Instantaneous Gas Hot Water Systems: How to Choose the Right Continuous Flow Unit

Nov 27, 2024

An instantaneous gas hot water system heats water only when a hot tap is turned on. Instead of storing hot water in a tank, it uses a gas burner and heat exchanger to warm water as it flows through the unit.

For the right home, instant gas hot water can be compact, practical and well suited to daily household use. But it is not as simple as choosing the biggest model. Gas type, gas pipe sizing, water pressure, number of bathrooms, shower habits, installation location and licensed gas fitting requirements all matter.

Installed Today helps customers compare instant gas hot water systems, 26L instant gas hot water systems, broader gas hot water systems and other hot water systems online.

Supply-only ordering is available across major Australian metro areas, with selected supplied-and-installed options available on eligible products in Brisbane and Gold Coast service areas where shown.

Quick Answer

An instantaneous gas hot water system may suit your home if:

You have natural gas or LPG available
The selected model matches the property’s gas type
Gas pipe sizing is suitable
Water pressure is suitable
The unit can be installed in a compliant outdoor location
You want a compact wall-mounted hot water system
You are replacing an existing continuous flow gas unit
Household demand matches the system’s L/min rating

Instant gas systems can provide continuous hot water within the unit’s rated capacity, but they are not unlimited in every situation. Multiple showers, high-flow fixtures or heavy simultaneous use can still exceed the system’s output.

For 26L systems, gas pipe sizing and water pressure are especially important. Bigger is not automatically better.

What Is an Instantaneous Gas Hot Water System?

An instantaneous gas hot water system, also called a continuous flow gas hot water system, heats water on demand.

When a hot tap opens, water flows through the unit. The system detects flow, ignites the gas burner and heats the water as it passes through the heat exchanger.

Unlike a storage tank, it does not keep a full cylinder of hot water heated throughout the day.

Instant gas hot water systems are commonly used for:

Family homes
Townhouses
Granny flats
Replacement of old continuous flow units
Homes with limited space for a storage tank
Properties with natural gas
Properties using LPG bottles
Outdoor wall-mounted hot water installations

How Does Instant Gas Hot Water Work?

The process is simple:

A hot tap is turned on
The unit detects water flow
The gas burner ignites
Water passes through the heat exchanger
The water is heated as it flows through the unit
Hot water travels to the tap or shower
The burner turns off when water flow stops

This means the system heats water only when hot water is being used.

Performance depends on the unit size, gas supply, pipe sizing, water pressure, temperature rise and number of outlets being used at the same time.

Instantaneous Gas vs Gas Storage Hot Water

Instant gas and gas storage systems both use gas, but they work differently.

Instantaneous Gas Hot Water

Instant gas systems heat water on demand and do not store hot water in a tank.

They may suit:

Homes wanting a compact wall-mounted unit
Properties with suitable gas pipe sizing
Homes with suitable water pressure
Like-for-like replacement of an existing continuous flow unit
Households wanting hot water heated as needed

Gas Storage Hot Water

Gas storage systems heat and store water in a tank.

They may suit:

Like-for-like gas storage replacements
Homes wanting stored hot water
Properties where a storage tank location already exists
Homes where continuous flow pipe sizing or pressure is not suitable
Customers replacing an existing gas storage system

Customers comparing storage options can browse 130/135L gas hot water systems and 160/170L gas hot water systems.

Benefits of Instantaneous Gas Hot Water

An instantaneous gas hot water system may offer several practical benefits when it suits the property.

Compact Wall-Mounted Design

Instant gas systems are usually smaller than storage tanks and are commonly mounted externally on a wall.

This can suit homes where ground space is limited or where replacing a bulky tank with a compact system is practical.

Hot Water Heated on Demand

Because the system heats water only when a hot tap is used, it avoids storing a full tank of hot water all day.

This can be useful for households wanting a continuous flow style of hot water, provided the unit is sized correctly.

Natural Gas and LPG Options

Instant gas systems are available in natural gas and LPG models.

This can make them suitable for homes connected to natural gas as well as properties using LPG bottles.

Natural gas and LPG are not interchangeable. The unit must match the property’s gas type.

Suitable for Many Household Sizes

Instant gas systems are available in different L/min ratings, including 16L, 20L and 26L options.

The right size depends on household use, not just the number printed on the product.

