What Size Instant Gas Hot Water System Do I Need?

Choosing the right size instant gas hot water system depends on your household size, number of bathrooms, shower habits, gas type, water pressure, flow demand and whether multiple hot water outlets are used at the same time. Instant gas hot water systems are sized by flow rate in L/min, not tank capacity, so choosing the right flow rate is one of the most important parts of buying a continuous flow gas hot water system.

At Hot Water Outlet, you can compare instant gas hot water systems online, along with gas hot water systems, hot water systems, electric hot water systems, heat pump hot water systems and common valves, pipework and fittings.

Quick Answer: What Size Instant Gas Hot Water System Do I Need?

The right size instant gas hot water system depends on how many hot water outlets may run at once and how much hot water your household needs during peak use.

As a general guide:

• Choose a 16 L/min instant gas hot water system for smaller homes, units or lower-demand one-bathroom properties where hot water use is usually limited to one outlet at a time.

• Choose a 20 L/min instant gas hot water system for moderate household demand, small families or homes where shower and kitchen use may overlap occasionally.

• Choose a 26 L/min instant gas hot water system for larger families, higher-demand homes, two-bathroom properties or households where multiple hot water outlets may be used close together.

• Always choose the correct natural gas or LPG model, because gas types are not interchangeable.

• Check controller compatibility, dimensions, connection positions, outdoor suitability, gas supply and product specifications before ordering.

• Do not choose the smallest unit just because it is cheaper if the household has high peak demand.

For most homes, the best starting point is the current instant gas system’s flow rate. If the current unit keeps up well, a similar flow rate may be practical. If it struggles when showers, kitchen or laundry use overlap, compare a higher-flow model before ordering.

Instant Gas Hot Water Sizing at a Glance

This table is a general guide only. The right size depends on actual household demand, gas type, gas supply, water pressure and product specifications.

How Instant Gas Hot Water Sizing Works

An instant gas hot water system heats water as it passes through the unit. Unlike a storage tank, it does not keep a large amount of hot water ready for later use.

Because it heats water on demand, instant gas sizing is based on flow rate in L/min.

Flow rate describes how much hot water the unit can deliver per minute under suitable conditions.

Common instant gas sizes include:

• 16 L/min

• 20 L/min

• 26 L/min

The right size depends on how much hot water is needed at peak times. A small home using one shower at a time may not need the same flow rate as a two-bathroom family home where showers, kitchen use and laundry may overlap.

Why Instant Gas Size Matters

Instant gas hot water size affects how well the system performs when hot water is being used.

If the unit is too small, it may struggle when demand is high. This can happen when more than one outlet is used at the same time, when showers run back-to-back or when the household has higher flow fixtures.

If the unit is larger than needed, it may cost more than necessary and still needs suitable gas supply, connection requirements and product compatibility.

Correct sizing helps avoid:

• Poor hot water performance during peak use

• Choosing too low a flow rate for a family home

• Buying the wrong natural gas or LPG model

• Choosing a system that does not suit two bathrooms

• Ordering a unit with unsuitable dimensions or connections

• Forgetting controller compatibility

• Forgetting required valves, fittings or accessories

The right instant gas system should match the home’s peak demand, not just the number of people living there.

16L vs 20L vs 26L Instant Gas Hot Water

The most common instant gas sizing comparison is 16 L/min, 20 L/min and 26 L/min.

A 16 L/min unit may be enough for some homes, but not every small home. A 26 L/min unit may suit many family homes, but it is not automatically required for every household.

The best size depends on real use.

What Size Instant Gas for 1 Person?

A one-person household may suit a lower-flow instant gas hot water system if hot water use is light and usually limited to one outlet at a time.

For one person, consider:

• Shower length

• Whether there is a bath

• Kitchen hot water use

• Laundry hot water use

• Whether guests regularly stay

• Number of bathrooms

• Natural gas or LPG availability

• Current system flow rate

• Outdoor wall location

A 16 L/min instant gas system may suit many low-demand one-person homes where gas is available, but the selected model still needs to match gas type, connection layout and product requirements.

What Size Instant Gas for 2 People?

