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UFAD vs. Displacement

Both Underfloor Air Distribution (UFAD) and Displacement Air Distribution are well defined in the 2021 ASHRAE Handbook Applications, Chapter 20, Space Air Diffusion. Generally, the idea for UFAD is to have a well-mixed lower level in the occupied space up to about 4.5 feet above the floor. From the thermocline at 4.5 feet to about 3 to 5 inches from the ceiling is a stratified space. Above the Stratified space, the upper region is the warmest and most polluted air in the room. It is being induced into the return air space or ducts at the ceiling and returned to the air handler.