16L vs 20L vs 26L Instant Gas Hot Water: Which Size Should You Choose?

Instant gas hot water systems are commonly sold in sizes like 16L, 20L and 26L. These numbers do not mean the unit stores that much hot water. They refer to the system’s approximate flow capacity in litres per minute under test conditions.

Choosing the right size matters. A system that is too small may struggle with showers, especially when more than one outlet is being used. A system that is too large may still perform poorly if the gas pipework, water pressure or installation conditions are not suitable.

This guide explains the difference between 16L, 20L and 26L instant gas hot water systems, what each size is best suited to, and what to check before ordering.

Quick answer: 16L vs 20L vs 26L instant gas hot water

For most small homes, a 16L or 20L instant gas hot water system may be enough. For family homes, homes with two bathrooms, or higher shower demand, a 26L instant gas hot water system is usually the safer starting point.

You can compare current options in Instant Gas Hot Water Systems or browse the broader Gas Hot Water Systems range.

What does 16L, 20L or 26L mean?

The litre rating on an instant gas hot water system refers to flow rate, not tank size.

A 26L instant gas system does not store 26 litres of hot water. It heats water as it passes through the unit. The rating is a guide to how much water the unit can heat per minute under specific test conditions.

Real-world performance depends on:

• Incoming cold water temperature

• Required outlet temperature

• Temperature rise

• Water pressure

• Water flow rate

• Gas pipe sizing

• Natural gas or LPG supply

• Number of outlets being used

• Distance from the unit to the bathrooms

• Whether temperature controllers are used

This is why a 26L system may still perform badly if the gas supply or water flow is not suitable.

16L instant gas hot water systems

A 16L instant gas hot water system is usually the entry-level size for whole-home instant gas use.

Best suited to

A 16L instant gas system may suit:

• 1–2 person homes

• Small units or townhouses

• One-bathroom homes

• Lower hot water demand

• Homes with normal shower flow

• Like-for-like replacement where the old 16L system worked well

Advantages of 16L instant gas

The main advantages are:

• Usually compact

• Suitable for lower-demand homes

• Can be a practical replacement for an existing 16L unit

• May suit properties with only one bathroom

• Lower capacity than larger models, which may suit some smaller gas supplies

When 16L may be too small

A 16L unit may not be suitable if:

• More than two people live in the home

• Two showers may run at the same time

• The home has high-flow shower heads

• Long showers are common

• The property has multiple bathrooms

• The existing 16L system already struggled

• The household wants stronger hot water performance during peak use

For many family homes, 16L is too limited.

20L instant gas hot water systems

A 20L instant gas system sits between a small 16L unit and a larger 26L unit.

Best suited to

A 20L instant gas system may suit:

• 2–3 person homes

• Small families with moderate use

• One main bathroom plus occasional second outlet use

• Homes where 16L is too small but 26L may not be required

• Like-for-like replacement where the old 20L system worked well

Advantages of 20L instant gas

A 20L system gives more capacity than a 16L unit without jumping to the largest common residential size.

It may be a good option where the home has moderate demand, normal shower habits and suitable gas supply.

When 20L may be too small

A 20L unit may not be enough if:

• Four or more people live in the home

• Two bathrooms are used regularly

• Multiple showers run close together

• Long showers are common

• A bath is filled often

• The household has high peak demand

• The old 20L system struggled

For many larger family homes, a 26L system is often the better option, provided the site is suitable.

26L instant gas hot water systems

A 26L instant gas hot water system is commonly chosen for larger homes and family use.

Best suited to

A 26L instant gas system may suit:

• Family homes

• 3–5 person households

• Homes with higher shower demand

• Two-bathroom homes

• Properties where two outlets may be used close together

• Homes wanting stronger instant gas capacity

• Replacements where the old smaller unit was not keeping up

Advantages of 26L instant gas

The main advantage is higher flow capacity. A 26L system can generally support more hot water demand than 16L or 20L systems, assuming the gas and water supply are suitable.

It is often the preferred size where the home has multiple people, more than one bathroom, or a higher chance of simultaneous hot water use.

When 26L may not solve the problem

A 26L system is not a magic fix.

It may still perform poorly if:

• Gas pipework is undersized

• LPG bottle or regulator setup is not suitable

• Water pressure is low

• Flow rate is poor

• Existing pipework is restricted

• The unit is installed too far from outlets

• Multiple high-flow showers run together

• The system is not installed or commissioned correctly

Before upgrading to 26L, the property needs to be suitable for the extra demand.

