Instant Gas Hot Water Replacement Guide

Replacing an instant gas hot water system is not just a matter of choosing the same brand or flow rate. You need to confirm the gas type, flow rate, water pressure, gas pipe sizing, controller setup, clearances, flue requirements, power supply, valves and whether the existing location is still suitable. At Hot Water Outlet, you can compare supply-only hot water systems online, including instant gas hot water systems, gas hot water systems, electric hot water systems, heat pump hot water systems and compatible valves, pipework and fittings.

Quick Answer

Before replacing an instant gas hot water system, check:

• Natural gas or LPG

• Existing flow rate, such as 16L, 20L or 26L

• Number of bathrooms

• Whether showers run at the same time

• Water pressure and flow

• Gas pipe sizing

• Gas meter or LPG regulator capacity

• External or internal/flued location

• Clearances to windows, doors, vents and openings

• Controller compatibility

• Power point or electrical supply

• Existing valves and fittings

• Whether a licensed gas fitter is required

A like-for-like instant gas hot water system may suit when the existing setup has worked well, the gas type is confirmed and the location is still suitable.

If you want to move away from gas, you may also compare heat pump hot water systems or electric hot water systems, but changing from instant gas to electric or heat pump is not a direct swap.

Supply-only pricing is not the same as total installed cost. Installation, gas fitting, plumbing labour, electrical work, controller wiring, valves, pipework changes, old unit removal, disposal and compliance upgrades may all be separate.

Instant Gas Replacement Checklist

Step 1: Identify the Existing Instant Gas Unit

Start by checking the current unit.

Look for:

• Brand

• Model number

• Flow rate

• Gas type

• Serial plate

• External or internal/flued rating

• Approximate age

• Controller setup

• Power connection

• Existing gas pipe size

• Existing water connection layout

• Installation location

• Clearance around the appliance

The model number and data plate are usually the best starting points. They can help confirm the flow rate, gas type and appliance category.

Common brands compared for instant gas replacements include Rinnai hot water systems, Bosch hot water systems and Rheem hot water systems.

Step 2: Confirm Natural Gas or LPG

This is one of the most important checks before ordering.

Natural gas and LPG are not interchangeable. A natural gas hot water unit must not be connected to LPG unless it is specifically converted and approved by a licensed gas fitter. An LPG unit must not be connected to natural gas unless the same applies.

Check:

• Appliance data plate

• Existing model number

• Gas meter

• LPG bottle setup

• Gas supplier information

• Previous paperwork

• Licensed gas fitter confirmation

If the property has gas bottles, it is usually LPG. If the property has a gas meter connected to the street supply, it is usually natural gas. However, the appliance label and a licensed gas fitter should confirm before replacement.

Ordering the wrong gas type can delay the replacement and may make the unit unsuitable.

Step 3: Check the Flow Rate

Instant gas hot water systems are commonly rated by flow rate in litres per minute.

Common sizes include:

The existing flow rate gives you a starting point, but it should not be the only factor.

If the old unit worked well, a similar flow rate may be suitable. If the old unit struggled when multiple showers were used, the replacement may need a higher flow rate, but only if the gas supply, water pressure and pipework can support it.

Step 4: Check the Number of Bathrooms

The number of bathrooms is a major sizing factor.

A one-bathroom home may not need the same flow rate as a two-bathroom home. A home with two showers, a bath and a busy kitchen may need more capacity.

Do not choose the largest unit automatically. Bigger units may need more gas supply and may not perform properly if pipe sizing, meter capacity or water flow are not suitable.

Step 5: Check Simultaneous Hot Water Use

Instant gas systems heat water as it flows through the appliance. This means simultaneous use matters.

Check whether the property may have:

• Two showers running at once

• Shower and kitchen sink running together

• Shower and laundry use together

• Bath filling while another fixture is used

• Multiple occupants using hot water at peak times

• Guest use or short-term rental demand

If only one shower is used at a time, a smaller unit may be fine. If two showers are commonly used at once, a higher-flow unit may be needed.

Step 6: Check Water Pressure and Flow

Instant gas systems need suitable water flow to activate and operate correctly.

