The practical answer
AS 1668.1 applies to mechanical smoke-control systems and also contains fire-mode requirements for some other air-handling systems. The building’s fire matrix and smoke-control design should be reviewed before the change is finalized.
The most common hidden risks are changed fan performance, altered damper logic, BMS/FDCIE interface changes, inaccessible smoke-control dampers and tenancy modifications that change air paths.
A same-size replacement is not necessarily functionally equivalent if controls, power, failure behaviour or fire-mode performance differ.
When this question usually matters
An AHU or fan is being replaced.
Motorized or smoke-control dampers are being relocated.
A tenancy fit-out changes ducts, partitions or VAV boxes.
The BMS control sequence is being rewritten.
A smoke-control or stair-pressurization test has failed.
Red flags that make the question more significant
No one has checked the fire matrix.
The replacement is being justified only by normal airflow.
Existing damper positions and status feedback are unclear.
The controls contractor is expected to infer the fire sequence.
The final commissioning scope tests components but not the whole sequence.
How to screen an HVAC modification for smoke-control impact
Follow the equipment through normal mode and fire mode before calling the change like-for-like.
Recover the fire basis
Find the fire matrix, smoke-control drawings and commissioning records.
Identify affected components
Fans, AHUs, dampers, VAVs, controls, power and indication.
Compare before and after behaviour
Normal duty, fire-mode duty, shutdown, override and failure conditions.
Coordinate disciplines
Mechanical, fire, electrical and controls information should agree.
Define commissioning
Test the installed sequence and system performance, not just individual components.
Detailed practical examples
These examples show how the question changes a real engineering decision. They are not universal answers; they show what needs to be clarified before the right answer becomes obvious.
New EC fan array replaces an old belt-driven fan
The new fan matches normal airflow and pressure but uses a different drive and control architecture.
The old fan also operated in a smoke-control sequence, so normal performance alone does not prove equivalence.
What made the job difficult
- Fire-mode duty is hidden in old commissioning records.
- Control and status interfaces change.
- Existing dampers have uncertain operation.
How the review should proceed
- Recover the fire-mode duty.
- Compare controls and failure behaviour.
- Verify damper interaction.
- Commission the complete sequence.
The project becomes a fire/smoke-control modification, not a simple mechanical replacement.
New partitions and VAV boxes change a smoke-control zone
A floor is divided into new tenancies and air distribution is reconfigured.
The central AHU stays the same, but the fire-mode air path may not.
What made the job difficult
- Damper locations change.
- VAV fire-mode positions are unclear.
- The fire matrix is not updated.
How the review should proceed
- Compare the new layout with smoke-control zones.
- Map damper functions.
- Define VAV behaviour in fire mode.
- Update and test the final sequence.
The fit-out requires smoke-control coordination even without central plant replacement.
Information to gather before deciding
Fire matrix
Smoke-control drawings
Fan and AHU schedules
Damper schedule
BMS/FDCIE interfaces
Power supply basis
Commissioning records
Proposed changes