The 16, 20 and 26 on the side of a continuous-flow water heater are litres per minute — but only in summer. Here is how to size one so it still works in a Kapiti July.
If you have decided on gas continuous-flow hot water, the next question is which size. Units are sold as 16, 20, 24 or 26. Most people assume the number is simply a bigger-is-better scale, with price the only trade-off.
The number is litres per minute, at a temperature rise
A "26" delivers 26 litres per minute. But that figure is quoted at a specific temperature rise — how much the unit has to lift the incoming water to reach your set temperature. Usually that is quoted at a 25°C rise.
This is the part that catches people out. In February, mains water on the Kapiti Coast might arrive at 18°C. Getting it to 50°C is a 32°C lift. In July, that same mains water might arrive at 10°C, so you need a 40°C lift, and the unit's real-world output drops accordingly.
A unit sized on its summer figure will run short in winter. That is the most common sizing mistake.
Rough litres-per-minute demand
Actual flow at the fixture, not the theoretical maximum:
- Kitchen tap: 6 to 8 L/min
- Bathroom basin: 4 to 6 L/min
- Standard shower: 8 to 10 L/min
- Water-saving shower head: 6 to 8 L/min
- Bath filling: 12 to 18 L/min
Add up what genuinely runs at once. Two showers together is 16 to 20 L/min before anyone touches a kitchen tap.
Matching size to household
- 16 L/min — one bathroom, one or two people, no simultaneous use. Fine for a unit, a small flat, or a sleepout.
- 20 L/min — the workhorse for most New Zealand family homes. One main bathroom plus an ensuite, with occasional overlap.
- 24 to 26 L/min — two-plus bathrooms with genuine simultaneous demand, or a household that fills a bath while someone showers. Also the sensible pick if you are on a colder winter supply and want the shower to hold temperature in August.
The gap between one size and the next is a few hundred dollars at install, against fifteen years of living with the result. If you are between two sizes, take the larger one.
LPG changes the maths
Most of the coast runs on bottled LPG rather than reticulated natural gas. A continuous-flow unit only draws gas while hot water is running, which suits bottles well — but a larger unit at full noise burns through a bottle faster. If you are on twin 45kg bottles with automatic changeover, that is generally a non-issue. If you are on a single bottle, size and usage matter more.
Reticulated natural gas, which runs through much of the Hutt Valley and Upper Hutt, removes the bottle question entirely.
The things that get missed at quoting
- Flue clearances. Units need specified clearances from windows, eaves and boundaries. This often decides where the unit can go, which in turn decides pipe runs.
- Distance to the bathroom. A unit mounted far from the fixtures means a longer wait for hot water and more water down the drain each time.
- Gas line capacity. An existing line sized for a hob may not carry a 26. Upsizing the run is part of the job, not an extra surprise.
- Water pressure. Continuous-flow needs a minimum flow to fire. On a property with low incoming pressure, that gets checked first.
Ballpark cost
A continuous-flow install typically lands around $3,500 to $6,000 + GST, depending on gas connection, flue runs and how far the unit sits from the fixtures. A straightforward like-for-like swap sits at the lower end and is usually a one-day job.
Tony installs Rinnai, Rheem and Bosch continuous-flow units across Paraparaumu, Waikanae, Otaki and the Hutt Valley, and issues the compliance certificate on completion. Sizing is part of the free on-site assessment — get a quote or call 021 198 3113.