Does Instant Gas Mean Endless Hot Water?

Instant gas systems do not run out in the same way as a storage tank, but they are not unlimited.

Every continuous flow unit has a maximum rated flow capacity. If the home uses more hot water than the unit can heat at once, performance can drop.

This can happen when:

Two showers run at the same time
A shower and kitchen tap run together
A bath is filling while another outlet is used
Showerheads have high flow rates
Water pressure is too low
Gas pipe sizing is too small
LPG regulator setup is not suitable
The unit is undersized for household demand

A 26L unit may suit higher demand, but the gas and water supply must support it.

Best Instantaneous Gas Hot Water Systems to Compare

There is no single best instantaneous gas hot water system for every home. The right model depends on gas type, household demand, water pressure, pipe sizing and installation location.

Below are common instant gas options customers may compare through Installed Today.

Rinnai Infinity B26 Natural Gas Hot Water System

The Rinnai Infinity B26 Natural Gas Hot Water System may suit households comparing a higher-capacity natural gas continuous flow system.

It may suit:

Medium to larger households
Homes with natural gas
Properties with suitable gas pipe sizing
Homes with suitable water pressure
Customers wanting a compact external wall-mounted unit
Like-for-like replacement of an existing 26L natural gas system

For 26L systems, gas pipe sizing and water pressure must be checked before ordering.

Rinnai Infinity B26 LPG Gas Hot Water System

The Rinnai Infinity B26 LPG Gas Hot Water System may suit homes using LPG bottles where higher hot water demand is expected.

It may suit:

LPG homes
Rural or acreage properties
Higher-demand households
Outdoor wall-mounted installations
Customers replacing an existing LPG continuous flow unit

LPG bottle setup, regulator suitability, gas pipe sizing and water pressure should be confirmed by a licensed gas fitter.

Rheem 26L Natural Gas Hot Water System

The Rheem 26L Natural Gas Hot Water System may suit natural gas homes comparing 26L continuous flow options.

It may suit:

Natural gas properties
Larger households where demand suits a 26L system
Homes with suitable gas pipework
Homes with suitable water pressure
Customers comparing Rheem instant gas systems

A 26L unit can still be exceeded if too many outlets run at the same time.

Rheem 26L LPG Gas Hot Water System

The Rheem 26L LPG Gas Hot Water System may suit LPG homes where the property setup supports a larger continuous flow unit.

It may suit:

Homes using LPG bottles
Medium to larger households
Outdoor installations
Customers wanting an LPG continuous flow option
Properties with suitable gas supply and water pressure

Gas type must be checked before ordering. LPG and natural gas models are different.

Rheem 20L Natural Gas Hot Water System

The Rheem 20L Natural Gas Hot Water System may suit small to medium households with moderate hot water demand.

It may suit:

Natural gas homes
Moderate shower use
Smaller to medium households
Like-for-like 20L replacement jobs
Customers who may not need a 26L system

A 20L system may be more appropriate than a 26L system where household demand is lower or the property setup does not support a larger unit.

Rheem 20L LPG Gas Hot Water System

The Rheem 20L LPG Gas Hot Water System may suit LPG homes with moderate hot water demand.

It may suit:

Smaller LPG homes
Outdoor installations
Moderate daily usage
Like-for-like LPG replacements
Customers comparing mid-size LPG continuous flow systems

Gas bottle setup and regulator suitability should be checked before ordering.

Rinnai Infinity B16 Natural Gas Hot Water System

The Rinnai Infinity B16 Natural Gas Hot Water System may suit lower-demand natural gas applications.

It may suit:

Small homes
Single bathroom homes
Granny flats
Lower simultaneous hot water use
Like-for-like replacement of a smaller instant gas unit

A 16L system may not suit homes where multiple showers or fixtures are used at once.

Rinnai Infinity B16 LPG Gas Hot Water System

The Rinnai Infinity B16 LPG Gas Hot Water System may suit lower-demand LPG applications.

It may suit:

Small LPG homes
Granny flats
Single bathroom setups
Lower water demand
Customers comparing compact LPG continuous flow options

LPG setup, regulator and pipework should be checked before purchase.

Bosch 16H Natural Gas and LPG Hot Water Systems

Customers comparing smaller continuous flow gas options may also look at Bosch models such as the Bosch 16H Natural Gas Hot Water System and Bosch 16H LPG Gas Hot Water System.

