Underfloor air distribution (UFAD) works well in theory but not always in application.
Conceptually, air conditioning is distributed through an underfloor cavity and floor-mounted diffusers rather than the conventional array of overhead ducting and diffusers. Building occupants can then regulate airflow within a controlled zone by adjusting the applicable air diffuser.
The raised floor configuration is also convenient for housing communication cabling, wiring and even utility piping, which can more easily be installed, maintained and retrofitted through removable floor panels.
However, design assumptions didn’t always translate into reality in many early UFAD systems. Disruptions in pressurization, turbulence and temperature gains (known as thermal decay) as air travelled through underfloor plenums compromised performance. Energy gain combined with poor distribution, resulting in temperature variables of as much as -12 C to -15 C at discharge points, which was far from the designers’ goal of a -16 C to -17 C variable across the entire space.
In a typical example, the HVAC system would be tasked unnecessarily to satisfy perimeter set-points in warmer areas requiring more cooling. Since building perimeter zones require roughly three times more cooling per square foot than interior space, occupants within zones nearest the HVAC system discharge would be chilled from the cooler temperatures.
As a solution, textile ducts are now being introduced inside the UFAD to transfer and distribute the supply air to exact zones of the system. Recent tests have shown textile ducts can be more energy-efficient because the linear dispersion configuration distributes air uniformly over the length. This results in better mixing than in metal ducting with registers spaced every 10 feet.
Properly mixed airflow within the plenum improves performance of floor-mounted diffusers by avoiding warmer temperatures that can cause vertical throw and de-stratification. A combination of non-vented and vented sections of an underfloor textile duct system can be configured to distribute airflow nearest the perimeter to serve the higher demand zone first. As airflow transfers to interior zones, the thermal decay warms up the airflow and reverses the cold core/warm perimeter syndrome that plagues UFAD systems.
Nick Paschke is new product sales manager with DuctSox Corp., a manufacturer of textile air dispersion products.