Gas Hot Water Size Guide: Storage vs Instant Gas

Choosing the right gas hot water system size depends on more than the number of people in the house. Gas hot water sizing changes depending on whether you are choosing a gas storage hot water system or an instant gas hot water system.

Gas storage systems are usually sized by tank capacity, such as 135L or 170L. Instant gas systems are usually sized by flow rate, such as 16L, 20L or 26L per minute.

This guide explains how gas hot water sizing works, how to compare storage and instant gas systems, and what to check before ordering a replacement.

Quick answer: what size gas hot water system do I need?

As a general guide:

For smaller homes, a 135L gas storage system or 16L–20L instant gas system may be suitable. For most family homes, 170L gas storage or a 20L–26L instant gas system is usually the better starting point.

You can compare options in Gas Hot Water Systems or browse Instant Gas Hot Water Systems.

Gas storage vs instant gas: what is the difference?

Gas storage and instant gas systems both use gas to heat water, but they work in different ways.

A gas storage system gives you a stored volume of hot water ready to use. An instant gas system does not store hot water in a tank. Instead, it heats water on demand when a hot tap is opened.

That means you cannot compare a 170L gas storage system directly with a 26L instant gas system. They are measured differently and behave differently in the home.

How gas storage hot water sizing works

Gas storage systems are sized by tank capacity.

Common residential sizes include:

Gas storage systems usually recover faster than electric storage systems, so a 135L gas storage system may perform better than a 135L electric storage unit. But if the household uses a lot of hot water at once, the tank can still run out.

135L gas storage

A 135L gas storage system may suit:

• 1–2 person homes

• Smaller townhouses

• Moderate shower use

• Homes without large baths

• Like-for-like replacement where the old unit kept up well

It may be too small if several people shower back-to-back, the home has teenagers, or the old 135L system regularly ran out.

170L gas storage

A 170L gas storage system may suit:

• 3–5 person homes

• Family households

• Homes with two bathrooms

• Back-to-back shower use

• Homes where 135L feels too tight

For many families, 170L gives a better buffer than 135L without jumping into a much larger storage system.

Larger gas storage

Larger gas storage may be worth considering for:

• Larger families

• Homes with high peak demand

• Multiple bathrooms

• Regular bath use

• Shared homes

• Short-stay accommodation

Before increasing tank size, the installation location, gas supply, drainage, access, valves and compliance requirements must be checked.

How instant gas hot water sizing works

Instant gas hot water systems are usually sized by litres per minute.

Common residential sizes include:

The L/min rating is not the same as stored hot water capacity. It refers to the unit’s flow capacity under test conditions. Real-world performance depends on water temperature rise, incoming water temperature, gas supply, water pressure and how many outlets are being used.

16L instant gas

A 16L instant gas system may suit:

• Small homes

• One-bathroom properties

• Lower hot water demand

• Basic shower and kitchen use

It may not be suitable for larger families or homes where multiple hot water outlets run at the same time.

20L instant gas

A 20L instant gas system may suit:

• 2–3 person homes

• Moderate shower use

• Smaller family homes

• Homes with one main bathroom

It gives more capacity than a 16L system but may still be limited if two showers run together.

26L instant gas

A 26L instant gas system is often considered for:

• Family homes

• Higher shower demand

• Larger houses

• Homes with two bathrooms

• Properties wanting stronger instant gas capacity

A 26L system still needs the right gas pipe sizing, water pressure and installation conditions. Oversizing the unit does not fix poor gas supply, low water pressure or unsuitable pipework.

Storage vs instant gas sizing comparison

Which is better: gas storage or instant gas?

Neither is automatically better. The right choice depends on the property.

Gas storage may be better where:

• The existing system is already gas storage

• The old system performed well before failing

• The installation location suits another storage unit

• The household likes having stored hot water available

• A straightforward replacement is preferred

Instant gas may be better where:

• Continuous hot water is preferred

• The property has suitable gas supply

• The water pressure and flow are suitable

• Space for a storage tank is limited

• The home’s hot water use suits on-demand heating

If the existing gas pipework is undersized or the water pressure is poor, instant gas may not perform properly without extra work.