These may suit smaller households or lower-demand applications where the gas type, pressure and installation location are suitable.

What About Stiebel Eltron?

Stiebel Eltron is more commonly relevant to instant electric hot water rather than gas instantaneous hot water.

If you are looking for compact electric point-of-use hot water for a sink, kitchenette, office or low-demand fixture, compare instant electric hot water systems and Stiebel Eltron hot water systems.

Instant electric is not the same as instant gas. Whole-home instant electric can require significant electrical capacity, dedicated circuits or three-phase power depending on the application.

What Size Instantaneous Gas Hot Water System Do You Need?

The right instant gas size depends on real household demand.

Consider:

Number of people in the home
Number of bathrooms
Shower length
Whether showers run at the same time
Bath use
Kitchen and laundry hot water use
Existing system performance
Natural gas or LPG
Gas pipe sizing
Water pressure
Distance from unit to outlets
Showerhead flow rates

16L Instant Gas Systems

A 16L instant gas system may suit:

Smaller homes
Single bathroom households
Granny flats
Low simultaneous hot water use
Like-for-like smaller instant gas replacements

20L Instant Gas Systems

A 20L instant gas system may suit:

Small to medium households
Moderate shower demand
Homes with one to two bathrooms depending on usage
Customers who do not need a 26L system
Like-for-like 20L replacements

26L Instant Gas Systems

A 26L instant gas system may suit:

Larger households
Higher hot water demand
Multiple bathroom homes where usage is moderate
Homes with suitable gas pipe sizing
Homes with suitable water pressure
Like-for-like 26L replacements

A 26L system is not automatically the best choice. If the gas supply or water pressure is not suitable, the unit may not perform properly.

Natural Gas vs LPG Instant Gas Systems

Natural gas and LPG systems are different.

Before ordering, confirm:

Whether the home has natural gas or LPG
Whether the selected model matches the gas type
Whether gas pipe sizing is suitable
Whether the gas meter or LPG bottle setup can support the unit
Whether other gas appliances are connected
Whether a licensed gas fitter has confirmed the setup

Do not order a natural gas unit for an LPG property, or an LPG unit for a natural gas property.

Installation Requirements for Instantaneous Gas Hot Water

Instantaneous gas hot water installation can involve plumbing, gas fitting and sometimes electrical work.

A licensed gas fitter should confirm:

Gas type
Gas pipe sizing
Gas meter capacity or LPG bottle setup
LPG regulator suitability
Appliance gas load
Outdoor location
Clearances from windows, doors, vents and boundaries
Ventilation requirements
Water pressure
Temperature control requirements
Compliance requirements

Gas work must be completed by a licensed gas fitter. Plumbing work should be completed by a licensed plumber where required. Electrical work must be completed by a licensed electrician where required.

External gas units are designed for outdoor installation unless the specific model is approved for internal flued installation.

Outdoor vs Indoor Installation

Most domestic external instantaneous gas hot water systems are designed for outdoor installation.

Do not install an external gas unit inside a cupboard, garage, roof space, enclosed patio, bathroom or internal room unless the specific model is approved for internal flued installation and installed with the correct flue system.

Before ordering, confirm:

Is the model external or internal flued?
Is the proposed location compliant?
Are clearances suitable?
Is there safe access for servicing?
Is the unit protected as required?
Are gas and water connections suitable?

Gas Pipe Sizing and Water Pressure

Gas pipe sizing and water pressure are two of the most important checks.

An instant gas system needs enough gas to heat water at the required flow rate. If gas pipework is undersized, the system may underperform, fault or affect other gas appliances.

Water pressure also matters. If pressure or flow is too low, the system may not activate properly or may deliver unstable hot water.

For 26L units, both gas pipe sizing and water pressure should be checked before purchase.

Temperature Controllers

Some instant gas systems can be paired with temperature controllers, depending on the model.

Temperature controllers may help with:

User control
Bathroom temperature limits
Kitchen temperature settings
Convenience
System operation

Controllers may not be included with the base product. Check product inclusions before ordering.

Temperature control requirements and safe delivery temperatures should be confirmed by a licensed plumber.

Instant Gas vs Electric Storage

Electric storage systems heat water in a tank using electricity. Instant gas systems heat water on demand using natural gas or LPG.

