The litres on a cylinder’s label are one number out of four. How many people live in the house is the start, but how they shower, how fast the cylinder reheats and what fits in the cupboard decide whether that number works in practice. Two households of four can sensibly need different sizes.
This guide takes the makers’ tables as the starting point, then shows how to adjust them for your house.
What size cylinder suits your household?
Start with the makers’ guides in the table below. Rinnai gives one size for each household size; Rheem rates each of its sizes for a range of households. Where yours lands within those ranges depends on how you use hot water: the showers, the busiest hour of the day and how the cylinder is heated.
| People | Rinnai’s guide | Rheem’s rating for that size (stainless, moderate climates) |
|---|---|---|
| Up to 3 | 135 litres | 135 litres: 1 to 3 people |
| 4 | 180 litres | 180 litres: 3 to 5 people |
| 5 | 215 litres | No size in that range |
| 6 | 250 or 270 litres | 250 litres: 3 to 6 people |
| 7 | 300 or 340 litres | 300 litres: 3 to 6 people |
The overlap is the point. Rheem rates its 180 litre cylinder for anything from three to five people: habits matter as much as headcount. BRANZ puts typical use at 40 to 60 litres of hot water per person a day, and says the cylinder must cover the household’s peak demand, not its daily average.
How much hot water does your household actually use?
Mostly what the showers use. Measure yours: Rinnai suggests holding a bucket under the shower for ten seconds and multiplying by six to get litres a minute, and puts a typical shower at 8 to 10. About half of that comes from the cylinder; the rest is cold water mixed in. Then count the showers taken in your busiest hour.
A worked example (our arithmetic). Take a 9 litre a minute shower at 42°C, with cold water at 15°C, Rinnai’s figure for an Auckland winter, and the cylinder at 65°C. Just over half the flow comes from the cylinder: about 5 litres a minute, or about 40 litres for an eight-minute shower.
The busiest hour. Four people showering back to back before school and work draw about 160 litres in that hour. A 180 litre cylinder covers it, roughly, because cold water starts mixing into what’s left near the end. A fifth shower, or a bath, tips it over.
What doesn’t count. Most dishwashers and washing machines heat their own water rather than drawing from the cylinder. A bath does count: a deep one takes as much hot water as several showers.
Does mains pressure change the size you need?
Yes. BRANZ puts a low pressure shower at about 7 litres a minute and a mains pressure shower at 12 to 20. By the same arithmetic, an eight-minute shower draws about 30 litres from the cylinder at 7 litres a minute, and about 65 at 15. A household that never ran out on low pressure can run out after upgrading, unless the cylinder grows or the shower heads limit flow.
The four back-to-back showers above become about 260 litres on unrestricted mains pressure: past a 180 or a 250 litre cylinder. Two fixes, often used together: a larger cylinder, and shower heads that limit flow. Watercare calls 9 litres a minute or less ideal, and a good flow-limited head keeps most of the feel of mains pressure.
If you’re changing from low to mains pressure at the same time, size the new cylinder for the new showers, not the old ones. Our guide to mains pressure vs low pressure cylinders covers what else the change involves.
How fast does a cylinder reheat, and why does it matter?
Slowly, compared with how fast you use it. By our arithmetic, a 3 kW element takes about three and a half hours to heat 180 litres from cold, and a 2 kW element about five; Vector puts a full reheat at around six hours. So the cylinder has to hold your busiest hour, not top it up during it.
Element size. Rheem’s cylinders come with 2 or 3 kW elements, and its larger sizes come with 3 kW. Rheem and Rinnai also make dual element models, with a second element partway up the tank.
Load control. Vector switches off electric water heating at peak times, typically for less than three hours, and on the North Shore it runs on the older pilot wire system. A cylinder sized with little to spare is the one that runs out during a control period. Our no hot water checklist explains how load control works.
