Textile Troubleshooting for Under-Floor Air Distribution
By Nick Paschke
Under-floor air distribution (UFAD) works well in theory, but not always in application. Conceptually, air conditioning is distributed through an under-floor 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 under-floor plenums compromised performance. Energy gain combined with poor distribution, resulting in temperature variables of as much as five to 10 degrees Fahrenheit at discharge points, which was far from the designers’ goal of a one- to three-degree 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.
Notably, an office building in Dallas, Texas reported a 12-degree Fahrenheit temperature differential within its 25,000-square-foot floor plate and UFAD. As a result, the air-conditioning system was deployed at 100% fan and cooling capacity, creating higher energy costs.
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 that 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 under-floor 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.
In the Dallas office building example, introduction of the under-floor textile duct narrowed the temperature differential from 12 to four degrees F. With the cool air being distributed at the perimeter zone, the building operator could reduce the fan speed to 70% and cooling capacity to 60%, providing some operational budget relief.
Colliers International recently specified textile duct inside the UFAD system in a new 120,000-square-foot office tower for a Toronto area software developer. This follows the experience in a circa-2005 property the same tenant occupies, where ongoing temperature fluctuations are attributed the UFAD design. The facilities managers are now contemplating a retrofit/upgrade to install textile ducting.
“We considered several options to improve UFAD's temperature control, but I really like the idea of textile duct because, so far, it appears to curb thermal degradation and offer so much more installation and reconfiguration flexibility than metal duct runs and other alternatives,” says Bill Blackburn, Vice President of Design and Construction with Colliers International.
Textile duct will also be used in the UFAD system slated for the Centre for Interactive Research on Sustainability (CIRS), now under construction on the University of British Columbia campus. The 62,000-square-foot (5,675 square metres) building will be one of a handful of regenerative buildings in the world and has been billed as the most sustainable building in North America.
The engineering firm, Stantec, specified UFAD for the living environmental laboratory's four floors. Additionally, textile duct was specified to increase air dispersion efficiency and indoor air comfort.
To the south, textile duct is part of the UFAD in Seattle University's $55 million library renovation and addition. This helps maintain tight temperature control tolerances, thus reducing energy costs, according to Seattle University's lead buildings control technician, Patrick McCurdy. While a conventional overhead system typically supplies 13 °C (55°F) air and cools from top to bottom, the library's HVAC equipment supplies warmer 18 °C (65°F) and uses air displacement to cool the bottom five feet of the addition's 18-foot-high areas.
Nick Paschke is New Product Sales Manager with DuctSox Corp, a manufacturer of textile air dispersion products. For more information, see the web site at www.ductsox.com. The Iowa State University study Thermal Comparison Between Ceiling Diffusers and Fabric Ductwork Diffusers for Green Buildings can be found at http://www3.me.iastate.edu/bglab.