16L vs 20L vs 26L comparison table

Can a 26L instant gas system run two showers?

A 26L instant gas system may be suitable for two-bathroom homes, but it does not guarantee perfect performance with two showers running at the same time.

Two-shower performance depends on:

• Shower flow rates

• Incoming water temperature

• Set temperature

• Gas pipe sizing

• Water pressure

• Distance to bathrooms

• Whether other hot outlets are running

• Natural gas or LPG setup

If both showers are high-flow, or the gas supply is marginal, a 26L system may still struggle.

How household size affects instant gas sizing

Number of people is only a guide. A two-person household with long showers may use more hot water than a four-person household with short showers.

How bathroom count affects instant gas sizing

Bathroom count is important because instant gas systems are affected by simultaneous demand. A home with two showers running at once needs more flow than a home where showers are used one after another.

Natural gas vs LPG instant gas hot water

Natural gas and LPG systems must be selected correctly.

A natural gas instant hot water system is not the same as an LPG instant hot water system. Do not assume they are interchangeable. The appliance must match the property’s gas type, and any gas work must be completed by a licensed gas fitter.

Before ordering, confirm:

• The property’s gas type

• The appliance gas type

• Whether the gas pipework is suitable

• Whether the LPG regulator and bottle setup can supply the unit

• Whether other gas appliances are connected

• Whether the system location is compliant

This is especially important when choosing a 26L unit, because larger instant gas systems usually place higher demand on the gas supply.

Gas pipe sizing is critical

Gas pipe sizing is one of the most important checks when choosing instant gas.

If the gas pipework is too small, the system may not receive enough gas under load. This can cause poor performance even if the unit itself is the correct size.

Possible signs of gas supply issues include:

• Hot water temperature fluctuating

• Poor hot water flow

• Burner issues

• System fault codes

• Other gas appliances affected when hot water is running

• System cutting in and out

• Hot water not reaching expected temperature

A licensed gas fitter should check gas pipe sizing before installation, especially if replacing a smaller unit with a larger one.

Water pressure and flow matter too

Instant gas systems need enough water flow to activate and operate properly.

If water pressure or flow is poor, the system may not perform as expected. This can be caused by:

• Low incoming pressure

• Restricted pipework

• Blocked strainers or filters

• Faulty mixer taps

• High pressure losses through old pipework

• Tempering valve issues

• Poor shower head selection

• Long pipe runs to bathrooms

If only one outlet has a problem, the issue may be with that tap, mixer, shower head or pipework rather than the hot water system.

Temperature rise affects performance

Instant gas sizing is affected by temperature rise.

Temperature rise is the difference between the incoming cold water temperature and the hot water temperature the system needs to deliver.

For example, if the incoming water is cold and the system needs to heat it to a comfortable shower temperature, the unit has to work harder. In colder regions or colder seasons, the same unit may deliver less usable hot water flow than it does in warmer conditions.

This is one reason instant gas ratings should be treated as a guide, not a guaranteed outlet flow in every home.

Should you replace like-for-like?

Like-for-like replacement may make sense if the old system performed well.

For example:

• If a 16L unit worked well in a small one-bathroom home, another 16L may be suitable.

• If a 20L unit kept up with a couple or small family, another 20L may be suitable.

• If a 26L unit handled family demand well, another 26L may be suitable.

But do not replace like-for-like without thinking if:

• The old unit struggled

• More people now live in the home

• A second bathroom has been added

• Shower habits have changed

• The old system had temperature fluctuations

• You are changing from storage to instant gas

• The gas supply has not been checked

• The old unit was a different gas type

If the old system was undersized, copying the size may repeat the same issue.

Instant gas vs gas storage

Instant gas and gas storage are sized differently.

Gas storage may be better where a simple replacement is preferred and the existing storage system worked well. Instant gas may be better where the home wants continuous hot water and the site can support it.

You can compare broader gas options in Gas Hot Water Systems.

Brand options for instant gas hot water

Common instant gas brands include Rinnai Hot Water Systems and Bosch Hot Water Systems, along with other gas hot water options depending on availability.

