Gas Hot Water System Replacement Guide

Replacing a gas hot water system needs more care than simply matching the brand or litre size. You need to confirm whether the system is natural gas or LPG, whether it is gas storage or instant gas, whether the location is suitable, and whether the gas supply, flue, clearances, valves and pipework are correct. At Hot Water Outlet, you can compare supply-only hot water systems online, including gas hot water systems, instant gas hot water systems, electric hot water systems, heat pump hot water systems and compatible valves, pipework and fittings.

Quick Answer

Before replacing a gas hot water system, check:

• Natural gas or LPG

• Existing system type

• Gas storage or instant gas

• Tank size or flow rate

• Indoor or outdoor location

• Flue requirements

• Gas pipe sizing

• Gas meter or LPG regulator capacity

• Clearances to windows, doors and openings

• Water pressure

• Controller wiring if applicable

• Existing valves and fittings

• Whether a licensed gas fitter is required

A like-for-like gas hot water system or instant gas hot water system is often the simplest replacement when the existing setup is suitable and compliant.

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 gas to electric is not a direct swap and may require electrical, plumbing and gas disconnection work.

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

Gas Hot Water Replacement Checklist

Step 1: Identify the Existing Gas Hot Water System

Start by checking what type of gas hot water system you currently have.

Common gas hot water systems include:

• Gas storage hot water

• Instant gas hot water

• Continuous flow gas hot water

• Gas boosted solar hot water

• Indoor flued gas hot water

• External gas hot water

• Natural gas hot water

• LPG hot water

The replacement path depends on the system type. Replacing gas storage with gas storage is different from replacing instant gas with instant gas, and both are different from changing gas to electric or heat pump.

Step 2: Confirm Natural Gas or LPG

This is one of the most important checks.

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 label

• Existing model number

• Gas bottle setup

• Gas meter

• Previous paperwork

• Product gas type before ordering

• Licensed gas fitter confirmation

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

Step 3: Check Gas Storage vs Instant Gas

Gas hot water systems are usually either storage or instant.

Gas Storage

A gas storage system stores hot water in a tank and uses gas to reheat the water.

You can compare gas hot water systems if you are replacing an existing gas storage unit.

Gas storage may suit when:

• The existing system is gas storage

• A tank-style system is preferred

• The current location is suitable

• The tank size has worked well

• A like-for-like replacement is preferred

Instant Gas

An instant gas system heats water as it flows through the unit. It is also called continuous flow gas hot water.

You can compare instant gas hot water systems if you are replacing an existing continuous flow unit.

Instant gas may suit when:

• The existing system is instant gas

• Storage tank space is limited

• Strong recovery is important

• Gas supply is suitable

• The correct flow rate is selected

• The unit location and clearances are suitable

Step 4: Check Tank Size or Flow Rate

For gas storage systems, check the tank size in litres.

Common gas storage sizes may include:

For instant gas systems, check the flow rate in litres per minute.

Common instant gas sizes may include:

Do not choose based on the old size alone. If the old system struggled, the replacement may need different sizing. If the household has changed, hot water demand may have changed too.

Step 5: Check the Existing Location

Gas hot water systems must be installed in suitable locations.

Check whether the existing system is:

• External

• Internal and flued

• In a garage

• In a cupboard

• On a wall

• On a slab

• In a plant area

• Near windows or doors

• Near vents or openings

• Under eaves

• Near boundaries or balconies

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

Do not assume an outdoor gas unit can be installed indoors or inside a cupboard.

Step 6: Check Clearances

Gas hot water systems need correct clearances from openings and other building features.

Clearance checks may include:

• Windows

• Doors

• Vents

• Air intakes

• Corners

• Boundaries

• Balconies

• Eaves

• Electrical equipment

• LPG bottles

• Other gas appliances

• Combustible surfaces

Clearance requirements can affect whether a like-for-like replacement is suitable. An old unit may not meet current requirements, even if it has been there for years.

A licensed gas fitter should confirm the installation requirements before replacement.

Step 7: Check Gas Pipe Sizing

Gas hot water systems need adequate gas supply.

Gas pipe sizing can affect:

• Burner performance

• Hot water temperature stability

• Instant gas flow performance

• Multiple appliance operation

• LPG regulator performance

• Compliance

If you are replacing a smaller unit with a larger instant gas unit, gas pipe sizing becomes especially important.

Check:

• Existing gas pipe size

• Gas meter capacity

• Other gas appliances on the property

• Distance from meter or LPG bottles

• LPG regulator capacity

• Whether the new unit has higher gas demand

• Whether gas upgrades may be needed

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

Step 8: Check LPG Bottle and Regulator Setup

If the property uses LPG, the bottle and regulator setup matters.

Check:

• Number of LPG bottles

• Bottle size

• Regulator condition

• Regulator capacity

• Gas pipe sizing

• Appliance gas demand

• Distance from bottles to the hot water system

• Bottle location clearances

• Whether other appliances use LPG

An LPG instant gas unit may need adequate bottle and regulator capacity to operate correctly. Low supply or pressure drop can affect performance.

