“The building has an HVAC or ventilation problem. Which part of AS 1668 should I actually be looking at?”
The phrase ‘AS 1668 compliance’ is too broad to be useful. A stale office, a naturally ventilated warehouse, an AHU that operates in fire mode and a poorly maintained cooling tower are different engineering problems and should start in different documents.
The practical answer
Use AS 1668.2 when the core question is mechanical ventilation of buildings or car parks, including outdoor air, exhaust, make-up air, contaminant control and vehicle-emission ventilation.
Use AS 1668.4 when the ventilation strategy relies on permanent natural openings and the question is whether those openings provide a valid natural-ventilation pathway.
Use AS 1668.1 when the HVAC system, fans, dampers, ducts or controls have a fire-mode or smoke-control role. Normal operating airflow does not prove that the fire-mode sequence is correct.
Use AS/NZS 3666.1 and 3666.2 where moisture, cleanliness, access, cooling water, microbial control, commissioning or ongoing maintenance is the real issue.
Where this Standard fits
- Office, retail, education or hospitality ventilation questions
- Exhaust systems for toilets, kitchens, plant rooms and other enclosures
- Car parks and vehicle facilities
- Warehouses and buildings relying on natural ventilation openings
- HVAC systems that shut down, pressurize or exhaust smoke during fire mode
- Air-handling systems where condensate, wet filters, drainage or cleanliness are concerns
- A single ventilation-rate table for every building problem
- A thermal-comfort Standard
- Proof that high CO2 alone establishes non-compliance
- A substitute for the NCC or project approval pathway
- Permission to modify an AHU without checking its fire-mode role
- A complete maintenance program without AS/NZS 3666 and other relevant service requirements
Which part of the AS 1668 / AS/NZS 3666 cluster should you investigate?
Define the system function first. Many existing buildings involve more than one pathway, but one question usually needs to lead the review.
AS 1668 series navigator
Mechanical ventilation, natural ventilation, smoke-control and hygiene questions start in different parts of the AS 1668 / AS/NZS 3666 cluster.
How to use this summary
- Mechanical ventilation questions usually start in AS 1668.2.
- Natural ventilation by permanent openings usually starts in AS 1668.4.
- Fire-mode or smoke-control functions bring in AS 1668.1.
- Cooling-tower hygiene and some contamination questions bring in AS/NZS 3666.
Real projects often touch more than one document family at once.
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
Ventilation is not the same as comfort
A system can meet a ventilation objective and still have temperature, humidity, noise or air-movement complaints. Keep the engineering question clear.
The same AHU can have several jobs
Normal ventilation, smoke-control operation, shutdown logic and microbial-control design can all apply to one air-handling system.
Occupancy change can change the basis
A room that once worked acceptably may need reassessment when density, use, operating hours or contaminant generation changes.
Natural ventilation is an engineered pathway
Openable windows, roller doors and louvres only help when the required opening geometry and operating assumptions are actually available.
Maintenance access begins at design
Filters, coils, drains, humidifiers and wet components cannot be maintained well if the original design makes them inaccessible.
Measurements need context
CO2, airflow, pressure and contaminant readings are evidence. They become useful only when connected to occupancy, system operation, outdoor conditions and the relevant design basis.
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.
Meeting room converted into a high-occupancy training room
An office meeting room that previously held eight people is reconfigured for twenty-five-person training sessions. Complaints of stuffiness begin, and portable CO2 monitors show rising concentrations during long sessions.
The first question is mechanical ventilation under AS 1668.2, but the assessment also needs the actual occupancy, room use, air distribution and system operating schedule. A CO2 reading by itself is not the design calculation.
What made the job difficult
- The original design documentation lists the old room use but not measured supply or outdoor-air quantities.
- The VAV box may reduce airflow when sensible cooling demand is low even though the room is fully occupied.
- The room door is often closed for acoustic reasons and sessions last several hours.
- The central AHU serves many spaces, so increasing one room’s demand can affect the wider system balance.
How the engineering review should proceed
- Confirm the actual occupancy pattern and intended room classification before selecting the ventilation basis.
- Review the air path from outdoor-air intake through the AHU and terminal system to the room, including operating schedules and control sequences.