Instant Gas May Suit:

Homes with suitable gas supply
Customers wanting compact continuous flow hot water
Properties with suitable gas pipe sizing
Homes with suitable water pressure
Outdoor wall-mounted installations

Electric Storage May Suit:

Homes without gas
Like-for-like electric replacements
Customers wanting a familiar tank system
Properties where gas installation is not practical
Homes where heat pump or instant gas is unsuitable

Customers comparing electric alternatives can browse electric hot water systems, including 250L electric hot water systems, 315L electric hot water systems and 400L electric hot water systems.

Instant Gas vs Heat Pump Hot Water

A heat pump hot water system is an efficient electric storage-style system that uses electricity to move heat from surrounding air into the water tank.

Instant Gas May Suit:

Homes with natural gas or LPG
Customers wanting a compact wall-mounted unit
Properties with suitable gas pipe sizing
Homes with suitable water pressure
Like-for-like continuous flow replacements

Heat Pump May Suit:

Homes without gas
Customers moving away from gas
Homes replacing older electric storage
Properties with outdoor space
Locations with clear airflow
Homes where condensate drainage can be managed
Solar PV homes where timer operation suits usage

Customers comparing heat pump alternatives can browse heat pump hot water systems, medium heat pump hot water systems from 200L to 280L and large heat pump hot water systems over 280L.

Running Costs and Energy Use

Instant gas systems heat water only when hot water is being used, so they avoid storage tank standby heat loss. However, running costs still vary by household.

Running costs can be affected by:

Natural gas prices
LPG bottle pricing
Daily hot water use
Shower length
Number of occupants
Number of bathrooms
Temperature settings
System size
Maintenance condition
Water pressure
Incoming water temperature
Gas supply setup
Other gas appliances

Avoid choosing a system based only on generic savings claims. The best system is the one that suits your property and usage.

Rebates and Incentives

Rebates and incentives can change and do not apply to every hot water system.

Instant gas systems are not automatically eligible for rebates. Some efficient hot water systems, particularly selected heat pump or solar systems, may be eligible for STCs, rebates or incentives depending on current rules, product type, location and installation details.

Before choosing a system based on incentives, confirm current eligibility and whether any discount is already included in the advertised price.

Maintenance for Instantaneous Gas Hot Water

Maintenance requirements depend on the model, water quality, gas supply, usage and manufacturer instructions.

Maintenance may include:

Cleaning inlet filters
Checking hot water flow
Monitoring error codes
Checking temperature stability
Inspecting for visible leaks
Servicing gas components where required
Checking flue or exhaust areas where applicable
Descaling where hard water affects performance
Following manufacturer service guidance

If the unit has temperature fluctuation, low flow, ignition problems or repeated error codes, the issue may involve gas supply, water pressure, scale, filters, sensors, valves or installation conditions.

Installed Today is positioned as an online supplier of hot water systems, water filters and pumps, with selected supplied-and-installed options on eligible products in Brisbane and Gold Coast service areas where shown. General repair diagnosis, emergency plumbing and ongoing servicing should be handled by licensed trades where required.

Brisbane and Gold Coast Considerations

In Brisbane and Gold Coast homes, instant gas hot water suitability can be affected by:

Natural gas availability
LPG bottle setup
Gas meter location
Gas pipe sizing
Water pressure
Narrow side access
Outdoor wall location
Eaves, windows and clearances
Existing valve condition
Coastal exposure on the Gold Coast
Apartment or townhouse restrictions
Whether the old system kept up with demand
Compliance upgrades
Old unit removal

Selected supplied-and-installed options are available on eligible Installed Today products in Brisbane and Gold Coast service areas where shown. Availability depends on the product, address and installation requirements.

Supply-Only vs Supplied-and-Installed

Installed Today offers supply-only ordering across major Australian metro areas. This can suit customers who already have a licensed installer organised and want to order the product online.

Selected supplied-and-installed options are available on eligible products in Brisbane and Gold Coast service areas. Availability depends on the product, address and installation requirements.

For supply-only products, the listed product price generally covers the product only.