Check:

• Water pressure

• Shower flow rate

• Minimum flow requirement of the new unit

• Whether the old unit failed to ignite at low flow

• Whether temperature fluctuated

• Whether other taps affected shower temperature

• Existing pressure limiting valve

• Existing tempering valve

• Pipework restrictions

• Blocked strainers or filters

If the old unit had poor temperature stability, the issue may not be the hot water unit alone. Water pressure, flow, gas supply, valves or pipework may also be involved.

Step 7: Check Gas Pipe Sizing

Instant gas systems can require a strong gas supply, especially higher-flow models.

Gas pipe sizing can affect:

• Burner performance

• Temperature stability

• Maximum flow rate

• Operation when other gas appliances are running

• LPG pressure stability

• Compliance

Check:

• Existing gas pipe size

• Distance from gas meter or LPG bottles

• Number of gas appliances on the property

• Gas meter capacity

• LPG regulator capacity

• Whether the new unit has higher gas demand

• Whether the existing system ever showed gas starvation symptoms

• Whether pipework upgrades may be needed

If you are replacing a 16L unit with a 26L unit, do not assume the existing gas pipework will be suitable.

Gas fitting work must be completed by a licensed gas fitter.

Step 8: Check LPG Bottle and Regulator Setup

For LPG properties, the bottle and regulator setup is critical.

Check:

• Number of LPG bottles

• Bottle size

• Regulator type

• Regulator capacity

• Pipe size

• Distance from bottles to appliance

• Bottle location clearances

• Other LPG appliances

• Whether bottles run out too quickly

• Whether gas pressure drops during use

An LPG instant gas unit may not perform correctly if the regulator or bottle setup cannot support the appliance demand.

If the existing system had ignition issues, popping, dropouts or poor temperature stability, gas pressure and supply should be checked before assuming the new unit will fix everything.

Step 9: Check the Installation Location

Instant gas units can be external or internal/flued, depending on the product.

External gas hot water units are outdoor/external only unless the product is specifically designed and approved for internal or flued installation.

Check whether the current unit is:

• Mounted externally

• Installed in a recess box

• Installed in a garage

• Installed in a cupboard

• Installed internally with a flue

• Installed under eaves

• Installed near windows or doors

• Installed near vents or air intakes

• Installed near a boundary or balcony

Do not assume an old location is automatically suitable for a new unit. A replacement may need compliance checks, clearance checks or flue changes.

Step 10: Check Clearances

Gas appliances need correct clearances from building openings and other features.

Clearance checks may include:

• Windows

• Doors

• Vents

• Air intakes

• Corners

• Boundaries

• Balconies

• Eaves

• Electrical equipment

• Drains

• Other gas appliances

• LPG bottles

• Combustible materials

Clearance requirements can affect whether a like-for-like replacement is suitable.

A licensed gas fitter should confirm whether the existing location can be reused.

Step 11: Check Internal and Flued Units

If the existing unit is installed indoors, in a cupboard, in a garage or in a semi-enclosed area, flue and ventilation requirements become important.

Check:

• Whether the appliance is approved for internal use

• Flue type

• Flue route

• Flue termination location

• Ventilation

• Room size

• Clearances

• Combustion air requirements

• Whether a replacement model is compatible with the existing flue

• Whether a new flue kit is required

Do not replace an internal or flued unit with an external-only unit in the same location.

Step 12: Check Power Supply

Many instant gas hot water systems need electricity for ignition, fan operation, controllers or electronics.

Check:

• Existing power point

• Weatherproof power point if external

• Electrical isolation

• Power lead condition

• Whether the new unit needs a power point

• Whether the old unit was battery ignition or mains powered

• Whether an electrician is required

• Whether controller wiring is separate from power wiring

Electrical work must be completed by a licensed electrician.

If the existing unit is old and does not have a nearby power point, a replacement model may need electrical work.

Step 13: Check Controller Compatibility

Some instant gas systems use wired temperature controllers.

Check:

• Whether controllers are installed

• Number of controllers

• Controller brand

• Controller model

• Whether they are bathroom or kitchen controllers

• Whether existing wiring is suitable

• Whether the new unit supports controllers

• Whether new controllers are required

• Whether old controllers need to be removed

Controller compatibility varies by brand and model. Existing controllers may not work with a different brand or newer model.

