UFAD Systems: Benefits & VAV Recommendations.pdf

VAV is better and the first cost premium is small compared to manual control options. Most buildings using UFAD systems have midsize to small floor plate sizes, with many LEED certified buildings being thin rectangles or assemblies of thin rectangles. This geometry provides better daylighting and is generally more attractive. For a 2500M2 building floor plate, the outer 5M on all four sides is likely a perimeter zone, requiring VAV, thermostatic controlled systems. Of the remaining 1150M2, 500M2 is devoted to core areas, leaving 650M2 for VAV or manual control diffusers. Manual diffusers are only attractive due to their low first cost, with no benefits not included in a VAV diffuser with manual option. Interior zones are candidates for manual diffusers due to lower loads and less variability. However, with average values and a sample floor plate, only 13% of diffusers could be manual control. The cost difference between all VAV and a VAV and manual system is less than 10%. On a "% of total construction cost" basis, the added cost of all VAV is 1/10th of 1%. The most expensive part of a building's operating cost is the people working in the occupied space. Improving the facility environment to improve people efficiency is more important than energy cost or first cost of construction. Raised access floors and UFAD have a positive and measurable impact on the facility environment. The most basic thing a VAV diffuser does is automatically adjust itself to adapt to changing air temperatures and loads in the occupied zone. These changes often move in opposite directions from desirable. The air temperature under a raised floor in a UFAD system is often set at 17°C, but can vary from 17°C to 23°C due to factors such as distance from the outlet, duct velocity, AHU setpoint, and cooling load above the floor. The interzone load might change from an unoccupied cubicle to a fully occupied one, with loads varying from 5 to 13 M3/hr/M2. With these average values and a sample floor plate, the perimeter zones require 4590 M3/hr and the interior zones require 585 M3/hr. If the perimeter and core area diffusers are the same size, only 13% of the diffusers could be manual control.