“We are replacing a fan, AHU or damper. How do I know whether this ordinary HVAC job also changes the building’s fire and smoke-control system?”
Many mechanical modifications are scoped around normal operating airflow only. The risk appears when the same equipment has a different role during a fire event — shutdown, smoke exhaust, stair pressurization, zone pressurization or manual fire-fan control.
The practical answer
AS 1668.1 sets minimum requirements for the design, construction, installation and commissioning of mechanical smoke-control systems. It does not decide which buildings require those systems; that is an NCC and project-approval question.
The Standard includes specific smoke-control methods and operational requirements for fans, dampers and controls. Systems not intended to operate in fire mode may still be required to shut down.
A like-for-like mechanical replacement can still create a fire-safety issue if fan performance, controls, power, damper logic, BMS interfaces or manual override arrangements change.
Where this Standard fits
- Stair or fire-isolated exit pressurization
- Smoke exhaust systems
- Zone pressurization systems
- AHUs and ventilation systems that change mode during fire alarm
- Fire fan control panels and manual override functions
- Tenancy alterations affecting ducts, dampers or compartment air paths
- Every HVAC system is a smoke-control system
- Normal airflow testing proves fire-mode performance
- A same-size replacement fan is automatically equivalent
- The BMS alone defines the fire matrix
- Maintenance requirements are contained entirely in AS 1668.1
- A tenancy change is harmless because the main plant is unchanged
Does the HVAC modification affect fire and smoke control?
Trace the equipment through the fire matrix and air path. The question is functional, not just whether the asset tag says ‘fire fan’.
A good result needs the equipment definition, loads, design method, fabrication, inspection and operating limits to describe the same real system.
Key engineering concepts to understand
The NCC decides where smoke control is required
AS 1668.1 provides the mechanical smoke-control requirements once the applicable building pathway has been established.
Normal mode and fire mode are different systems
The same fan may stop, start, reverse duty or operate with different damper positions when fire mode is initiated.
Controls are part of the engineering design
Automatic initiation, manual override, indication and component status need to be coordinated, not left as a generic controls contractor task.
Air paths matter in fire mode
Changing a duct, damper or tenancy layout can affect compartment pressure and smoke movement even when the main fan is untouched.
Power and wiring can be functional requirements
Equipment required to operate in fire mode may have specific power, wiring and mechanical-protection expectations.
Commissioning is system-level evidence
A fan factory test or individual damper test does not prove the installed smoke-control sequence works as a whole.
Detailed engineering case studies
The examples below show how the Standard changes a real engineering decision. They are not clause summaries or universal answers; each case starts with the equipment, task and evidence available.
New supply fan matches normal airflow but not the original fire-mode basis
A commercial building replaces an AHU fan with a modern EC fan array. Normal supply-air duty is matched and energy performance improves, but the old fan also operated as part of a zone pressurization arrangement.
AS 1668.1 becomes central because fire-mode performance, controls and failure behaviour may differ even when normal airflow is equivalent.
What made the job difficult
- The original fire-mode duty is only shown on old commissioning sheets.
- The new fan array has different drive and control architecture.
- The BMS contractor proposes reusing the old sequence without full review.
- Existing dampers have uncertain stroke and position feedback.
How the engineering review should proceed
- Recover the fire-mode sequence and required system performance before final fan selection.
- Compare the new fan arrangement under normal, fire and failure conditions, including control and power interfaces.
- Verify damper operation, status indication and manual control functions that interact with the new plant.
- Commission the complete fire-mode sequence rather than only the fan airflow.
The fan replacement becomes a fire/smoke-control modification requiring coordinated mechanical and controls verification. A normal-duty match alone is not enough.
New partitions and duct branches alter a smoke-control zone
A large office floor is subdivided into new tenancies. Duct branches, VAV boxes and motorized dampers are relocated to suit the fit-out.
The main AHU is unchanged, but the air paths and controls within the smoke-control zone may no longer match the original zone pressurization design.
What made the job difficult
- The fit-out drawings focus on comfort air distribution.
- Some existing smoke-control dampers are hidden above new ceilings.
- VAV terminal behaviour during fire mode is not documented in the tenancy package.
- The fire matrix has not been updated for the new layout.
How the engineering review should proceed
- Compare the new tenancy layout with the original smoke-control zones and compartment boundaries.
- Identify all affected motorized and smoke-control dampers and their fire-mode positions.
- Review VAV and terminal behaviour so normal controls do not prevent the required fire-mode pressure relationships.
- Update the fire matrix and commissioning scope before occupation.
The fit-out requires a coordinated smoke-control review even though no major plant is replaced. The key deliverable is a verified final air path and control sequence for the altered floor.
Stair pressurization system fails field testing after years of alterations
A multi-storey building has recurring problems during periodic testing: some stair doors are difficult to open while upper levels do not achieve expected pressure performance.
The issue cannot be reduced to ‘increase the fan speed’. AS 1668.1 smoke-control performance depends on leakage paths, relief, doors, controls and the complete building system.
What made the job difficult
- Door replacements and sealing works have changed leakage over time.
- Several tenancy refurbishments altered corridor and lobby arrangements.
- The fan VSD settings were changed after earlier complaints.
- The original commissioning report is incomplete.
How the engineering review should proceed
- Review the current building configuration and the original pressurization basis together.
- Measure pressure and door forces under representative fire-mode conditions and identify where the system loses or traps air.
- Check fan control, relief paths, doors and compartment leakage before changing capacity.
- Document any building changes that need correction or incorporation into a revised engineering basis.
The solution may involve balancing, relief, door or leakage-path corrections rather than a larger fan. The final outcome should restore a coherent system-level basis and commissioning record.
Common mistakes
Equipment may have a different role, duty or control sequence during fire mode.
The matrix should represent the mechanical design intent and actual component behaviour.
Fire-mode performance, controls, redundancy and failure behaviour may differ.
Partitions, ducts and dampers can alter smoke-control zones without changing central plant.
The installed system needs sequence-level verification.
That trigger is an NCC and project-approval question.
Information to gather before making the decision
Building classification and NCC approval basis
Fire engineering report where applicable
Fire matrix
Smoke-control drawings and schematics
Fan and AHU schedules
Damper schedule and locations
FDCIE/FFCP/BMS interface information
Electrical supply and wiring basis
Original commissioning results
Current maintenance test records
Tenancy and building alterations
Proposed equipment or control changes
What should happen next?
Related AS Applied content
Source basis & limitations
- The page does not reproduce smoke-control operating tables, wiring tables, detailed system figures or commissioning criteria.
- The NCC is treated as the trigger for where smoke control is required, consistent with the Standard’s own scope notes.
Need the Standard applied to real equipment?
AS Applied helps frame the question. Detailed assessment, design, FEA, repair design, verification and RPEQ services are provided through XPO Engineers.