your household, so mention the fire or the panels at the start.
Rules that apply whatever the size
Size doesn't change the Building Code. Under clause G12, any hot water system must be able to be controlled to stop legionella bacteria growing, and water for personal washing must arrive at a temperature that avoids the likelihood of scalding. A bigger or smaller cylinder is no reason to change the setting yourself; Rheem says temperature changes are for an authorised person.
Any replacement, the same size or not, is exempt from building consent only in the conditions Schedule 1 of the Building Act sets out, and only when an authorised person does it. See building consent for a hot water cylinder.
Questions to ask the plumber
- What did you base this size on: people, showers or the old cylinder?
- Will it cope with two showers at once, or a shower and the kitchen tap?
- How long will it take to heat up again once it's run out?
- Is there room to get it in, and to reach the valves later for servicing?
- If we're moving to mains pressure or a heat pump, does that change the size you'd suggest?
What we would do
Treat your current cylinder as the baseline. If it keeps up, the same capacity is a sensible place to start, and the bigger decision is the type. If it doesn't, write down when it runs out and who was using water, then ask the plumber to walk you through how they reached their number. A tradie who specialises in hot water cylinders should be happy to show the working before you commit.