What Size Gas Hot Water System Do I Need? 16L vs 20L vs 26L Explained (NZ)
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TLDR: Continuous flow gas water heaters are sized by how many litres of hot water they can put out each minute, and the number in the model name tells you exactly that. As a rule of thumb, a 16 L/min unit suits 1–2 people, a 20 L/min unit suits 2–3, and a 26 L/min unit suits 3–6 or more, or any house where two showers often run at once. Cold water in winter means any unit makes less hot water, so if you’re torn between two sizes, go up. Here’s how it all works.
How continuous flow gas hot water works
A continuous flow gas water heater (you’ll also hear “instantaneous” or “instant” gas) doesn’t have a tank. When you turn on a hot tap, water runs through the unit, a gas burner lights and heats it on the way through. Turn the tap off and the burner shuts down. With no tank, you can’t drain it like a cylinder. What limits it is how much water it can heat at once, and that’s what the size number is about.
How long does gas take to heat water? Only seconds once the unit fires up. Most of the wait at the tap is the cold water already sitting in the pipe between the heater and the tap, so the further away the tap, the longer you’ll wait.
For the bigger picture on gas, including running costs and storage systems, see our complete guide to gas water heating in NZ.
What 16, 20 and 26 actually mean
On the Rinnai INFINITY range, the model number is the unit’s hot water capacity in litres per minute. An A16 puts out up to 16 litres of hot water a minute, an A20 up to 20, and an A26 up to 26. Rinnai rates those numbers at a 25°C temperature rise, meaning the water comes out 25°C warmer than it went in.
That’s important, because the water coming into your place isn’t the same temperature all year. In winter it’s colder, so the heater has to work harder to warm it up and can heat fewer litres a minute. The table shows roughly how much shower-temperature water (about 42°C) each size can manage at different incoming water temperatures.
| Incoming water temperature | A16 | A20 | A26 |
|---|---|---|---|
| 18°C (warmer months) | ~17 L/min | ~21 L/min | ~27 L/min |
| 12°C (cooler months) | ~13 L/min | ~17 L/min | ~22 L/min |
| 8°C (coldest winter conditions) | ~12 L/min | ~15 L/min | ~19 L/min |
Note: these are rough figures, worked out from each unit’s rated flow at a 25°C rise and a 42°C shower.
Rheem Stellar Mains Pressure Gas Water Heater 160L Natural Gas External
$3,177.33 Incl GST
Rheem Mains Pressure Gas Indoor Storage Tank 160L Natural Gas Indoor
$2,527.58 Incl GST
Rinnai INFINITY A20 Gas Continuous Flow Water Heater
$1,853.72 Incl GST
Rinnai INFINITY A16 Gas Continuous Flow Water Heater
$1,660.53 Incl GST
Which size suits your home?
| Household | What it needs to handle | Our recommendation |
|---|---|---|
| 1–2 people | One shower at a time, with the odd tap running | Rinnai INFINITY A16 → |
| 2–3 people | A shower going while someone’s using the kitchen or laundry | Rinnai INFINITY A20 → |
| 3–6+ people | Two showers at once, plus other taps | Rinnai INFINITY A26 → |
If you’re stuck between two sizes, go for the bigger one. A unit that’s a bit oversized just fires harder when it needs to. One that’s too small loses temperature every time someone else turns on a tap, and that’s the number one complaint we hear about continuous flow.
Other things to think about
- Natural gas or LPG. All three Rinnai INFINITY units we stock come in natural gas and LPG versions, and you pick which when you order. If you’re running off LPG bottles, have a read of rising gas costs and your options before you commit.
- Your gas supply. Bigger units use more gas when they’re flat out. At full flow the A26 uses up to 199 MJ/h, against 124 MJ/h for the A16, so your gas pipe, meter or regulator has to be able to keep up. Your gasfitter checks this as part of the install.
- Where it goes. These units go on an outside wall, so there’s no flue running through the house. The closer it is to your main bathroom, the less time you’ll spend waiting for the water to warm up.
- Temperature control. A wall controller lets you set the temperature at the tap, which is handy if you’ve got young kids.
- Power. These units need electricity to run, so there’s no hot water in a power cut. The built-in frost protection also needs power. It kicks in when the inside of the unit drops below 3.5°C, so leave it plugged in over winter.
When a gas storage system makes more sense
Continuous flow isn’t right for every house. If lots of taps regularly run at once, or your gas supply can’t keep up with a big continuous flow unit, a gas storage water heater may suit you better. It keeps a tank of hot water ready to go, so it copes well with busy spells. The Rheem Stellar 160L → is designed for households of around 3–4 people with more than one bathroom.
See the full range on our continuous flow gas hot water page, or check current prices on our price list.
Not sure which size is right for you? We supply and install gas hot water right around New Zealand, on a fixed price. Give us a call on 0800 234 800 or get an online quote.
Which size suits your home?
| Household | What it needs to handle | Our recommendation |
|---|---|---|
| 1–2 people | One shower at a time, with the odd tap running | Rinnai INFINITY A16 → |
| 2–3 people | A shower going while someone’s using the kitchen or laundry | Rinnai INFINITY A20 → |
| 3–6+ people | Two showers at once, plus other taps | Rinnai INFINITY A26 → |
If you’re stuck between two sizes, go for the bigger one. A unit that’s a bit oversized just fires harder when it needs to. One that’s too small loses temperature every time someone else turns on a tap, and that’s the number one complaint we hear about continuous flow.
Other things to think about
- Natural gas or LPG. All three Rinnai INFINITY units we stock come in natural gas and LPG versions, and you pick which when you order. If you’re running off LPG bottles, have a read of rising gas costs and your options before you commit.
- Your gas supply. Bigger units use more gas when they’re flat out. At full flow the A26 uses up to 199 MJ/h, against 124 MJ/h for the A16, so your gas pipe, meter or regulator has to be able to keep up. Your gasfitter checks this as part of the install.
- Where it goes. These units go on an outside wall, so there’s no flue running through the house. The closer it is to your main bathroom, the less time you’ll spend waiting for the water to warm up.
- Temperature control. A wall controller lets you set the temperature at the tap, which is handy if you’ve got young kids.
- Power. These units need electricity to run, so there’s no hot water in a power cut. The built-in frost protection also needs power. It kicks in when the inside of the unit drops below 3.5°C, so leave it plugged in over winter.
When a gas storage system makes more sense
Continuous flow isn’t right for every house. If lots of taps regularly run at once, or your gas supply can’t keep up with a big continuous flow unit, a gas storage water heater may suit you better. It keeps a tank of hot water ready to go, so it copes well with busy spells. The Rheem Stellar 160L → is designed for households of around 3–4 people with more than one bathroom.
See the full range on our continuous flow gas hot water page, or check current prices on our price list.
Not sure which size is right for you? We supply and install gas hot water right around New Zealand, on a fixed price. Give us a call on 0800 234 800 or get an online quote.
FAQ
As a rule of thumb, an A16 (16 L/min) suits 1–2 people, an A20 (20 L/min) suits 2–3, and an A26 (26 L/min) suits 3–6 or more. Go up a size if your showers are high-flow or you live somewhere cold.