Natural gas vs LPG sizing

Natural gas and LPG must be treated separately.

A gas hot water system must be ordered for the correct gas type. Natural gas units and LPG units are not interchangeable unless specifically designed and converted in line with the manufacturer’s requirements by a licensed gas fitter.

Before ordering, confirm:

• Whether the property has natural gas or LPG

• Whether the appliance matches that gas type

• Whether the existing gas pipework is suitable

• Whether the LPG bottle and regulator setup can support the appliance

• Whether the gas installation is compliant

• Whether a licensed gas fitter is completing the work

This is especially important for instant gas systems because high-demand units can need more gas flow than older or smaller appliances.

Gas pipe sizing matters

Gas pipe sizing is one of the biggest issues when changing gas hot water systems.

A larger gas hot water unit may need more gas than the old unit. If the pipework is too small, the system may not receive enough gas under load.

Possible symptoms of poor gas supply include:

• Temperature fluctuation

• Poor hot water performance

• Burner issues

• System fault codes

• Other gas appliances being affected

• Inconsistent operation when multiple appliances run

A licensed gas fitter should assess gas pipe sizing, especially when upgrading from a smaller unit, changing to instant gas, or adding extra gas appliances.

Water pressure and flow also matter

Gas hot water performance depends on water supply as well as gas supply.

For storage systems, water pressure can affect delivery at outlets. For instant gas systems, water flow and pressure are critical because the unit needs enough flow to activate and maintain stable heating.

Important checks include:

• Incoming water pressure

• Shower flow rate

• Existing pipework condition

• Tempering valve condition

• Mixer tap performance

• Whether multiple outlets are used at once

• Distance from the system to bathrooms

If only one tap has poor hot water, the fault may not be the hot water system. It may be a tap, mixer, valve or pipework issue.

How many people can each gas hot water size support?

This table is a guide only. Actual suitability depends on usage.

For high-use homes, size by peak demand rather than just the number of occupants.

Sizing by bathroom count

Bathroom count is important because it affects peak demand.

A 26L instant gas unit may suit many two-bathroom homes, but it does not guarantee two strong showers at once in every property. Gas supply, water pressure and temperature rise still matter.

Sizing for showers, baths and peak demand

Hot water use is often concentrated into short periods.

A home may use most of its hot water between 6 am and 8 am, then very little for the rest of the day. This matters because systems need to cope with peak demand, not just total daily use.

Consider:

• How many showers happen back-to-back?

• How long are the showers?

• Are two showers used at once?

• Is there a bath?

• Are children or teenagers using long showers?

• Is hot water used heavily for cleaning or laundry?

• Do guests stay regularly?

A small system may look fine on paper but fail during the real peak-use window.

Should I replace my old gas system with the same size?

Like-for-like replacement can make sense if the old system worked well.

For example:

• If a 135L gas storage system served a couple without running out, another 135L may be suitable.

• If a 170L gas storage system served a family well, another 170L may be suitable.

• If a 20L instant gas system worked well for years, another similar-size unit may be suitable.

But you should rethink the size if:

• The old system regularly ran out

• More people now live in the home

• A second bathroom has been added

• Shower habits have changed

• You are changing from storage to instant gas

• The old gas pipework may not suit the new appliance

• You are changing from natural gas to LPG or LPG to natural gas

Do not choose a new system only because it matches the sticker on the old unit. Choose based on current household demand and site suitability.

When gas storage may be the better choice

Gas storage can be a good choice when the existing setup already suits it.

It may be the better option if:

• You already have a gas storage unit in place

• The existing unit kept up with demand

• The installation area suits another tank

• You want stored hot water available

• The gas pipework is already suitable for a similar replacement

• You do not want to change system type

Gas storage is often simpler where the current installation is already compliant and the same style of system is being replaced.

When instant gas may be the better choice

Instant gas can be a good choice where the home and installation conditions suit it.

