Air duct replacement is the appropriate step when cleaning and sealing are no longer the right solution — when the duct material itself has deteriorated beyond serviceability. Melbourne’s large stock of homes with original 1970s to 1990s ducted systems means duct replacement is a common discussion point, and many homeowners are uncertain about when replacement actually makes economic and practical sense compared to continued cleaning and maintenance.
This guide covers the indicators that replacement is warranted rather than cleaning, the replacement process and costs for Melbourne homes, partial vs full replacement decisions, and how to plan the work.
When Duct Replacement Is the Right Decision
Replacement becomes the appropriate action when the cost and disruption of continued cleaning and maintenance outweighs the cost of replacement, or when the duct condition is so degraded that cleaning cannot achieve meaningful results.
Age-related widespread deterioration
Flexible duct installed in Melbourne homes in the 1970s through late 1990s is now 25 to 50 years old. At this age, the outer jacket is typically brittle and crumbling, the insulation layer has degraded or compressed to near-zero thermal resistance, and the inner liner may be intact but is at the end of its service life. When multiple sections across the system are in this condition, full replacement delivers better long-term value than cleaning and patching a system that will require replacement within the next few years regardless.
Extensive pest damage
Significant rodent or pest damage across multiple duct sections — multiple liner tears, widespread outer jacket damage, and contamination requiring specialist remediation — can make replacement of damaged sections more cost-effective than extensive repair and cleaning. See our guide on pest infestation in Melbourne air ducts.
Multiple collapsed or kinked sections
Flex duct that has been physically compressed beyond restoration — particularly in roof spaces that have been heavily used for storage or where structural settling has occurred — cannot be restored to designed airflow capacity by any cleaning or repair. Sections that are permanently collapsed require replacement of the affected runs. See our guide on collapsed air duct in Melbourne.
Post-asbestos removal
Melbourne homes where the original duct system included asbestos-containing duct insulation, tape, or plenum materials require removal of those ACMs by a licensed asbestos removalist. In many cases, the ACM is integral to the original duct assembly — the duct insulation wrap IS the asbestos. After licensed removal, a full duct replacement with modern materials is the appropriate rebuild. See our guide on asbestos in old Melbourne ductwork.
System upgrade or conversion
When a Melbourne home is converting from ducted gas heating to a new ducted reverse-cycle system, or significantly upgrading system capacity, the new system’s airflow requirements may not match the existing duct network’s design. In these cases, full duct replacement designed for the new system capacity is often included in the system upgrade scope.
Partial vs Full Duct Replacement
Not every Melbourne home requiring replacement work needs a full duct system replacement. Partial replacement is a cost-effective option when damage is localised:
Partial replacement: when it makes sense
Replacing 2 to 5 specific damaged flex duct sections while retaining the majority of the duct network that is in serviceable condition. This is appropriate when pest damage or physical disturbance has affected specific branches, when specific long runs have suffered age-related failure while shorter runs are intact, or when a single zone has experienced a pattern of repeated problems that suggest that zone’s duct network needs replacement.
Full replacement: when it makes sense
When the majority of duct sections are at or near end-of-life, when the whole-system condition report from a professional inspection shows widespread issues across multiple sections and zones, or when a system upgrade (new heater, new reverse-cycle system) makes full duct replacement the logical accompanying scope.
The condition report as planning tool
A written condition report from FreshDuct following a professional inspection and cleaning documents the condition of each duct section, identifying which sections are serviceable, which are marginal, and which warrant immediate replacement. This report is the planning input for a staged replacement approach — replacing the worst sections now and planning the remainder over subsequent years as budget allows.
Replacement Process and Costs in Melbourne
The installation process
Duct replacement is performed by licensed HVAC or mechanical services contractors. For a standard Melbourne home, the process involves: disconnecting and removing existing duct sections from the roof space, installing new flexible duct runs of appropriate diameter and insulation rating (typically R1.0 or R1.5 for Melbourne roof space applications), connecting new sections to the existing trunk line or replacing the trunk line as required, and testing all joins for integrity before commissioning. The home is typically without heating or cooling for the duration of the installation work.
Cost ranges for Melbourne
Partial replacement (3 to 5 sections): $300 to $800, typically completed in one day.
Full duct network replacement (all flex branches, 3 to 4 bedroom Melbourne home): $3,000 to $5,500.
Full duct network replacement including trunk line (complex systems, larger homes): $5,000 to $8,000.
Asbestos-inclusive replacement (ACM removal plus new duct installation): $6,000 to $12,000 depending on ACM extent and removal complexity.
What to ask a Melbourne duct replacement contractor
Ask for the specification of replacement flexible duct — inner liner material, insulation R-value, and outer jacket material. Verify that all joins will be sealed with mastic or proper foil tape (not grey cloth tape). Request a completion report documenting what was installed. See our guide on choosing a duct cleaning company in Melbourne for the general principles of evaluating HVAC service providers.
Cleaning New Replacement Ductwork
New ductwork does not typically require cleaning immediately after installation if the installation was performed cleanly and register openings were protected during the build. However, if the installation was performed in a roof space with significant debris, or if the home was also undergoing construction work at the same time, a post-installation clean is a sensible precaution before the system is commissioned for regular use. See our guide on new home duct cleaning in Melbourne for the parallel situation with newly built homes.