The practical answer
The best modification scopes prevent hidden interfaces from emerging during commissioning. That means recovering the current ventilation basis, fire matrix, control sequence and maintenance condition before the new equipment is locked in.
AS 1668.2, AS 1668.1 and AS/NZS 3666.1 can all apply to one AHU for different reasons. The project should show which requirements are being checked rather than saying simply ‘designed to AS 1668’.
The final verification should test the changed system in the modes that matter. Individual factory data and component checks do not prove the installed building system performs correctly.
When this guide is useful
- Replacing an AHU, supply fan or exhaust fan.
- Changing VAV boxes, dampers or controls.
- Reconfiguring ductwork during a tenancy fit-out.
- Adding kitchen or process exhaust.
- Upgrading plant where access, drainage or hygiene problems already exist.
- Routine filter replacement or minor maintenance with no change to the engineering basis.
- Assuming the current documentation is correct without field verification.
- Treating the controls contractor as the sole owner of fire-mode logic.
- Avoiding recommissioning because the replacement equipment has a factory test certificate.
A seven-part HVAC modification review
Each part asks what function could change. The scope should close the loop from design basis to field commissioning.
Define the modification boundary
Identify equipment, ducts, dampers, controls, power, drains and spaces affected directly or indirectly.
Confirm current building use
Check whether occupancy and room functions still match the original ventilation design.
Review normal ventilation
Outdoor air, supply, exhaust, make-up air, pressure and operating schedules under AS 1668.2 as relevant.
Review natural and mixed-mode assumptions
Check AS 1668.4 where the design relies on permanent openings or interaction with natural ventilation.
Review fire mode
Use the fire matrix and AS 1668.1 to identify shutdown, pressurization, smoke exhaust, damper and override functions.
Review hygiene and maintenance
Use AS/NZS 3666.1 to address drainage, wet components, access, intake location and cleanability.
Commission and update records
Test the changed normal and fire modes, update drawings, manuals, asset data and maintenance tasks.
Detailed engineering examples
These cases show how the framework changes a real engineering decision. They are deliberately written around uncertainty, options and evidence rather than one universal answer.
Old AHU replaced with a compact high-efficiency unit
A new AHU is smaller, more efficient and can meet the normal supply duty. The project team wants to reuse the existing ducts and controls.
What makes the decision difficult
- The old unit supplies outdoor air to several changed occupancies.
- It participates in fire-mode operation.
- The new compact arrangement reduces access around filters and drains.
How the guide should be applied
- Confirm current ventilation demand and outdoor-air strategy.
- Recover the fire-mode sequence and compare control/power interfaces.
- Check condensate drainage, filter access and coil cleaning space.
- Define normal-mode and fire-mode commissioning tests before procurement.
The replacement becomes a coordinated building-services modification with a clear acceptance plan, avoiding late discovery of fire-mode or maintenance problems.
Larger exhaust fan creates building pressure problems
A food tenancy increases exhaust duty without coordinated supply or make-up air changes.
What makes the decision difficult
- Door forces increase.
- Odours migrate into dining areas.
- Shared mall air systems are affected.
How the guide should be applied
- Review exhaust and replenishment air together.
- Map pressure relationships across the tenancy boundary.
- Check controls and interlocking.
- Commission the final airflow and pressure system.
The remedy addresses make-up air and controls rather than simply changing the fan again.
Duct and VAV changes affect fire-mode operation
A floor is subdivided and VAV boxes are relocated. The fit-out is treated as a comfort-only mechanical project.
What makes the decision difficult
- The floor forms part of a zone pressurization system.
- Damper functions are unclear.
- The fire matrix is not updated.
How the guide should be applied
- Map the fit-out against smoke-control zones.
- Define VAV and damper fire-mode positions.
- Coordinate the fire matrix.
- Test the final sequence after completion.
The project includes smoke-control coordination and commissioning before occupancy, preventing a hidden fire-mode defect.
Common traps
The same airflow or footprint does not prove equivalent controls, fire mode or maintainability.
Existing ducts, dampers, power and controls can govern whether the modification works.
The old plant may already be serving a building that has drifted away from the original occupancy basis.
Acceptance tests should be defined during design so sensors, access and controls support them.
A replacement is the best time to correct poor drainage and inaccessible components.
Evidence to gather before deciding
Current building use and occupancy
Mechanical drawings and schedules
Fire matrix and smoke-control drawings
BMS and control sequences
Fan, AHU and damper data
Power and wiring interfaces
Drainage and condensate arrangement
Maintenance access requirements
Commissioning history
Proposed equipment and control changes