For example, changing from an old unit to a new unit may require new controllers, controller wiring changes or removal of old controller wiring.

Step 14: Check Pipework Connections

Even if the new unit is similar, connection positions may differ.

Check:

• Cold water inlet position

• Hot water outlet position

• Gas connection position

• Relief drain requirements

• Mounting bracket location

• Unit height and width

• Distance from existing pipework

• Whether pipework changes are needed

• Whether the unit can mount to the existing wall surface

A same-brand replacement may still have different connection points depending on model and age.

Step 15: Check Valves and Fittings

Instant gas replacement may require more than the appliance itself.

You may need compatible valves, pipework and fittings, such as:

• Isolation valve

• Cold water valve

• Gas isolation valve

• Non-return valve

• Pressure limiting valve

• Tempering valve where required

• Relief valve drain materials

• Connection fittings

• Pipework materials

• Controller wiring components

• Weatherproof electrical components if required

Existing valves may be old, seized, leaking, non-compliant or unsuitable for the replacement unit.

The correct valve setup depends on system type, water pressure, temperature settings, installation location and local compliance requirements.

Step 16: Check Tempering Valve Requirements

Some instant gas systems are factory temperature limited. Others may require additional temperature control depending on the model, installation and connected outlets.

A tempering valve or other temperature control may be required depending on:

• Appliance maximum delivery temperature

• Bathroom outlet requirements

• Existing plumbing setup

• Whether the unit is set above sanitary fixture temperature limits

• Local compliance requirements

• Whether controllers are used

• Whether the system supplies bathrooms, kitchens or laundries

A licensed plumber should confirm what is required for the specific property and system.

Replacing Instant Gas Like-for-Like

A like-for-like instant gas replacement may suit when:

• The existing system worked well

• The flow rate was suitable

• The gas type is confirmed

• The existing location is compliant

• Gas pipe sizing is suitable

• Water pressure is suitable

• Controller setup is compatible or being updated

• The buyer wants a simpler replacement

However, even like-for-like replacements need checks. The new unit may have different dimensions, clearances, flue requirements, power requirements or controller compatibility.

Replacing a 16L Instant Gas Unit

A 16L instant gas unit may suit smaller homes, units or one-bathroom properties.

Before replacing, check:

• Whether the old 16L unit was adequate

• Whether the household has grown

• Whether a second bathroom has been added

• Whether showers ran cold during peak use

• Whether the gas supply can support a larger unit if upgrading

• Whether the location has enough clearance

If the existing 16L unit struggled, a larger unit may be worth comparing, but gas supply and water flow need to suit.

Replacing a 20L Instant Gas Unit

A 20L unit may suit small to medium homes, depending on climate, shower flow rates and simultaneous use.

Before replacing, check:

• Existing performance

• Number of bathrooms

• Whether multiple outlets are used at once

• Gas pipe sizing

• Controller compatibility

• Clearances

• Whether a 26L upgrade is practical or needed

A 20L to 26L upgrade may suit some homes, but only if gas supply and site conditions are suitable.

Replacing a 26L Instant Gas Unit

A 26L unit is commonly compared for larger homes, higher demand and two-bathroom properties.

Before replacing, check:

• Existing gas pipe sizing

• Gas meter capacity

• LPG regulator capacity if applicable

• Water pressure and flow

• Simultaneous shower use

• Controller setup

• Clearances

• Whether the old unit was actually performing correctly

If a 26L system was still not enough, the issue may be demand, gas supply, water flow, pipework, controllers or a need for a designed multi-unit setup.

Replacing Instant Gas With Gas Storage

Some buyers may compare gas hot water systems if they want to move from instant gas back to storage.

This may suit when:

• Stored hot water is preferred

• There is enough space for a tank

• Gas supply suits a storage system

• Drainage can be managed

• A tank-style replacement fits the property

However, changing from instant gas to gas storage is not a direct product swap.