Night rate. Rheem notes that on an off-peak tariff, power to the cylinder “may not be available at certain times of the day”. If your cylinder heats only overnight, it has to hold the whole day’s hot water, which usually means a size up.
Heat pumps. A heat pump water heater reheats more slowly still: Rinnai’s 265 litre unit recovers about 60 litres an hour on the heat pump alone. Its tank has to be sized for your peak use. Our guide to gas vs electric hot water compares running costs, and califont or cylinder covers sizing a califont, which has no tank at all.
Is a bigger cylinder always better?
No. BRANZ says a cylinder that’s too large heats more water than you need, and every cylinder loses heat through its walls around the clock; a bigger one has more wall. Size for your busiest hour with some margin for guests or a growing family, and no further. A larger cylinder also needs the space and floor to carry it.
BRANZ recommends cylinders that are short and broad rather than tall and slim, because less surface area means less heat lost. Cylinders installed since 2002 must have A grade insulation; an older one that’s warm to the touch is losing heat that a wrap would keep in.
Whatever the size, the water must be heated to at least 60°C every day to control Legionella, and a bigger cylinder takes longer to get there. Our guide to the 60°C rule and tempering valves explains why.
Will it fit?
Check the diameter, the height and the floor. BRANZ puts most cylinders at 450 to 590 millimetres across, so the height is usually what changes between sizes. A full 180 litre cylinder weighs about 244 kilograms and a 300 litre about 400, and it must stand on a level base strong enough for that, in a drained tray.
Rheem’s stainless range shows the pattern. Every size is 560 millimetres across so it fits most cylinder cupboards, while the height runs from about one metre for 135 litres to over two metres for 300. Going from 135 to 180 litres in the same cupboard often works; going to 250 or 300 often doesn’t.
Leave room around it, too. The cylinder needs a tray with a drain, seismic restraint, space for its valves above or beside it, and enough access to replace it one day.
If the cupboard can’t take the size you need, a cylinder built for outdoors frees the cupboard and removes the height limit. An integrated heat pump unit also goes outside.
What about a wetback or a distant kitchen?
Both change the answer. A wetback cylinder is usually larger: Rinnai says wetback installations tend to be 250 to 300 litres where space allows, so they can store the fire’s heat, and some have the electric element high in the tank. A kitchen far from the cylinder may suit a small under-sink cylinder of its own.
Wetbacks. A fire heats water whenever it’s burning, whether or not you need it, so a larger tank captures more of that heat instead of venting it. With the element in the upper part of the tank, only the top is heated electrically in summer, so you aren’t paying to heat the extra volume. BRANZ notes that taller cylinders suit wetbacks better.
Distant outlets. BRANZ advises putting the cylinder as close as possible to where most hot water is used, because every metre of pipe holds water that cools between uses. Rinnai makes under-sink cylinders for kitchens a long way from the bathrooms, and BRANZ suggests a continuous flow heater where one outlet is isolated.
How do you know what size you have now?
Read the rating label on the side of the cylinder. It lists the capacity in litres, along with the element’s power rating and the date of manufacture. If the cylinder seemed big enough for years and now runs out, check for a fault before buying a bigger one.
A failed element or thermostat can leave you with half a tank of hot water, which feels exactly like a cylinder that’s too small. Our no hot water checklist covers what to look for.
If the cylinder is working properly and the household has simply grown, or you’ve moved to mains pressure showers, then it’s a size question, and replacement time is when to answer it.
What we do when you ask us to size a new cylinder
We look at the cylinder you have, the cupboard and how the household uses hot water, then tell you whether the same size is right or a different one makes sense. A like-for-like swap is often the simplest answer; if a mains pressure upgrade or an outdoor cylinder suits the house better, you hear that too, with the price before any work starts.
Every cylinder we install is done to code: seismic restraint, a tempering valve, a tray and drain, and the right relief valves. Our plumbers are licensed, so the person doing the job is the person who signs it off.
Our hot water cylinders page covers repairs, replacements and new installations in more detail.