A two-person household may suit a 16 L/min or 20 L/min instant gas system depending on shower habits and whether hot water is used in more than one area at once.

For two people, consider:

• Back-to-back showers

• Morning and evening routines

• Kitchen hot water use

• Laundry use

• Whether two outlets may run together

• Number of bathrooms

• Current flow rate

• Current system performance

If both people shower close together and kitchen or laundry use overlaps, a 20 L/min system may be worth comparing. If demand is lighter and there is one bathroom, a 16 L/min model may suit some homes.

What Size Instant Gas for 3 People?

A three-person household often needs more careful sizing than a couple, especially if there are children, regular bath use or two bathrooms.

For three people, compare:

• 16 L/min for lower-demand one-bathroom homes

• 20 L/min for moderate household demand

• 26 L/min where demand is higher or two bathrooms may be used

• Current system flow rate

• Shower habits

• Whether multiple outlets overlap

A 20 L/min instant gas system is often worth comparing for many three-person homes, but the right choice depends on gas type, bathroom count and peak demand.

What Size Instant Gas for a Family of 4?

A family of 4 usually needs stronger instant gas performance than a smaller household. The right size depends on whether the home has one bathroom or two, how close together showers happen and whether kitchen or laundry use overlaps.

For a family of 4, compare:

• 20 L/min where demand is moderate

• 26 L/min where demand is higher

• Number of bathrooms

• Back-to-back showers

• Bath use

• Morning and evening peak use

• Current system flow rate

• Whether the current system struggles

A 26 L/min instant gas system is commonly compared for family homes, especially where there are two bathrooms or higher peak demand. A 20 L/min system may still suit some lower-demand homes.

What Size Instant Gas for a Family of 5 or More?

A family of 5 or more usually needs higher-flow instant gas hot water, especially if there are multiple bathrooms, teenagers, long showers or overlapping hot water use.

For larger families, consider:

• 26 L/min instant gas systems

• Number of bathrooms

• Whether two showers may run close together

• Back-to-back showers

• Bath use

• Kitchen and laundry overlap

• Gas supply suitability

• Current system performance

A 16 L/min system may be too small for many larger households. A 20 L/min system may suit some moderate-use homes, but 26 L/min is often the main category larger families compare.

What Size Instant Gas for One Bathroom?

A one-bathroom home may not need the same flow rate as a two-bathroom home, but usage still matters.

For one-bathroom homes, consider:

• Number of people

• Shower length

• Whether showers happen back-to-back

• Whether kitchen or laundry use overlaps with showers

• Bath use

• Current system performance

A 16 L/min system may suit some one-bathroom homes with lower demand. A 20 L/min system may be worth comparing if more people live in the home or hot water use is heavier.

What Size Instant Gas for Two Bathrooms?

A two-bathroom home usually needs more careful instant gas sizing because there is a higher chance of simultaneous or back-to-back hot water use.

For two-bathroom homes, consider:

• Whether two showers may run at once

• Whether one shower and one basin may run together

• Number of people in the home

• Morning and evening peak demand

• Bath use

• Kitchen and laundry use

• Current system flow rate

• Gas supply suitability

A 20 L/min or 26 L/min instant gas system is often worth comparing for two-bathroom homes. If two showers may run close together or at the same time, a higher-flow system may be more suitable.

What Size Instant Gas for Homes With a Bath?

Bath use can increase hot water demand significantly. Although instant gas does not store hot water in a tank, it still needs enough flow rate to supply the fixture properly.

For homes with a bath, consider:

• Bath size

• How often the bath is used

• Whether showers are used soon after bath use

• Whether the bath and other outlets may overlap

• Number of occupants

• Current system performance

A lower-flow instant gas system may not suit homes with regular bath use and higher household demand. Compare flow rate carefully before ordering.

What Size Instant Gas for a Unit?

Some units may suit instant gas hot water where natural gas or LPG is available and the building setup allows it. However, body corporate rules, external wall location, gas type and access need to be checked.

For units, consider:

• Existing system type

• Natural gas or LPG availability

• External wall location

• Number of occupants

• Number of bathrooms

• Body corporate or building rules

• Flow rate requirements

• Product dimensions

• Controller requirements

A 16 L/min or 20 L/min unit may suit some smaller unit setups, but the correct size depends on demand and building suitability.