Check:

• Space for a storage tank

• Drainage and relief discharge

• Gas pipework changes

• Water pipework changes

• Flue and ventilation requirements

• Tempering valve requirements

• Access for delivery

• Old unit removal

Replacing Instant Gas With Heat Pump

A heat pump hot water system may be worth comparing if you want to move away from gas.

This may suit when:

• The home has solar power

• LPG bottles are inconvenient

• The household wants an electric option

• There is suitable outdoor airflow

• Condensate drainage can be managed

• Noise placement is acceptable

• Electrical supply is suitable

• There is enough space for a storage tank

Changing from instant gas to heat pump may require:

• Gas disconnection or capping

• Electrical circuit work

• Plumbing changes

• Drainage changes

• Condensate drain

• New base or slab

• Valve changes

• Old gas unit removal

Heat pump rebates, STCs and incentives can change and depend on the product, installer, property, purchase type and program rules. Do not assume a rebate applies without checking current eligibility.

Replacing Instant Gas With Electric Storage

An electric hot water system may suit buyers who want to move away from gas but prefer a simple electric storage tank.

This may suit when:

• Heat pump placement does not suit

• Noise needs to be minimal

• Lower upfront product cost is a priority

• Electrical supply can support the system

• There is enough space for a tank

• The home has moderate hot water demand

Changing from instant gas to electric storage may require:

• New electrical circuit

• Switchboard check

• Gas disconnection or capping

• Plumbing changes

• Relief valve drainage

• Tempering valve setup

• Tank base or safe tray

• Old unit removal

Replacing Gas-Boosted Solar With Instant Gas

Some homes have solar hot water that feeds a gas booster. If the solar side is being removed or bypassed, an instant gas system may be compared as a replacement.

Check:

• Whether the solar collectors are staying

• Whether the solar tank is staying

• Whether the gas booster is compatible

• Whether the replacement is standalone instant gas

• Natural gas or LPG

• Gas pipe sizing

• Water pipework layout

• Controller setup

• Roof collector removal if required

• Solar pipework isolation

Solar-boosted gas setups can be more complex than standard instant gas replacements. A licensed plumber and gas fitter should confirm the correct approach.

Instant Gas vs Heat Pump Replacement

Some buyers replacing instant gas compare staying with gas against moving to heat pump.

Instant gas may suit homes staying with gas. Heat pump may suit homes moving away from gas, especially where solar power and suitable outdoor space are available.

Instant Gas vs Electric Storage Replacement

Electric storage may be simpler where gas is being removed, but it must be sized carefully because it stores a limited amount of hot water.

Best Instant Gas Brands to Compare

The best brand depends on gas type, flow rate, controller needs, budget and site suitability.

Rinnai

Rinnai hot water systems are commonly compared for instant gas and continuous flow hot water systems. Rinnai may suit buyers comparing natural gas and LPG options across common flow rates.

Bosch

Bosch hot water systems are commonly compared for compact instant gas hot water replacements and continuous flow applications.

Rheem

Rheem hot water systems are commonly compared across instant gas, gas storage, electric storage and heat pump categories.

Common Mistakes When Replacing Instant Gas

Buying the Wrong Gas Type

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

Choosing a Larger Unit Without Checking Gas Supply

A higher flow rate may require more gas supply. The gas pipework, meter or LPG regulator must be suitable.

Assuming Controllers Will Work

Existing controllers may not be compatible with the new unit, especially if changing brand or model.

Ignoring Water Flow

Instant gas systems need suitable water flow to activate and maintain stable temperature.

Using an External Unit Indoors

External gas hot water units are outdoor/external only unless specifically designed and approved for internal or flued installation.

Not Checking Clearances

Windows, doors, vents, eaves, boundaries and air intakes can affect whether the location is suitable.

Forgetting Power Requirements

Some old units may differ from newer models. A replacement may need a suitable power point or electrical work.

Comparing Supply-Only Price to Installed Cost

Supply-only pricing does not include gas fitting, plumbing labour, electrical work, controller wiring, valves, fittings, old unit removal, disposal or compliance upgrades.

Assuming the Old Installation Is Still Compliant

An old unit may have been installed under older rules or in a location that needs review during replacement.