It may be the better option if:

• You want continuous hot water

• You have limited space for a tank

• The gas supply is suitable

• The water pressure and flow are suitable

• The system can be installed in a compliant location

• The household uses hot water across the day rather than all from stored volume

Instant gas is not always a simple swap. Gas pipe sizing, water flow, controller options, flueing/location requirements and electrical supply may need checking.

Installation and compliance reminders

Gas hot water installation is not DIY work.

Use a licensed plumber and gas fitter for gas hot water system installation. Electrical work, where required, must be completed by a licensed electrician.

Important checks may include:

• Correct natural gas or LPG appliance selection

• Gas pipe sizing

• Water pressure and flow

• Tempering valve requirements

• Isolation valves

• Relief valve discharge

• Flueing or ventilation requirements

• Outdoor location and clearances

• LPG bottle and regulator setup

• Electrical connection where required

• Safe access for delivery and installation

Extra parts or site work may be needed depending on the installation. Relevant options may be listed under Installation Add-Ons.

Supply-only vs supplied-and-installed

Installed Today offers gas hot water products 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 gas hot water systems because plumbing and gas fitting 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.

Do not order a gas hot water system unless you have confirmed the correct gas type, size and installation requirements.

Common mistakes when sizing gas hot water

Comparing storage litres with instant gas litres per minute

A 170L gas storage system and a 26L instant gas system are not directly comparable. One is stored volume, the other is flow rate.

Choosing 16L instant gas for a large family

A 16L system may be too small for a family home with higher demand or multiple bathrooms.

Assuming 26L fixes every problem

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

Ordering the wrong gas type

Natural gas and LPG are not interchangeable. The unit must match the gas supply.

Ignoring gas pipe sizing

Changing to a higher-demand unit may require a gas pipe sizing assessment.

Replacing like-for-like when the old system was too small

If the old system constantly ran out or struggled, copying the old size may repeat the issue.

Forgetting installation add-ons

Valves, drainage, base work, electrical connection or gas upgrades may be required depending on the site.

Final verdict

For smaller households, a 135L gas storage system or 16L–20L instant gas system may be suitable.

For family homes, a 170L gas storage system or 20L–26L instant gas system is usually a better starting point.

For larger homes, multiple bathrooms or heavy hot water use, sizing needs more care. A larger storage unit, 26L instant gas system, or another hot water type may be more suitable depending on gas supply, water pressure, location and installation requirements.

The best gas hot water size depends on the system type, household demand, natural gas or LPG supply, pipe sizing, flow rate and compliance requirements.

FAQs

What size gas hot water system do I need for a family of four?

A family of four will often look at a 170L gas storage system or a 20L–26L instant gas system. The right choice depends on shower habits, bathroom count, gas supply and water pressure.

Is 135L gas storage enough?

A 135L gas storage system may be enough for smaller households or moderate use. It may be too small for families with long or back-to-back showers.

Is 170L gas storage enough for five people?

A 170L system may suit some five-person homes, but it depends on usage. If several people shower close together or baths are used regularly, larger storage or another option may be better.

Is 16L instant gas enough for a house?

A 16L instant gas system is generally better suited to smaller homes with lower demand. Larger homes usually need a higher-capacity option.

Is 20L instant gas enough for a family?

A 20L instant gas system may suit some small-to-medium families, but homes with multiple bathrooms or higher peak demand may need a 26L system.

Is 26L instant gas enough for two bathrooms?

A 26L instant gas system is commonly considered for two-bathroom homes, but performance depends on gas pipe sizing, water pressure, flow rate and how the bathrooms are used.

Can I change from gas storage to instant gas?

Yes, but it must be assessed properly. Gas pipe sizing, water pressure, installation location, electrical requirements and compliance clearances may all need checking.

Can I change from instant gas to gas storage?

Yes, but the site must have enough space, suitable drainage, compliant positioning and the correct gas supply for the selected storage unit.

Are natural gas and LPG hot water systems the same?

No. Natural gas and LPG systems are not interchangeable. The appliance must match the gas type and be installed by a licensed gas fitter.

Do I need a licensed installer for gas hot water?

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