Air Conditioning Ductwork Guide for Melbourne Homeowners

Why Your Air Conditioning Ductwork is the Hidden Highway of Home Comfort

Ductwork is the network of tubes (ducts) that carries conditioned air from your central heating or cooling unit to the various rooms in your home. Think of it as a highway system for air. Just as potholes and traffic jams slow down vehicles and waste fuel, leaks, restrictions, and poor design in your ductwork waste energy and reduce comfort.

There are two types:

Flexible Ducts: Made from a wire coil covered with flexible plastic and wrapped in insulation. Common in residential air conditioning installations due to ease of installation and lower cost. Must comply with AS4254.1:2021 standards.

Rigid Duct: Constructed from galvanised steel or other sheet metal. More durable and generally preferred for longer runs usually found in commercial air conditioning installations. Must comply with AS4254.2:2012 standards.

Independent studies have found that flexible duct systems can experience thermal energy losses of 20% to 40% due to ineffective sealing, poor installation practices, and insufficient insulation. This means up to 40 cents of every dollar you spend on heating or cooling could be leaking into your roof space rather than conditioning your living areas.

When Ductwork Needs to be Replaced

One of the most common scenarios requiring complete ductwork replacement in Melbourne is upgrading from evaporative cooling to reverse cycle air conditioning. These two systems operate on fundamentally different principles and have different ductwork requirements.

Why Evaporative Ductwork Won’t Work for Refrigerated Systems

Different Airflow Requirements: Evaporative cooling systems move massive volumes of air to create a cooling effect. They rely on high airflow rates to distribute moist, cool air throughout your home. Refrigerated air conditioning, on the other hand, uses much less airflow and operates as a closed-circuit system, recirculating and conditioning the air already inside your home.

This difference manifests in several ways:

Duct Diameter:

• Evaporative cooling typically uses 250mm to 300mm diameter ducts
• Refrigerated air conditioning uses 150mm to 200mm diameter ducts

The larger evaporative ducts are oversized for refrigerated systems, leading to reduced air velocity, poor air circulation, and inadequate cooling performance.

Outlet Size and Positioning:

• Evaporative systems use large vents (often 300mm square outlets) positioned to maximise air distribution from a high-volume, low-pressure system
• Refrigerated systems require smaller, strategically positioned vents (typically 200mm outlets) optimised for directional airflow from a high-pressure system

Your existing evaporative vents will be too large and in the wrong positions for effective refrigerated cooling. This isn’t just about aesthetics – incorrect vent sizing and positioning directly impacts system performance and energy efficiency.

Insulation Standards: Evaporative cooling ducts in Melbourne often have minimal insulation (sometimes as low as R0.6) because the air being distributed is already cool and moist. Refrigerated air conditioning requires insulated ductwork to:

• Prevent condensation forming on the outside of cold ducts
• Minimise heat gain as cooled air travels through hot roof spaces
• Maintain energy efficiency

According to the National Construction Code (NCC), refrigerated air conditioning ductwork in Melbourne (Climate Zone 6) running through unconditioned spaces requires a minimum R-value of R1.5 to R2.0, depending on the application and duct location.

Moisture Management: Evaporative cooling adds moisture to the air, which over time can be trapped in ductwork, potentially leading to:

• Mould growth inside existing ducts
• Degradation of duct materials
• Persistent musty odours

Refrigerated systems remove moisture from the air, providing drier, more comfortable conditions. Using old evaporative ductwork introduces contamination risks and moisture-related problems that can affect your indoor air quality.

Return Air Requirements: Evaporative systems typically don’t have return air ducts – they simply push air through the house and rely on open windows and doors for air to escape. Refrigerated systems require properly sized return air ducting to complete the air circulation loop. This means not only replacing supply ducts but also installing an entirely new return air system.

The Complete Upgrade Process

When converting from evaporative to refrigerated cooling, your HVAC installer will need to:

1. Remove the existing evaporative system from your roof, including the cooler unit itself
2. Remove all evaporative ductwork from your ceiling space
3. Patch ceiling vent holes left by the larger evaporative outlets
4. Install new, correctly sized ductwork designed for refrigerated air conditioning
5. Install smaller refrigerated vents in optimal positions for each room
6. Create return air ducting (which didn’t exist with evaporative)
7. Seal and insulate all ductwork to AS 4254 standards

Some Melbourne homeowners choose to leave the evaporative cooler on the roof (with a metal plate covering the roof penetration) rather than pay for removal. While this saves a few hundred dollars initially, removing it completely provides a cleaner finish and eliminates potential future roof leaks.

You Can Use the Same Ducts When Upgrading Evaporative Cooling Models

If you’re replacing an ageing evaporative cooling system with a newer, more efficient evaporative model, you can usually reuse your existing ductwork and vents, provided they’re in good condition.

Before reusing evaporative ductwork, your installer should check for:

• Physical damage (holes, tears, deterioration)
• Mould or contamination inside the ducts
• Proper duct support and securing
• Adequate insulation for current standards
• Vent condition and secure mounting

If your evaporative ductwork is less than 15 years old, properly maintained, and shows no signs of damage or contamination, reusing it can save several thousand dollars on your upgrade cost.