Ignoring Pipework Changes

Connection positions can change between models, even when the replacement is similar.

Buyer Checklist Before Ordering Online

Before ordering a supply-only instant gas hot water replacement, confirm:

• Existing brand and model

• Existing flow rate

• Natural gas or LPG

• Number of bathrooms

• Number of people in the home

• Whether showers run at the same time

• Whether the home has a bath

• Existing water pressure and flow

• Gas pipe sizing

• Gas meter capacity

• LPG bottle and regulator setup if applicable

• Existing unit location

• External or internal/flued installation

• Clearances to windows, doors, vents and openings

• Flue requirements if internal

• Controller compatibility

• Power point or electrical requirements

• New unit dimensions

• Pipework connection locations

• Required valves and fittings

• Whether changing to heat pump or electric is being considered

• Licensed plumbing, gas fitting and electrical requirements

Supply-Only vs Installed Cost

Hot Water Outlet sells supply-only hot water systems and accessories online. Supply-only pricing is useful for comparing product costs, but it is not the same as total installed cost.

Supply-only pricing usually does not include:

• Installation

• Plumbing labour

• Gas fitting

• Electrical work

• Old unit removal

• Disposal

• Valves

• Fittings

• Pipework changes

• Gas upgrades

• Regulator changes

• LPG bottle work

• Controller wiring

• Power point installation

• Flue work

• Drainage work

• Compliance upgrades

• Concrete bases or platforms

• Delivery beyond what is shown at checkout

Where appropriate, Hot Water Outlet may help organise quotes from reputable installers.

FAQ: Instant Gas Hot Water Replacement Guide

What should I check before replacing an instant gas hot water system?

Check the gas type, flow rate, water pressure, gas pipe sizing, gas meter or LPG regulator capacity, appliance location, clearances, controller compatibility, power supply, valves, fittings and licensed gas fitting requirements.

Can I replace instant gas like-for-like?

Yes, if the existing system is suitable, compliant and correctly sized. However, gas type, pipe sizing, clearances, controllers and power requirements still need to be checked.

How do I know if I need natural gas or LPG?

Check the appliance label, gas meter, LPG bottles or previous paperwork. A licensed gas fitter should confirm the gas type before replacement.

What size instant gas hot water system do I need?

Sizing depends on bathrooms, shower flow rates, simultaneous use, climate and household demand. One-bathroom homes may suit smaller units, while two-bathroom homes may need higher-flow options.

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

Sometimes, but the gas pipe sizing, gas meter or LPG regulator, water pressure and appliance location must be suitable for the larger unit.

Can I replace instant gas with a heat pump?

Yes, if the property has suitable outdoor airflow, drainage, noise placement, electrical supply and space for a tank. Gas disconnection and plumbing changes may be required.

Can I replace instant gas with electric storage?

Yes, but it is not a direct swap. You may need electrical work, gas disconnection, plumbing changes, drainage and a suitable tank location.

Do existing controllers work with a new instant gas unit?

Not always. Controller compatibility depends on brand and model. Existing controllers may need replacement or rewiring.

Can an external instant gas unit be installed inside?

No. External gas hot water units are outdoor/external only unless the product is specifically designed and approved for internal or flued installation.

Does supply-only pricing include installation?

No. Supply-only pricing generally does not include installation, plumbing labour, gas fitting, electrical work, valves, fittings, pipework changes, old unit removal, disposal or compliance upgrades.

Can Hot Water Outlet install the system?

Hot Water Outlet sells supply-only hot water systems and accessories online. Plumbing, gas fitting and electrical work must be completed by appropriately licensed tradespeople. Where appropriate, Hot Water Outlet may help organise quotes from reputable installers.

Final Thoughts

Replacing an instant gas hot water system starts with confirming the gas type, flow rate, gas supply, water flow, appliance location, clearances, controllers, power and required valves. A like-for-like instant gas replacement may suit when the existing setup works well, but upgrading size or changing system type needs extra checks.

Browse instant gas hot water systems, compare gas hot water systems, electric hot water systems, heat pump hot water systems and compatible valves, pipework and fittings online at Hot Water Outlet.