Design Guide to Underfloor Air Distribution 1 Design Guide to Underfloor Air Distribution INTRODUCTION types of office moves is often referred to as churn. The concept of underfloor air is not new, however changes in office space usage, The costs associated with these churn moves sustainable design and indoor air quality are one of the largest costs an owner / tenant issues have sparked considerable recent may face. The costs associated with cabling interest in the concept. Underfloor air moves and to a lesser extent HVAC system conditioning had its start in computer rooms, are the major portion of these move costs. at a time when mainframe computers So a system that could greatly simplify the generated considerable heat and had a costs associated with churn moves could considerable amount of complex cabling help reduce total system life cycle cost. The required by the computers. Access floor advantage of access flooring is that cable systems allowed plenty of open space to run and HVAC moves are greatly simplified. cabling and a generous pathway to supply large quantities of cooling air under the intense heat of the electronics. The natural Improved comfort convection currents of warm air rising allowed cool air to enter low, cool the The second feature of underfloor air equipment and remove the warm air near the distribution (UFAD) was that the same ceiling. Of course today’s computer rooms principle of warm air rising from hot have changed and the need for this type of electrical gear could also be applied to warm cooling has decreased. However, access air around people. BOMA (Building Owners floor systems have found new markets. and Managers Association) reported in their “What Tenants Want Survey” that thermal and indoor air quality concerns were two of Reduced life cycle cost the top concerns and least met expectations of tenants. Traditional overhead air The recent interest in access floor systems distribution systems are designed to do a has been spurred by other applications that very good job of mixing the air in the space can make use of the two principles that these and prevent stratification in the room. earlier computer room systems addressed. Without increased complexity and cost First, the large open floor plenum was a delivering thermal comfort to every convenient space to run large amounts of occupant while also providing personal cable for power and communications. The ventilation may be cost prohibitive. greatest change in offices in the last 15 years Overhead systems may require more fan has been an ever-growing need for voice, energy to overcome static losses of data and power connections to every generating the mixing, airflow patterns worker’s workstation. This coupled with one required within the space. other trend in office space design that is the use of more open plan space and a trend to Delivery of ventilation air requires sufficient move workers into cross-functional mixing to assure that the ventilation workgroups with great regularity. IFMA component is delivered to the occupied (International Facility Management breathing zone. By applying the warm air Association) reported in one of their recent rising principle, air can be provided below surveys that on average an office worker the occupants and discharged directly into experiences a move every 5-½ months; this the breathing zone at relatively low velocity. is 44 percent move rate. The term for these The people warm the air and by natural 2 Design Guide to Underfloor Air Distribution convection the air rises toward the ceiling. comfortable, and provides better ventilation Since people only breath air in a zone from and improved indoor air quality. approximately the floor to 6 feet, the space above this zone can be treated as a stratified The application of an underfloor air air layer and the load components in this distribution system requires the system zone treated differently. The result is that air designer to view the design very differently. provided underfloor can be supplied at low It requires special consideration throughout pressure and the energy for space the design process from design schematic conditioning can be reduced. The ventilation stage, to load estimate, to commissioning. air can be provided in the zone where it is This document is intended to provide the needed most with pollutants moved gently designer with some of the guidelines to toward the return. Additionally, adjustable follow in systems design of underfloor air diffusers, which discharge air to small areas distribution, UFAD. Further in depth in the space, may be adjustable by the analysis of all aspects of UFAD design can occupant and can result in an improvement be found in the ASHRAE UFAD Design in personal thermal comfort. This results in Guide. a system that can be more energy efficient, Figure 1 – Typical Underfloor Air Distribution System 3 Design Guide to Underfloor Air Distribution UFAD System Benefits without primary usage of the flooring systems is to special air-handling provide a service and utility space to run cabling for voice, data and power. As a Several types of underfloor system designs result, the floors can be as little as 4 inches exist. This manual covers a system offered over the structural floor or they may be by Carrier Corporation (see Fig. 3c). This much higher. When the floor space is to be approach employs standard air handling used as a supply plenum for air distribution, equipment and uses duct supply floor heights are normally at 12 to 18 inches temperatures in the normal air conditioning high. range, which can best handle spot cooling requirements that often arise on every project. The air is distributed in the space through a pressurized plenum with swirl style floor grilles. The plenum is supplied through a zone parallel fan power box with a special DDC control to modulate temperature and controls plenum pressure. The primary damper provides the minimum ventilation air to the zone and maintains plenum pressure. The ECM style fan motor in the box modulates to mix return and Figure 2 – Access Flooring System primary air to maintain required temperature. Heating and cooling Access floor systems are successfully requirements for specialized high load applied to many types of buildings both new spaces and perimeter zones are handled with construction and renovation. However, when conventional overhead systems, underfloor applied to retrofit situations a few special series fan boxes or fan coil units. considerations are required. Elevator lobbies and bathrooms which are built on the APPLICATION original structural floor need to be raised to the new floor elevation or ramps provided to CONSIDERATIONS get from the structural floor heights to the raised floor height. Access flooring systems An access floor is a flooring system that is In new construction one architectural installed above the standard structural floor consideration of the floor systems is that a of the building. We will reference Haworth ceiling space could be reduced allowing a Access Flooring System in our discussions floor-to-floor height one foot less than (see Fig 2). The systems consist of a matrix standard. This can be accomplished by of adjustable pedestals mounted normally on either the use of open ceiling space with two-foot centers, which support the weight only lighting and sprinklers in the space, or of the access floor panels and the loads on by reducing the ceiling plenum space since the floor above. The two-by-two, large supply ducts are not required. Then for approximately 1-1/8 inches thick concrete every 12 stories of building an additional floor panels are laid into this grid system. story could be added with minimal The structural floor below is normally sealed additional cost in the building shell cost. In to provide a moisture and dust barrier. The buildings less than 12 stories the reduction 4 Design Guide to Underfloor Air Distribution in height would result in a reduction of 3.) The third concept is to provide overall building shell cost. airflow from underfloor that is controlled at the workstation. This method is called Task Ambient Basic Concepts Conditioning. The control is A traditional underfloor air distribution normally accomplished with a small system uses the floor space between the fan that directs an air jet at the structural floor and the access floor as a occupant within the workstation. supply air plenum. We will use the UFAD approach in this There are three concepts normally manual. The plenum may be configured associated with providing floor level using one of two approaches. Either the cooling, each with distinctive entire underfloor plenum is pressurized to a characteristics: relative low pressure of approximately 0.10 1.) Displacement ventilation discharges inch WG or air is discharged into the air horizontally near the floor at very plenum at zero or neutral pressure and fans low velocities and near laminar flow near the outlets draw the supply air from the conditions. The goal is to use only plenum and discharge it to the space. the buoyancy effects to create air Neutral pressure plenums have a reduction motion within the space and in leakage around floor tiles and plenum maintain the stratification layer penetrations, but fan wiring and energy costs above the controlled zone that is not negate some of the system’s inherent mixed. While this provides excellent benefits. The use of the underfloor plenum ventilation effectiveness in the has two major impacts on system design. occupied zone it is not the optimum solution for thermal comfort. This is First, since the air is brought in at floor because of wide temperature level, a chance exits for a cold floor or for variance, greater than the, ASHRAE cold discharge temperatures near the (Std 55) accepted, 3 to 5 degree occupant’s feet.