What Size Instant Gas for a Townhouse?

Townhouses can be a good fit for instant gas hot water where natural gas or LPG is available and a compact wall-mounted unit is preferred.

For townhouses, consider:

• Number of occupants

• Number of bathrooms

• Whether the townhouse has one or two bathrooms

• Whether showers happen close together

• Natural gas or LPG type

• Outdoor wall location

• Gas supply suitability

• Product dimensions

• Body corporate or complex rules if relevant

A 20 L/min or 26 L/min instant gas system may be worth comparing for many townhouses, especially where there are multiple occupants or two bathrooms.

What Size Instant Gas for a Granny Flat?

A granny flat may suit a lower-flow instant gas system if natural gas or LPG is available and demand is modest.

For granny flats, consider:

• Number of occupants

• Bathroom use

• Kitchen use

• Laundry use

• Whether the system is dedicated or shared

• Gas availability near the granny flat

• Flow demand

• Outdoor wall location

A 16 L/min instant gas system may suit some granny flats with one bathroom and low to moderate use. A larger model may be needed if demand is higher or fixtures may overlap.

What Size Instant Gas for a Rental Property?

Rental properties should be sized for likely tenant demand, not just the current tenant.

For rentals, consider:

• Number of bedrooms

• Number of bathrooms

• Likely number of occupants

• Whether tenants have reported hot water issues

• Existing instant gas flow rate

• Gas type

• Warranty information

• Product availability

• Required valves and fittings

A low-flow unit may suit a small rental, but a family rental or two-bathroom property may need a higher-flow system. Avoid undersizing rental hot water just to reduce upfront cost.

What Size Instant Gas for a Renovation?

Renovations should be sized for the finished home, not the old layout.

For renovations, consider:

• Future number of bathrooms

• New bath or larger shower

• Kitchen changes

• Laundry changes

• Whether two outlets may run together

• Whether the system location is moving

• Natural gas or LPG type

• Gas supply suitability

• Required valves, pipework and fittings

If you are adding a second bathroom, larger bath or extra occupants, the old instant gas flow rate may no longer be suitable.

Natural Gas vs LPG Instant Gas Sizing

Gas type is one of the most important checks before ordering an instant gas hot water system.

Instant gas systems are sold as natural gas models or LPG models. They are not interchangeable.

Before ordering, confirm whether the property uses:

• Natural gas

• LPG bottles

• Bulk LPG

Do not guess based on appearance. Check the current unit label, gas meter, LPG bottle setup or ask a licensed gas fitter if you are unsure.

Choosing the wrong gas type can cause delays, compatibility issues and extra costs.

Why Gas Supply Matters With Instant Gas Size

A higher-flow instant gas system may need suitable gas supply to perform properly. This is especially important when comparing larger 26 L/min units or changing from gas storage to instant gas.

Gas supply suitability can be affected by:

• Existing gas line sizing

• Distance from gas supply

• Other gas appliances

• Natural gas or LPG setup

• Existing regulator setup

• System location

• Product requirements

Hot Water Outlet sells supply-only products online. A licensed gas fitter should confirm gas supply suitability before installation or connection work.

Why Water Pressure Matters

Instant gas performance can also depend on water pressure and flow conditions. A system with a high flow rating still needs the property’s water supply and plumbing setup to support the expected demand.

Before choosing a larger instant gas system, consider:

• Existing water pressure

• Shower flow rates

• Number of outlets

• Whether multiple fixtures run together

• Pipework condition

• Product specifications

If the home has low water pressure or flow issues, choosing a larger hot water unit may not solve the underlying problem. A licensed tradesperson should assess the setup where needed.

Controller Compatibility

Some instant gas hot water systems may support temperature controllers. Controllers can be useful, but they are model-specific and may not be included.

Before choosing an instant gas unit based on controllers, check:

• Whether the unit supports controllers

• Which controller models are compatible

• How many controllers can be connected

• Whether existing controllers can be reused

• Whether extra wiring is required

• Whether controllers are included or sold separately

Do not assume an old controller will work with a new instant gas unit. Always check compatibility before ordering.