It does not usually include:

Installation labour
Plumbing alterations
Electrical work
Gas work
New valves
Pipework changes
Gas pipe upgrades
Drainage changes
Tempering valve upgrades
Pressure limiting valve upgrades
Expansion control valves
Old unit removal
Disposal
Compliance upgrades
Concrete bases or slab work
Flue kits unless listed
Temperature controllers unless listed
LPG bottle supply
LPG regulator upgrades
Relocation of the system
Rebate administration
Site assessment

For instant gas systems, confirm gas type, gas pipe sizing, water pressure, outdoor location, clearances and installation suitability with a licensed gas fitter before ordering.

Common Instantaneous Gas Buying Mistakes

Assuming Endless Means Unlimited

Instant gas systems can provide continuous hot water within their rated capacity. Too many outlets can still exceed performance.

Choosing 26L Without Checking Gas Pipe Sizing

A 26L system may need more gas supply than the existing pipework can provide.

Ordering the Wrong Gas Type

Natural gas and LPG are different. The selected unit must match the property.

Installing an External Unit Indoors

External gas units are for outdoor installation unless the model is specifically approved for internal flued use.

Ignoring Water Pressure

Poor pressure can affect activation, temperature stability and performance.

Choosing Based Only on Brand

Brand matters, but sizing, gas type, pressure and installation conditions matter more.

Assuming Supply-Only Includes Installation

Supply-only pricing generally covers the product only unless installation is clearly listed or selected.

Before Ordering Checklist

Before choosing an instantaneous gas hot water system, check:

Is the home natural gas or LPG?
Does the selected model match the gas type?
Is the existing unit continuous flow, gas storage or electric?
Is the replacement like-for-like?
How many people live in the home?
How many bathrooms are used daily?
Are showers used at the same time?
Does the household use a bath?
Did the old system keep up with demand?
Is gas pipe sizing suitable?
Is LPG regulator setup suitable if applicable?
Is water pressure suitable?
Is the outdoor location compliant?
Are clearances suitable?
Are temperature controllers required?
Is a tempering valve or temperature control device required?
Is old unit removal included or separate?
Is the product supply-only or supplied-and-installed?
Has a licensed gas fitter confirmed suitability?

Frequently Asked Questions

What is an instantaneous gas hot water system?

An instantaneous gas hot water system heats water as it flows through the unit. It does not store hot water in a tank.

Is instantaneous gas the same as continuous flow?

Yes. Continuous flow gas hot water is another name for instantaneous gas hot water.

Does an instant gas hot water system run out?

It does not run out like a storage tank, but it can be overloaded if hot water demand exceeds the unit’s rated flow capacity.

What size instant gas hot water system do I need?

Common sizes include 16L, 20L and 26L. The right size depends on bathrooms, shower habits, simultaneous use, gas pipe sizing and water pressure.

Is a 26L instant gas system best for every home?

No. A 26L system may suit some larger households, but only where gas supply and water pressure are suitable. Bigger is not automatically better.

Can I use an LPG unit on natural gas?

No. LPG and natural gas units are different. The selected model must match the property’s gas type.

Can an external instant gas unit be installed inside?

No, unless the specific model is approved for internal flued installation and installed with the correct flue system.

Do instant gas systems need electricity?

Some instant gas systems require electricity for ignition, controls or temperature controllers. Check the product specifications and installation requirements.

Can I install an instant gas hot water system myself?

No. Instant gas hot water installation involves gas fitting and plumbing work. Licensed trades must complete the work where required.

Does supply-only pricing include installation?

No. Supply-only pricing generally covers the product only. It does not usually include plumbing work, gas fitting, electrical work, valves, pipework changes, gas pipe upgrades, drainage, old unit removal, disposal or compliance upgrades unless clearly listed.

Final Word

An instantaneous gas hot water system can be a compact and practical option for homes with natural gas or LPG, but it needs to be selected carefully.

The best unit depends on gas type, household demand, water pressure, gas pipe sizing, outdoor location, clearances and whether the system is replacing an existing continuous flow unit.

A 16L system may suit smaller homes, a 20L system may suit moderate demand, and a 26L system may suit higher-demand households where the property can support it. Bigger is not automatically better.

Installed Today makes it easy to compare instant gas hot water systems, 26L instant gas hot water systems, gas hot water systems and broader hot water systems online.

For supply-only orders, confirm gas type, gas pipe sizing, water pressure, clearances and installation requirements with licensed trades before purchasing.

Back to News