Underfloor Air Distribution (UFAD) Systems

What You Need to Know

Underfloor air distribution (UFAD) supplies cool air from a pressurised plenum beneath a raised floor, instead of through ceiling ducts. Air enters the room at floor level through small diffusers and rises naturally as it absorbs heat from people and equipment. This cuts HVAC energy by up to 30% and gives occupants direct control over their comfort. The NCC, AS 1668.2, and ASHRAE 62.1 all apply to UFAD design in Australia, though no Australian code treats it as a separate system type.

• Outdoor air must reach at least 10 L/s per person for offices, with a floor rate of 0.35 L/s per m² as the minimum (AS 1668.2-2012, Table A1 and Cl 3.3)
• Air-conditioning systems above 1,000 L/s must use a variable speed fan (NCC 2025, J6D3(1)(e))
• Ductwork and plenums in systems of 3,000 L/s or more must be sealed per AS 4254.1 and AS 4254.2 for the operating static pressure (NCC 2025, Part J6)
• Supply air diffusers must not exceed 40 Pa pressure drop (NCC 2025, Part J6)
• Economy cycles are required when total airflow exceeds the thresholds in NCC Table J6D3, ranging from 2,000 to 9,000 L/s depending on climate zone (NCC 2025, J6D3(1)(c))
• UFAD floor supply with ceiling return earns a zone air distribution effectiveness (Ez) of 1.2 in cooling mode, provided the supply jet reaches 0.25 m/s at 1.4 m above the floor (ASHRAE 62.1-2022, Table 6-2 — international reference)

For a 2,000 m² open-plan office with 200 people, the outdoor air requirement is about 2,000 L/s under AS 1668.2. With UFAD's higher ventilation effectiveness (Ez = 1.2 per ASHRAE 62.1), the design outdoor air quantity at the AHU can drop by roughly 17% compared to a conventional overhead mixing system. That means a smaller outside air intake, a smaller heating coil, and lower energy use.

The raised floor sits 300–450 mm above the structural slab. A central AHU pushes conditioned air into this plenum at 12–25 Pa, far less pressure than a ducted ceiling system. Swirl diffusers in the floor panels release air into the occupied zone at 17–19°C. The air warms as it rises, picks up heat from lights and equipment, and returns through ceiling grilles. This thermal stratification means roughly half of overhead lighting heat stays above the occupied zone and does not add to the cooling load at desk level.

Thermal decay is a key design factor. Industry estimates suggest roughly 1°C of temperature rise for every 10 m of plenum travel, though actual values depend on slab temperature and airflow rate. Overall thermal decay of 2–5°C is typical across a full floor plate. Size the AHU discharge temperature low enough so the farthest diffuser still delivers adequate cooling.