How do I calculate space cooling and heating loads in a building with UFAD?
A. In his book Underfloor Air Distribution Guide, (ref pages 158-162) Fred Bauman points out that while cooling and heating loads are calculated in a similar fashion as that of overhead distribution, there are differences. The most important is that because UFAD systems rely on stratification, it is not correct to assume a completely mixed room. Stratification will divide the room into an upper and lower zone. In general, once the room air has risen above this stratification height, it will not reenter the lower zone, and herein lies the fundamental difference from the fully mixed room assumed in an overhead system. As such, convective heat gains from sources above the stratification height will be exhausted directly into the ceiling return and not included in the airside calculations. The solution is to use the assumption of two distinct mixed zones, which should yield results that the total CFM required is basically the same as an overhead distributed system.