When comparing brands, check:

• Gas type

• Flow rating

• Warranty terms

• Controller compatibility

• Installation requirements

• Model dimensions

• Required clearances

• Natural gas or LPG version

• Whether the system suits the existing site

Do not choose only by brand or litre rating. The correct size and site suitability matter just as much.

Installation and compliance reminders

Instant gas hot water installation is not DIY work.

Use a licensed plumber and gas fitter for installation. Any electrical work, where required, must be completed by a licensed electrician.

Important installation checks include:

• Correct natural gas or LPG model

• Gas pipe sizing

• Water pressure and flow

• Compliant installation location

• Clearances from openings, boundaries and services

• Flueing or ventilation requirements where applicable

• Tempering valve requirements where applicable

• Isolation valves

• Controller wiring if required

• LPG bottle and regulator setup

• Safe access for installation and servicing

Extra site work may be required depending on the property. Relevant options may be available under Installation Add-Ons.

Supply-only vs supplied-and-installed

Installed Today offers instant gas hot water systems online with supply-only options across major metro areas.

For supply-only orders, you need to arrange your own suitable licensed installer. This is especially important for instant gas systems because gas fitting and plumbing must be completed by licensed trades.

Supplied-and-installed options are only available where shown in eligible Brisbane and Gold Coast service areas. Availability depends on the product, location and installation conditions shown at the time of ordering.

Before ordering, confirm the correct gas type, system size and installation requirements.

Common mistakes when choosing instant gas size

Choosing 16L because it is cheaper

A 16L unit may be too small for a family home. Choosing only by upfront price can lead to poor performance.

Assuming 26L works for every property

A 26L unit still needs suitable gas pipework, water pressure and installation conditions.

Ignoring natural gas vs LPG

Natural gas and LPG models are not interchangeable. Always choose the correct gas type.

Not checking gas pipe sizing

Upgrading from 16L or 20L to 26L may require gas supply checks.

Expecting two perfect showers from any 26L system

Two-shower performance depends on flow rates, gas supply, water pressure and temperature rise.

Replacing like-for-like when the old system struggled

If the old unit did not keep up, the new unit should be selected around current demand, not just the old model number.

Forgetting installation extras

Controllers, valves, pipework, electrical work, gas upgrades or access requirements may affect the final installation.

Final verdict

A 16L instant gas hot water system can suit small, low-demand homes with one bathroom.

A 20L instant gas hot water system is a better fit for small-to-medium homes with moderate hot water use.

A 26L instant gas hot water system is usually the safer choice for family homes, higher demand, and two-bathroom properties, provided the gas supply, water pressure and installation conditions are suitable.

The best instant gas size depends on household demand, bathroom count, natural gas or LPG supply, gas pipe sizing, water flow, temperature rise and compliant installation.

FAQs

What does 16L mean on an instant gas hot water system?

It refers to the system’s approximate litres-per-minute flow capacity under test conditions. It does not mean the unit stores 16 litres of hot water.

Is a 16L instant gas system enough for a family?

Usually not for larger families. A 16L system is generally better suited to smaller homes with one bathroom and lower hot water demand.

Is a 20L instant gas system enough for a family of four?

A 20L system may suit some moderate-use households, but many families of four will be better suited to a 26L unit, subject to gas and water supply.

Is a 26L instant gas system enough for two bathrooms?

A 26L unit is commonly considered for two-bathroom homes, but performance depends on gas pipe sizing, water pressure, shower flow rates and whether outlets are used at the same time.

Can I upgrade from 16L to 26L instant gas?

Often yes, but the gas pipework, water pressure, gas type, installation location and compliance requirements need to be checked first.

Do I need natural gas or LPG?

You need the unit that matches your property’s gas supply. Natural gas and LPG appliances are not interchangeable unless specifically converted in accordance with manufacturer requirements by a licensed gas fitter.

Why does my instant gas temperature fluctuate?

Possible causes include poor water flow, gas supply issues, undersized pipework, mixer tap problems, blocked filters, incorrect setup or the system being undersized for demand.

Does a bigger instant gas system use more gas?

A larger unit can use more gas when operating at higher output, but actual usage depends on hot water use, flow rate, temperature setting and household habits.

Do I need a licensed installer?

Yes. Instant gas hot water systems require licensed plumbing and gas fitting. Any required electrical work must be completed by a licensed electrician.

Where can I compare instant gas systems?

You can compare available options in Instant Gas Hot Water Systems or view broader Gas Hot Water Systems.