Outdoor Location and Dimensions

Many instant gas hot water systems are external units designed for outdoor installation. The selected model still needs a suitable location.

Before ordering, check:

• Height

• Width

• Depth

• Wall space

• Clearance requirements

• Existing pipework

• Gas connection position

• Water connection positions

• Power requirements where relevant

• Access for licensed tradespeople

• Distance from windows, doors and openings where relevant

• Product specifications

A unit with the right flow rate may still be unsuitable if it does not fit the available location or connection layout.

Replacing an Existing Instant Gas System

If you are replacing an old instant gas hot water system, start by checking the existing unit.

Look for:

• Brand

• Model number

• Flow rate in L/min

• Natural gas or LPG

• Controller setup

• Existing location

• Dimensions

• Connection positions

• Power requirements

• Manufacturing date

• Product code

If the old unit kept up well, a similar flow rate may be a useful starting point. If it struggled during peak use, compare a higher-flow model where the property’s gas supply and water supply can support it.

Replacing Gas Storage With Instant Gas

Some homes compare instant gas when replacing an old gas storage tank. This can be practical in the right property, but storage and instant gas systems are different.

Before changing from gas storage to instant gas, check:

• Natural gas or LPG type

• Gas supply suitability

• Flow rate required

• Number of bathrooms

• Household demand

• Outdoor wall location

• Existing pipework

• Required valves and fittings

• Controller preferences

• Product dimensions

Changing from storage to instant gas should be assessed by licensed tradespeople because the connection requirements may differ.

Instant Gas vs Gas Storage Sizing

Gas storage systems and instant gas systems are sized differently.

Gas storage systems are sized by tank capacity and recovery rate. Instant gas systems are sized by flow rate in L/min.

Do not compare litres and L/min as if they are the same measurement. They describe different hot water system types.

Instant Gas vs Electric Storage Sizing

Instant gas and electric storage systems are also sized differently.

Electric storage systems use tank capacity in litres. Instant gas systems use flow rate in L/min.

If the property does not have natural gas or LPG, compare electric hot water systems or heat pump hot water systems instead.

Should You Choose a Bigger Instant Gas System?

A bigger instant gas system may be worth comparing if the current unit struggles or the household has higher peak demand.

A larger flow rate may suit when:

• More people now live in the home

• A second bathroom has been added

• Two showers may run close together

• Shower and kitchen use overlap often

• The current system struggles during peak use

• The property is a family rental

• Renovations will increase hot water demand

However, bigger is not automatically better. The larger unit still needs suitable gas supply, water flow, dimensions and product compatibility.

Should You Choose a Smaller Instant Gas System?

A smaller instant gas system may suit lower-demand homes, but only if peak use is modest.

A smaller flow rate may suit when:

• The home has one bathroom

• There are 1 or 2 occupants

• Hot water use is light

• Outlets are rarely used together

• The current higher-flow system is more than needed

• A compact lower-demand replacement is suitable

Do not choose smaller by price alone. If the system is undersized, hot water performance may suffer during daily use.

What Happens If Instant Gas Is Too Small?

An undersized instant gas system can create hot water performance issues.

Signs the system may be too small include:

• Hot water struggles when more than one outlet is used

• Temperature fluctuates during peak demand

• Showers are affected when kitchen taps are used

• The system cannot keep up with household use

• Two bathrooms cannot be used comfortably

• Tenants or family members complain about hot water performance

If your current instant gas system has these issues, compare a higher-flow model and have the gas and water supply checked before ordering.

Common Mistakes When Choosing Instant Gas Size

Choosing the Cheapest Flow Rate

A 16 L/min unit may cost less than a 26 L/min unit, but it may not suit a family home or two-bathroom property.

Ignoring Natural Gas vs LPG

Natural gas and LPG models are not interchangeable. Always confirm gas type before ordering.

Choosing by Number of People Only

Bathroom count, shower habits and simultaneous outlet use matter just as much as household size.