- Measure relevant airflow and trend CO2 as supporting evidence, but interpret the data against occupancy and outdoor concentration.
- Check whether changes are needed at room, terminal, AHU or controls level rather than simply opening the outdoor-air damper further.
A strong outcome is a documented ventilation adequacy assessment that identifies the changed occupancy as the trigger, verifies the existing system capacity and defines balancing or control changes. The conclusion should not be ‘CO2 was high, therefore non-compliant’ without the system evidence.
Warehouse relies on louvres, roller doors and local exhaust fans
A warehouse has permanent wall louvres, large roller doors that are usually open during the day and several local exhaust fans. A refurbishment encloses part of the building and adds racking near the louvres.
This is not a simple ‘natural or mechanical’ binary. The review needs to identify which spaces rely on AS 1668.4, which depend on mechanical exhaust under AS 1668.2 and whether the refurbishment blocks the original air path.
What made the job difficult
- Roller doors are operational openings and may not be a reliable permanent ventilation assumption.
- New internal partitions create deeper zones with weaker natural air paths.
- Racking and stored goods partly obstruct existing louvres.
- Local exhaust fans require a dependable source of make-up air.
How the engineering review should proceed
- Map each enclosure and identify the intended ventilation method rather than assessing the building as one large volume.
- Separate permanent natural openings from openings that depend on operator behaviour.
- Check how obstructions, adjacent buildings and internal partitions affect the opening arrangement and cross-ventilation path.
- Review mechanical exhaust and make-up air together so extraction does not create unintended pressure or performance problems.
The likely result is a mixed ventilation basis: some zones remain naturally ventilated, while enclosed or process-affected areas require mechanical ventilation. The refurbishment should document the new air paths rather than inherit the original assumptions blindly.
Replacing an old AHU in a multi-storey commercial building
An ageing air-handling unit is to be replaced with a modern high-efficiency unit. The new selection can meet normal supply-air duty, but the original documentation shows that the system also participates in fire-mode operation.
The replacement cannot be reviewed only as an energy or comfort upgrade. AS 1668.1 fire-mode functions, AS 1668.2 ventilation performance and AS/NZS 3666 maintainability all intersect at the same piece of plant.
What made the job difficult
- The new fan has a different drive and control architecture.
- Existing dampers and fire-mode sequences are poorly documented.
- The plantroom layout provides limited access to filters, coils and drains.
- The owner wants to reuse existing controls to reduce shutdown time.
How the engineering review should proceed
- Separate the AHU requirements into normal mode, fire mode, shutdown/override logic and maintenance/hygiene functions.
- Recover the fire matrix, damper schedule and control interfaces before final equipment selection.
- Confirm outdoor-air and ventilation duty under the current building use, not only the historical design.
- Use the replacement opportunity to correct access, drainage and cleaning limitations that would otherwise remain for the life of the new unit.
The replacement becomes a coordinated building-services modification package, not a fan-for-fan swap. The deliverables should show normal operation, fire-mode sequence, commissioning tests and maintainability of the new arrangement.
Common mistakes
The phrase hides whether the real issue is mechanical ventilation, natural ventilation or smoke control.
A sensor reading is useful evidence but needs occupancy, outdoor concentration, airflow and operating context.
An AHU or fan may appear ordinary in daily operation but have critical shutdown or smoke-control functions.
Operational behaviour should not be substituted for a documented natural-ventilation basis.
Poor access to coils, filters, drains and wet components undermines future maintenance and microbial control.
Increasing airflow in one zone can create balance, pressure or capacity problems elsewhere.
Information to gather before making the decision
Building classification and approval basis
Current space uses and occupant densities
Mechanical and fire-services drawings
Airflow schedules and commissioning records
BMS sequences and trend data
Outdoor-air intake and exhaust locations
Natural ventilation opening sizes and obstructions
Fire matrix and smoke-control sequence
AHU, fan, damper and control equipment data
Condensate, drainage and moisture history
Maintenance manuals and service records
Recent alterations, complaints and measured data
What should happen next?
Related AS Applied content
Source basis & limitations
- This page is intentionally problem-led and does not reproduce ventilation-rate tables, smoke-control operating tables, figures or detailed formulae from the licensed Standards.
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