1. What You Need to Know
Every room that gets supply air needs a return air path. No exceptions. When you close a door with no return path, pressure builds up. That pressure buildup causes noise, poor airflow, and wasted energy.
A return air pathway moves air from a conditioned room back to the air handling unit. This happens through ducted returns, transfer grilles, jump ducts, or door undercuts. The method you pick depends on the airflow rate, noise limits, and fire separation requirements.
Transfer air is different from return air. Transfer air moves between rooms in a predictable but uncontrolled way. AS 1668.2 lets you count transfer air toward ventilation requirements. This matters for rooms that borrow fresh air from adjacent spaces.
The pressure target: keep the room-to-corridor difference at or below 3 Pa with doors closed. Go above that and you get whistling doors, starved coils, and tenant complaints.
2. The Rules
AS 1668.2:2012 (and 2024 revision) sets the requirements for mechanical ventilation in Australian buildings.
- Transfer air between enclosures counts toward the effective outdoor airflow rate (AS 1668.2 Section 3).
- You cannot borrow air from carparks, spaces listed in Appendix B, certain healthcare areas, or any space with harmful contaminants.
- A ceiling void used as a return air plenum must not contain materials that contaminate the air.
- A duct or plenum must not contain a tundish or floor waste unless it stays permanently charged.
- All materials in a plenum space must meet fire hazard properties under AS 1530.
AS/NZS 1668.1 covers fire and smoke control for ventilation systems. Transfer openings through fire-rated walls need fire dampers and smoke dampers.
NCC Volume One, Part J6 references AS 1668.2 for air-conditioning and ventilation energy efficiency. Ductwork, plenums, and static components must meet insulation requirements.
AIRAH DA03 provides guidance on duct design, including return air sizing, velocity limits, and noise criteria.
3. What This Means in Practice
Sizing Transfer Grilles
Size transfer grilles at a face velocity of 1.5 to 2.5 m/s. Use the lower end for occupied rooms where noise matters. The formula:
Most commercial grilles have a free area ratio between 0.60 and 0.75. A typical example: 100 L/s at 2.0 m/s with 0.70 free area needs a gross grille area of 0.071 m². That works out to about a 300 mm × 250 mm grille.
Door Undercuts
A 25 mm undercut on an 820 mm wide door gives you roughly 205 cm² of free area. At 3 Pa or less, that moves about 30 L/s. For rooms with supply air above 40 L/s, a door undercut alone is not enough. You need a transfer grille or ducted return.
Carpet at the threshold kills undercut performance. Account for finished floor coverings in your clearance calculation.
Ceiling Void as Return Plenum
Using the ceiling void as a return air plenum saves ductwork cost. But the rules are strict. Every cable, pipe, and fitting in that void must comply with fire hazard property requirements (AS 1530). Standard PVC conduit will not comply. No combustible insulation on pipes unless tested and rated. The ceiling tiles must have the right fire rating.
Fire dampers are required where the plenum crosses a fire-rated wall or floor. Size the plenum opening to keep velocity below 2.5 m/s at the return grille face.
Ducted Returns
Ducted returns give you the most control. Size return ducts for 3.5 to 5.0 m/s. Keep branches at the low end and mains at the high end. Every return grille needs a clear, unobstructed path to the air handling unit.
4. Key Design Decisions
Ducted returns cost more but give precise air balance. Use them for rooms with supply air above 80 L/s, rooms far from the AHU, or spaces with strict noise criteria (NC 30 or below).
Transfer grilles cost less and install faster. Use them for rooms with supply air under 80 L/s that sit adjacent to a corridor with a return grille.
Tradeoff: Transfer grilles allow sound transfer between rooms. Baffled grilles reduce this but add pressure drop. Verify the acoustic consultant accepts the grille location before you order.
Ceiling plenums cut material and labour costs. They work well in open-plan offices and retail spaces with few fire compartments.
Ducted returns keep the ceiling void free from airflow restrictions. They suit buildings with many fire compartments, labs, or healthcare spaces where cross-contamination matters.
Tradeoff: Ceiling plenums push cost to other trades. Every electrician and plumber working in that void must use plenum-rated materials. This typically adds 15 to 25 percent to ductwork savings from the electrical and plumbing budgets.
Door undercuts cost nothing extra during construction. They work for rooms under 40 L/s of supply air with hard floors.
Transfer grilles give a defined, calculable air path. They cost $50 to $150 per opening installed. Use them for any room above 40 L/s or where you need a reliable pressure result at commissioning.
Tradeoff: Door undercuts give you no control after the door is hung. Carpet changes will reduce performance. A 25 mm undercut also drops the door STC rating by 5 to 10 points.
5. Callouts
- Check the mechanical drawings for return air path details at every room. If a room has supply air but no marked return path, raise an RFI before you close up walls.
- Door undercuts must be measured from the finished floor, not the slab. Confirm floor finishes before you hang doors.
- Transfer grilles through fire-rated walls need a fire damper on each side. Do not install a transfer grille in a fire wall without checking the fire engineer's report.
- When using the ceiling void as a plenum, confirm all cables and fittings are plenum-rated before closing the ceiling. The fire inspector will check this.
- Size transfer grilles to the schedule. Do not swap for a smaller grille because it fits the stud cavity better. Undersized grilles create noise and pressure problems.
- Return air pathways affect floor-to-floor height. Ducted returns need 150 to 250 mm of ceiling void space above the ductwork. Plan for this during concept design.
- Ceiling plenum returns typically reduce ductwork cost by 15 to 25 percent on typical office fitouts. But the savings shift to other trades that must use plenum-rated materials.
- Transfer air strategies reduce the amount of outdoor air you need to condition. This cuts AHU size and running costs. Ask your mechanical engineer about borrowed ventilation under AS 1668.2.
- Every enclosed room on your floor plan needs a return air strategy. Mark it on the reflected ceiling plan or note it in the room data sheets.
- Door undercuts affect door aesthetics and acoustic ratings. A 25 mm undercut drops the STC rating of a standard door by 5 to 10 points. Coordinate with the acoustic consultant.
- Transfer grilles are visible elements. Specify the grille type, colour, and location on elevations. Wall-mounted grilles at high level (2100 mm AFL) reduce visual impact.
- Ceiling plenum returns mean no lay-in tiles can sit above fire-rated partitions without fire dampers. This affects partition layout and ceiling grid planning.
6. References
- Standard AS 1668.2:2012, The use of ventilation and airconditioning in buildings — Part 2: Mechanical ventilation in buildings (Standards Australia)
- Standard AS 1668.2:2024, Revised edition (Standards Australia)
- Standard AS/NZS 1668.1:2015, The use of ventilation and airconditioning in buildings — Part 1: Fire and smoke control in buildings (Standards Australia / Standards New Zealand)
- Standard AS 1530.3, Methods for fire tests on building materials, components and structures — Part 3: Simultaneous determination of ignitability, flame propagation, heat release and smoke release (Standards Australia)
- Code NCC 2025, Volume One, Part J6: Air-conditioning and ventilation (ABCB)
- Guide AIRAH DA03, Duct Design (AIRAH)
- Guide ASHRAE Handbook - HVAC Systems and Equipment, Chapter 21: Duct Design (ASHRAE, 2020)
This memo provides general guidance for building services design and construction. Detailed design requires project-specific calculations, compliance checks against NCC and relevant Australian Standards, and coordination with the full design team. Always confirm requirements with the project mechanical engineer and fire engineer.