💨 Estimated Plenum Pressure
📏 Stratification Height
Sizing Breakdown
Full Calculation Table
| Component | Basis | Value | Result |
|---|
How to Use the Underfloor Air Distribution Calculator
Enter the zone sensible cooling load and floor area served by the raised floor plenum.
Enter room design temperature and UFAD supply air temperature, typically warmer than overhead system supply air.
Enter raised floor height, longest plenum travel distance, and select a floor diffuser type for airflow assumptions.
Click Calculate UFAD Sizing to see required supply airflow, diffuser count, estimated plenum pressure, and stratification comfort zone height.
Understanding Underfloor Air Distribution
Underfloor air distribution delivers conditioned air through a raised floor plenum and floor-mounted diffusers directly into the occupied zone, rather than from ceiling diffusers as conventional overhead systems do. This approach shares some conceptual similarity with the displacement ventilation calculator approach, since both rely on buoyancy-driven air movement through the occupied space, a design category the ASHRAE Handbook, HVAC Systems and Equipment volume covers alongside conventional overhead distribution, but UFAD delivers air through floor diffusers within the occupied zone itself rather than low sidewall units at the room perimeter.
Low-Pressure Plenum Design
UFAD plenums typically operate at low static pressure, commonly 12 to 25 pascals, considerably lower than conventional overhead ductwork running at hundreds of pascals. This lower pressure reduces fan energy meaningfully, but it also means plenum design needs careful attention to minimizing pressure drop, since obstructions from structural elements, cabling, and long distribution distances within the plenum cavity can quickly consume the limited pressure budget available, a balance worth checking against the air balance calculator once individual diffuser airflows are set.
Why UFAD Supply Air Runs Warmer
UFAD systems typically supply air at 18 to 20 degrees Celsius, noticeably warmer than the 12 to 14 degrees Celsius common in overhead systems. Because floor diffusers deliver air directly at floor level near occupants' feet, a colder supply temperature would create noticeable cold drafts, the kind of localized discomfort ASHRAE Standard 55 addresses through its draft risk criteria. UFAD compensates for this warmer supply temperature by delivering a larger total airflow at a smaller temperature difference to achieve the same cooling capacity, similar to the tradeoff seen in displacement ventilation design.
Stratification and Occupant Comfort
Like displacement ventilation, UFAD relies on buoyancy from heat sources to carry air upward through the space, developing a stratified temperature and air quality profile from floor to ceiling. The stratification comfort zone height needs to clear the occupied zone, typically head height for seated or standing occupants, for the system to deliver its intended comfort and air quality benefit. This calculator estimates stratification height using a similar buoyancy-based approach to the displacement ventilation calculator, adjusted for UFAD's typical floor-level delivery pattern.
Practical Advantages for Office Buildings
UFAD systems let occupants adjust individual floor diffusers near their workstation, giving personal comfort control that overhead systems generally cannot match. The raised floor cavity also commonly shares space with power and data cabling, simplifying future office reconfiguration since furniture and wall moves do not require relocating overhead ductwork. Fresh conditioned air entering near the breathing zone can also improve ventilation effectiveness, a metric the ventilation effectiveness calculator quantifies directly, compared to overhead mixing systems that dilute fresh air throughout the entire room volume before it reaches occupants.
From This Calculator to Air Handler Sizing
Once you know required supply airflow from this calculator, feed that number into the AHU sizing calculator for the central air handling equipment serving the raised floor plenum, keeping in mind the plenum's low static pressure requirement differs meaningfully from a conventional ducted system's fan sizing assumptions.
Frequently Asked Questions
Underfloor air distribution supplies conditioned air through a raised floor plenum into the occupied zone through floor-mounted diffusers, rather than delivering air from ceiling diffusers as conventional overhead systems do. Air rises through the space via natural buoyancy from heat sources before being exhausted near the ceiling, similar in concept to displacement ventilation but delivered through floor diffusers rather than low sidewall units, and typically at a somewhat higher supply air temperature than displacement systems.
Underfloor air distribution plenums typically operate at low static pressure, commonly 12 to 25 pascals, considerably lower than conventional overhead ductwork. This lower pressure requirement reduces fan energy but means plenum design needs careful attention to minimize pressure drop from obstructions like cabling, structural elements, and long distribution distances within the plenum cavity.
UFAD systems typically supply air at 18 to 20 degrees Celsius, warmer than the 12 to 14 degrees Celsius common in overhead systems, because the air enters directly into the occupied zone near people's feet rather than being thrown and mixed from ceiling height. A colder supply temperature at floor level would create noticeable cold drafts, so UFAD relies on a larger total airflow at a smaller temperature difference to deliver the same cooling capacity.
UFAD systems offer occupants individual control over floor diffusers near their workstation, simplify future office reconfiguration since diffusers and often power and data cabling share the same raised floor cavity, and can improve ventilation effectiveness since fresh air enters near the breathing zone rather than mixing with the entire room volume from overhead. The tradeoff includes plenum construction cost, careful attention to plenum air leakage and cleanliness, and coordination between mechanical and structural design for the raised floor system.
Related HVAC Tools
Related buoyancy-driven strategy
Central air handler for the plenum
Zone cooling load input
Common UFAD application
Ventilation air pairing option
Free cooling for your climate
Alternative zone control approach
Reduce ventilation energy penalty