Underfloor Air Distribution (UFAD) supplies conditioned air through a plenum beneath a raised floor and discharges it at floor level, using the natural rise of warm air to stratify the room; supply is typically 17–20°C (warmer than overhead), delivered at 3–6 m/s through swirl or linear diffusers at 20–60 Pa. It cuts fan and cooling energy 10–30% while improving comfort near occupants, and it lets a workspace be reconfigured without touching the ceiling.
Why UFAD Saves Energy
Because supply air can be 17–20°C instead of 12–14°C, the cooling coil works at a higher evaporating temperature, raising chiller COP by 5–15%. The stratified layer puts cooling where people are (0–1.8 m) and lets the upper zone run warmer, so you condition only the occupied slice. Fan energy drops because ducts are short, low-pressure (20–60 Pa at the diffuser), and air travels less vertically through long ceiling risers.
A second benefit is personal control. Each diffuser is locally adjustable, so a warm occupant opens theirs and a cold one closes theirs without a work-order. In high-churn offices this removes a major complaint driver and lets the system track the actual desk layout rather than a design drawing that is obsolete the day people move in.
Plenum and Diffuser Design
The underfloor plenum is both return-path buffer and supply duct; keep it 300–500 mm deep with sealed, clean surfaces and a pressure of 10–30 Pa. Diffusers are placed on a 9–15 m² grid, each serving 50–150 m³/h at 3–6 m/s. Swirl diffusers throw air in a controlled cone for personal zones; linear slot diffusers suit perimeter glass lines. A typical floor tile zone handles 0.05–0.15 m³/s of supply air without draught.
Comparing UFAD and Overhead
| Aspect | UFAD | Overhead VAV | Implication |
|---|---|---|---|
| Supply temp | 17–20°C | 12–14°C | UFAD raises chiller COP |
| Diffuser pressure | 20–60 Pa | 250–500 Pa duct | UFAD fan savings |
| Flexibility | Very high | Low–medium | UFAD reconfigures fast |
| First cost | +5–15% | Baseline | UFAD pays back 3–8 yr |
| Best use | Open offices, labs | Standard floors | Match to churn |
Key Design Numbers
Target a plenum velocity under 2 m/s and a supply of 5–10 L/s per person (roughly 20–40 m³/h each). Cooling load served per diffuser is 100–300 W of sensible gain; a 1,000 m² office at 50 W/m² needs about 50,000 m³/h total. Stratification height of 1.8–2.2 m keeps the occupied layer 2–4°C cooler than the ceiling, improving comfort and cutting cooling 10–30% versus mixing the entire volume uniformly.
Worked Example: An Open Office Fit-Out
A 2,000 m² office at 60 W/m² needs 120 kW sensible, served at 18°C supply with a 6°C rise, giving 120,000 / (1.2 × 1000 × 6) ≈ 16.7 m³/s (60,000 m³/h). On a 12 m² diffuser grid that is 167 diffusers at 360 m³/h each. Plenum at 15 Pa and 1.5 m/s handles it with a small 4-79 fan at 40 Pa. Chiller COP rises ~10% from the warmer supply, saving roughly 25,000 kWh a year versus an overhead 13°C design.
Decision Checklist: Choose UFAD When
- Choose UFAD for open-plan offices with high churn where reconfiguring air points matters more than first cost.
- Choose UFAD in labs or raised-floor data-adjacent spaces that already have a plenum to use, making the duct effectively free.
- Choose overhead VAV when ceiling space exists and churn is low, to keep first cost down and simplify the build.
- Choose UFAD with careful sealing when indoor air quality and personalized control are priorities for recruitment and retention.
Pitfalls to Avoid
Cable and pipe clutter in the plenum can block airflow and raise pressure 20–50 Pa above design, so keep the plenum 50% clear. Leaky tile gaskets waste 5–10% of volume. Without proper return at the ceiling, stratification collapses and you lose the savings. Commission diffuser flow within ±10% and verify the plenum stays at 10–30 Pa, because the whole concept depends on a stable, low-pressure underfloor field.
Standards and Commissioning
Design to ASHRAE 62.1 for underfloor ventilation (note the 5–10 L/s per person still applies at the warmer supply) and to ASHRAE 90.1 for fan power. Return air should be taken high on the wall or ceiling to preserve stratification; low returns destroy the effect. Pressure-test the plenum and balance diffusers before occupancy, logging flow at a representative 10% of locations.
Equipment Note
Keruite manufactures HVAC fans and air-handling equipment at an 80,000 m² facility in Dezhou, China, with an in-house AMCA-210-style test laboratory; every unit is balanced to G2.5 and performance-tested before shipment. Pairing UFAD with a low-pressure 4-79 supply fan rated to the 20–60 Pa diffuser duty keeps the plenum stable and the energy benefit intact, and the factory G2.5 balance prevents the small fan from adding vibration into the raised floor that occupants would feel through their chairs.
Underfloor vs Displacement Ventilation
UFAD is sometimes confused with displacement ventilation, but they differ in supply temperature and air pattern. Displacement supplies cool, low-velocity air (about 19–22°C) at floor level that rises as a slow piston, keeping contaminants below head height; it needs very low throw (under 0.5 m/s) and works best with heat loads above the occupied zone. UFAD supplies 17–20°C through active diffusers with more throw and tolerates higher velocities, making it more flexible for mixed office layouts where desks sit anywhere on the floor plate.
The practical split is: choose displacement for spaces with a clear heat source above people—labs, theatres, industrial bays—where the piston effect is strong and the air stays remarkably still. Choose UFAD for general offices where you want the energy and reconfiguration benefits but also need to push air a few metres to an occupant who just moved. Both rely on stratification and both beat overhead mixing for comfort near the floor, but UFAD is the more forgiving of the two when the layout is not perfectly predictable, which is most commercial buildings.
FAQ
What temperature does UFAD supply?
UFAD supplies 17–20°C air, warmer than the 12–14°C of overhead systems, because it relies on stratification rather than flooding the whole volume. The warmer coil temperature raises chiller COP by 5–15% and conditions only the occupied 0–1.8 m layer, cutting cooling energy 10–30% while keeping occupants comfortable at the desk.
Is UFAD worth the higher first cost?
First cost runs 5–15% above overhead VAV, but payback is typically 3–8 years from fan and chiller savings plus faster reconfiguration. It suits high-churn offices and labs where moving air points quarterly would otherwise mean ductwork changes. Low-churn buildings with stable layouts may not recoup the premium and are better with overhead VAV.
How deep must the plenum be?
Keep the underfloor plenum 300–500 mm deep, sealed and kept at least 50% clear of cables and pipes. Plenum pressure should sit 10–30 Pa with velocity under 2 m/s. Blockage can add 20–50 Pa and erase the fan savings, so cable management matters as much as the diffusers themselves and should be part of the commissioning checklist.
How many diffusers per person?
Plan 5–10 L/s per person (20–40 m³/h), with one swirl or linear diffuser per 9–15 m² serving 50–150 m³/h at 3–6 m/s. A 1,000 m² office at 50 W/m² needs roughly 50,000 m³/h total. Verify each diffuser within ±10% at commissioning and keep returns high on the wall to preserve the stratified layer that makes UFAD work.