Step 9: Check Water Pressure and Flow

Gas hot water systems need suitable water pressure and flow.

This is especially important for instant gas systems because they need minimum flow to activate.

Check:

• Water pressure

• Flow rate

• Number of fixtures

• Shower flow rates

• Whether multiple showers run at the same time

• Existing pressure limiting valve

• Existing tempering valve

• Pipework restrictions

• Signs of poor flow or fluctuating temperature

If the old instant gas system struggled to activate or temperature changed during use, the issue may not be the unit alone. Water flow, pressure, gas supply or pipework may also need checking.

Step 10: Check Controllers and Wiring

Some instant gas hot water systems use temperature controllers.

Before replacing the unit, check:

• Whether controllers are installed

• Number of controllers

• Controller brand

• Compatibility with the replacement unit

• Existing controller wiring

• Whether wiring is damaged

• Whether the customer wants controllers retained

• Whether new controllers are required

Controller compatibility can vary by brand and model. Do not assume existing controllers will work with a new unit.

Step 11: Check Valves and Fittings

Gas hot water replacement often requires more than just the unit.

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

• Tempering valve

• Pressure limiting valve

• Duo valve

• Expansion control valve

• Isolation valve

• Non-return valve

• Relief valve drain materials

• Gas isolation valve

• Connection fittings

• Pipework materials

• Safe tray components if applicable

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

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

Step 12: Check Drainage and Relief Discharge

Gas storage systems need safe relief valve discharge. Some replacement setups may also require drainage changes.

Check:

• Existing relief drain

• Safe discharge point

• Drainage location

• Whether discharge could cause damage

• Safe tray requirements if indoors

• Whether pipework needs replacement

• Whether the new system changes valve locations

Poor drainage can create water damage, nuisance discharge or compliance issues.

Step 13: Decide Whether to Replace Like-for-Like or Change System Type

A like-for-like gas replacement may be the simplest option, but it is not always the best option.

Like-for-Like May Suit When

• The existing gas setup worked well

• The system location is suitable

• The gas type is confirmed

• Gas pipe sizing is suitable

• The old system size was adequate

• The buyer wants a simple replacement

• The household is not moving away from gas

Changing System Type May Suit When

• The home is moving away from gas

• LPG bottle supply is inconvenient

• The home has solar power

• The old gas location is no longer suitable

• Gas pipework needs major upgrades

• A heat pump is preferred

• Electric storage is preferred

• Renovation work is already happening

Changing from gas to electric or heat pump can be worthwhile, but it usually involves more checks and trade work.

Replacing Gas Storage With Gas Storage

Replacing gas storage with gas storage may be practical when the existing location, gas type and tank capacity are suitable.

Brands commonly compared for storage-style gas replacements include Rheem hot water systems, Dux hot water systems and Vulcan hot water systems.

Key Checks

Check:

• Natural gas or LPG

• Existing tank size

• New tank dimensions

• Indoor or outdoor rating

• Flue and ventilation requirements

• Gas pipe sizing

• Drainage

• Relief valve discharge

• Tempering valve requirements

• Access for removal and replacement

Gas storage tanks can have different dimensions even if the litre size is similar, so measure the space before ordering.

Replacing Instant Gas With Instant Gas

Replacing instant gas with instant gas may be a good option when the existing continuous flow setup has worked well.

Common brands include Rinnai hot water systems, Bosch hot water systems and Rheem hot water systems.

Key Checks

Check:

• Natural gas or LPG

• Existing flow rate

• Required replacement flow rate

• Gas pipe sizing

• Water pressure

• Minimum flow rate

• Existing controller wiring

• External clearances

• Flue requirements if internal

• Mounting position

• Pipework changes

• Power point requirements if applicable

If the old unit was undersized, upgrading flow rate may help, but only if the gas supply and installation conditions suit.

Replacing Gas With Heat Pump

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

Heat pumps use electricity and do not require natural gas or LPG. They may suit homes with solar power, suitable outdoor space and regular hot water demand.

Brands commonly compared include iStore hot water systems, Stiebel Eltron hot water systems, Envirosun hot water systems and Emerald Energy.

Heat Pump Buyer Checks

Check:

• Outdoor airflow

• Condensate drainage

• Noise placement

• Distance to bedrooms and neighbours

• Electrical supply

• Switchboard capacity

• Tank size

• Access for delivery

• New system location

• Gas disconnection or capping requirements

• Pipework changes

• Required valves and fittings

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 Gas With Electric Storage

An electric hot water system may suit homes moving away from gas where a simple storage tank is preferred.

Electric storage may suit when:

• A familiar tank-style system is wanted

• There is enough space for a storage tank

• Electrical supply can support the system

• Heat pump placement does not suit

• Noise needs to be minimal

• Lower upfront product cost is a priority

Electric Replacement Checks

Check:

• Tank size

• Element rating

• Electrical circuit requirements

• Switchboard capacity

• Indoor or outdoor location

• Drainage

• Safe tray if indoors

• Gas disconnection or capping requirements

• Pipework changes

• Tempering valve requirements

Changing from gas to electric is not a direct product swap. Licensed plumbing, electrical and gas fitting work may be required.