Upgrading Ducted Gas Heating: The Add-On Cooling Complication

If you currently have ducted gas heating and want to add air conditioning, the question of whether you need new ductwork depends entirely on how your system was originally designed.

Modern Add-On Cooling Ready Systems

Many modern gas ducted heating systems installed in Melbourne over the past 10-15 years were designed with future cooling in mind. These systems can accommodate add-on cooling units without complete ductwork replacement, but they still require modifications:

Existing Requirements for Add-On Cooling Compatibility:

Your system needs:

• Compatible heater model: Not all gas heaters can accept add-on cooling. Check your model specifications.
• Larger ductwork: Ducts must be one size larger than standard heating-only systems (typically 200mm instead of 150mm)
• R1.5 insulation: Ductwork must be rated to R1.5 minimum to prevent condensation
• Zoning capability: The system must support zoning, as add-on cooling doesn’t cool the whole house simultaneously
• Adequate return air: A larger return air grille with filtration suitable for air conditioning
• Electrical capacity: Sufficient electrical infrastructure to power the cooling compressor

If your gas ducted heating was installed with these specifications, you may be able to add refrigerated cooling without replacing all the ductwork. However, you’ll likely still need:

• Larger or additional vents in some rooms
• A larger return air grille
• Modifications to duct runs in specific areas
• Professional assessment of duct sealing and insulation

Older Gas Heating Systems: Complete Replacement Often Required

For most Melbourne homes with older gas ducted heating (installed before 2010), the ductwork wasn’t designed to handle air conditioning loads. These systems generally have:

Undersized Ducts: Older gas heating systems use 150mm diameter ducts or smaller, sufficient for heating (hot air rises naturally) but inadequate for cooling (cold air sinks and requires higher pressure for effective distribution).

Floor-Based Distribution: Many older gas heating systems blow warm air through floor vents. While this works well for heating, it’s highly inefficient for cooling. Cold air blown from floor vents tends to pool at ground level rather than mixing effectively with room air, creating uncomfortable stratification.

For cooling to work effectively, you need ceiling-mounted vents that allow cooled air to fall and mix naturally with warmer air. This often requires complete ductwork redesign and relocation.

Insufficient Insulation: Heating ducts often have minimal insulation since heat loss from ducts in roof spaces isn’t as critical when you’re distributing hot air. Air conditioning ducts in Melbourne’s hot roof spaces (which can reach 60-70°C on summer days) need substantial insulation to prevent heat gain and condensation.

Lack of Return Air: Older gas heating systems in Melbourne often use a simple return air grille in a central hallway. Air conditioning requires a much larger return air system, often necessitating additional return air points in different zones of the house.

Why You Can’t Reuse Heating Ducts for Cooling

We’ve written a detailed article explaining why you can’t effectively use gas heating ducts for air conditioning, but the key reasons are:

1. Physics: Heating and cooling work differently. Hot air rises and disperses naturally; cold air sinks and requires pressurised distribution.
2. Duct sizing: Cooling requires higher airflow rates and larger ducts than heating.
3. Insulation: Cooling in hot roof spaces demands much better insulation to prevent energy loss and condensation.
4. Distribution points: Ceiling outlets for cooling vs. floor outlets for heating require completely different duct layouts.

While the upfront cost of new ductwork might seem daunting, attempting to force air conditioning through inadequate heating ducts results in:

• Poor cooling performance
• High energy bills (system running constantly without achieving comfort)
• Potential coil icing from insufficient airflow
• Shortened equipment lifespan
• Uncomfortable hot and cold spots throughout your home

The 15 Year Rule for Older Ductwork

Even if you’re not changing your cooling or heating system type, ductwork itself has a limited lifespan and will eventually need replacement.

Ductwork Lifespan in Melbourne Conditions

Australian and international studies suggest:

Flexible Ductwork: Around 15 years before deterioration impacts performance 

Rigid Sheet Metal Ductwork: 20-25 years when properly maintained

However, Melbourne’s extreme temperature fluctuations accelerate ductwork deterioration. Your roof space might experience:

• Summer temperatures: 60-70°C
• Winter temperatures: Near-freezing on frosty mornings
• Daily temperature swings: 20-30°C

This constant expansion and contraction causes:

• Seams to pull apart, creating air leaks
• Flexible duct jackets to crack and deteriorate
• Insulation to compress and lose R-value
• Connection points to loosen

By the 15-year mark, most flexible ductwork systems in Melbourne roof spaces show signs of deterioration that significantly reduce HVAC efficiency.

Australian Ductwork Standards: What Your New Ductwork Must Meet

When installing new ductwork in Melbourne, your HVAC installer must comply with current Australian Standards and the National Construction Code (NCC). Understanding these requirements helps ensure you receive a quality installation.

Ductwork Repairs vs Replacement: Making the Right Decision

Not every ductwork problem requires complete replacement. Sometimes, small repairs are sufficient and can extend the life of your ducts. 

The Professional Installation Process: What to Expect

When you choose to replace your ductwork, understanding the process helps you work effectively with your HVAC contractor.

Maintaining Your New Ductwork: Getting Maximum Lifespan

Once you’ve invested in new ductwork, proper maintenance extends its lifespan and maintains efficiency.