Forgetting Gas Supply Suitability

A higher-flow unit still needs suitable gas supply. This should be checked by licensed tradespeople.

Forgetting Water Pressure and Flow

Poor water pressure or flow may affect performance. A larger unit may not solve an underlying water supply issue.

Assuming Controllers Are Compatible

Existing controllers may not work with a new unit. Check controller compatibility before ordering.

Comparing L/min to Litres

Instant gas systems are sized by L/min. Storage tanks are sized by litres. These are different measurements.

Forgetting Valves and Fittings

The main unit may not be the full order. Valves, pipework, fittings or accessories may also be required.

Instant Gas Hot Water Size Buyer Checklist

Before ordering an instant gas hot water system online, check:

• Current hot water system type

• Current brand and model

• Current flow rate in L/min

• Whether the current system keeps up

• Natural gas or LPG

• Number of people in the home

• Number of bathrooms

• Number of showers

• Whether two outlets may run together

• Shower habits

• Bath use

• Kitchen hot water use

• Laundry use

• Desired flow rate

• Gas supply suitability

• Water pressure and flow

• Controller requirements

• Product dimensions

• Connection positions

• Outdoor location suitability

• Required valves and fittings

• Delivery access

• Warranty information

• Product specifications

Compare Instant Gas Hot Water Sizes Online

Hot Water Outlet makes it easier to compare instant gas hot water systems by flow rate, brand, gas type and product specifications.

Start with:

• Instant Gas Hot Water Systems

• Gas Hot Water Systems

• Hot Water Systems

• Valves, Pipework and Fittings

If your home does not have natural gas or LPG, compare electric hot water systems and heat pump hot water systems instead.

FAQs About Instant Gas Hot Water System Size

What size instant gas hot water system do I need?

The right size depends on household size, number of bathrooms, shower habits, gas type and whether multiple outlets may run at the same time. Instant gas systems are sized by flow rate in L/min.

Is 16L instant gas hot water enough?

A 16 L/min instant gas system may suit smaller homes, units or lower-demand one-bathroom properties. It may not suit larger families, two bathrooms or frequent simultaneous hot water use.

Is 20L instant gas hot water enough?

A 20 L/min instant gas system may suit moderate household demand, small families or homes where hot water use occasionally overlaps. Suitability depends on bathroom count, shower habits and gas type.

Is 26L instant gas hot water enough?

A 26 L/min instant gas system is commonly compared for larger homes, families, two-bathroom properties and higher peak demand. Gas supply and product suitability still need to be confirmed.

What size instant gas hot water system does a family of 4 need?

A family of 4 may need a 20 L/min or 26 L/min instant gas system depending on the number of bathrooms, shower habits, bath use and whether multiple outlets run at once.

What size instant gas hot water system does a family of 5 need?

A family of 5 or more often compares 26 L/min instant gas systems, especially where there are two bathrooms, teenagers, long showers or overlapping hot water use.

What size instant gas hot water system do I need for two bathrooms?

Two-bathroom homes often need a 20 L/min or 26 L/min instant gas system, depending on whether two showers may run close together and how much hot water the household uses during peak times.

Can I replace gas storage with instant gas?

Some homes may be able to replace gas storage with instant gas, but suitability depends on gas type, gas supply, flow rate needs, location and connection requirements. Licensed tradespeople should assess the setup.

Are natural gas and LPG instant gas systems the same?

No. Natural gas and LPG systems are different and are not interchangeable. Always choose the model that matches the property’s gas type.

Can I install an instant gas hot water system myself?

No. Instant gas hot water systems involve gas and plumbing work that should be completed by appropriately licensed tradespeople. Hot Water Outlet sells supply-only hot water systems and accessories online, and may help organise quotes from reputable installers where appropriate.

Find the Right Size Instant Gas Hot Water System

The right instant gas hot water system should match your household demand, gas type, number of bathrooms, flow rate requirements and replacement setup. Before ordering, compare flow rate in L/min, natural gas or LPG model, controller compatibility, dimensions, connection positions, warranty information and required accessories.

Browse instant gas hot water systems online with Hot Water Outlet to compare continuous flow gas hot water options by brand, flow rate, gas type and product specifications.