Replacing Gas-Boosted Solar Hot Water

Some solar hot water systems use a gas booster.

Replacement options may include:

• Like-for-like gas booster replacement if compatible

• Full replacement with instant gas

• Full replacement with heat pump

• Full replacement with electric storage

• New solar hot water setup where suitable

Check:

• Whether the solar collectors are staying

• Whether the solar tank is still serviceable

• Whether the booster is the only failed part

• Natural gas or LPG

• Gas pipe sizing

• Electrical controls

• Roof collectors

• Solar pipework

• Whether old solar components need removal

Gas-boosted solar replacements can be more complex than standard gas replacements because the solar side may still affect the system design.

What Size Gas Hot Water System Do You Need?

Sizing depends on household demand, system type and number of bathrooms.

For instant gas, consider simultaneous use. A one-bathroom home has different needs from a two-bathroom home where two showers may run at once.

Best Gas Hot Water Brands to Compare

The best brand depends on whether you are replacing gas storage or instant gas, and whether the system needs to be natural gas or LPG.

Rinnai

Rinnai hot water systems are commonly compared for instant gas and continuous flow hot water systems.

Bosch

Bosch hot water systems are often compared for instant gas applications.

Rheem

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

Dux

Dux hot water systems are often compared for gas storage and electric storage replacements.

Vulcan

Vulcan hot water systems are often considered for budget-conscious gas storage and electric storage replacements.

Common Mistakes When Replacing Gas Hot Water

Buying the Wrong Gas Type

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

Assuming External Units Can Go Indoors

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

Ignoring Gas Pipe Sizing

A larger gas unit may need more gas supply. Undersized pipework can affect performance.

Replacing Instant Gas With a Bigger Unit Without Checking Supply

A higher flow rate may not help if the gas pipework, water pressure or meter capacity cannot support it.

Forgetting Controller Compatibility

Existing controllers may not suit the new unit.

Not Checking Clearances

Windows, doors, vents, boundaries and other openings can affect where a gas appliance can be installed.

Comparing Supply-Only Price to Installed Cost

Supply-only pricing does not include installation, gas fitting, plumbing labour, valves, pipework changes, old unit removal, disposal or compliance upgrades.

Ignoring Water Pressure

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

Forgetting LPG Regulator Capacity

LPG systems need the correct bottle and regulator setup.

Assuming the Old Installation Is Still Compliant

An old system may have been installed under different conditions. A replacement may require compliance upgrades.

Buyer Checklist Before Ordering Online

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

• Existing system type

• Gas storage or instant gas

• Natural gas or LPG

• Existing tank size or flow rate

• Number of people in the home

• Number of bathrooms

• Whether showers run at the same time

• Whether the home has a bath

• Existing unit dimensions

• New unit dimensions

• Indoor or outdoor location

• Flue requirements

• External clearances

• Gas pipe sizing

• Gas meter capacity

• LPG bottle and regulator setup if applicable

• Water pressure

• Controller compatibility

• Drainage and relief discharge

• Required valves and fittings

• Access for delivery and replacement

• Whether changing to electric or heat pump is being considered

• Licensed plumbing and gas fitting requirements

• Electrical requirements if applicable

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

• 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: Gas Hot Water System Replacement Guide

What should I check before replacing a gas hot water system?

Check whether the system is natural gas or LPG, gas storage or instant gas, the existing size, installation location, gas pipe sizing, clearances, water pressure, controller wiring, valves, drainage and licensed gas fitting requirements.

Can I replace gas hot water like-for-like?

Yes, if the existing setup is suitable and compliant. Like-for-like replacement is often simpler, but gas type, location, clearances, gas pipe sizing and valves still need to be checked.

How do I know if my hot water system is natural gas or LPG?

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

Can I replace gas storage with instant gas?

Sometimes, but it is not always a direct swap. Instant gas may require different pipework, gas supply, water pressure, clearances and controller setup.

Can I replace instant gas with gas storage?

Sometimes, if there is enough space, drainage, gas supply and a suitable location. A licensed gas fitter and plumber should confirm whether the site suits.

Can I replace gas hot water with a heat pump?

Yes, if the site has suitable outdoor airflow, drainage, noise placement and electrical supply. Gas disconnection or capping may also be required.

Can I replace gas hot water with electric storage?

Yes, but electrical work may be needed. The old gas system must also be disconnected or isolated safely by a licensed gas fitter.

Are natural gas and LPG hot water systems the same?

No. Natural gas and LPG are different gas types. They are not interchangeable unless the unit is specifically converted and approved by a licensed gas fitter.

Can an external gas hot water system 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 a gas hot water system starts with confirming the gas type, system type, size, location, gas supply and compliance requirements. A like-for-like gas storage or instant gas replacement may be the simplest option when the existing setup suits, but changing to electric or heat pump may also be worth comparing if you want to move away from gas.

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