Heat transfer pathways in underfloor air distribution (UFAD) systems
This paper reports on a modeling study to investigate the primary pathways for heat to be removed from a room with underfloor air distribution (UFAD) under cooling operation. Compared to the standard assumption of a well-mixed room air condition, stratification produces higher temperatures at the ceiling level that change the dynamics of heat transfer within a room as well as between floors of a multi-story building. A simplified first-law model has been used to estimate and compare the relative magnitudes of the heat being removed from a room through two primary pathways: (1) heat extraction via warm return air exiting the room at ceiling level or through the return plenum and (2) heat entering the underfloor supply plenum either through the slab from the floor below or through the raised floor panels from the room above. Surprisingly, it is shown that up to 40% of the total room cooling load is transferred into the supply plenum and only about 60% is accounted for by the return air extraction rate. The implications for the design and operation of UFAD systems are discussed.