INNOVATIONS
Volume 2, 2004

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We're Building System Performance™ ·  TE High-Performance Buildings™ 
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 TOP 10 INNOVATIVE ENGINEERING APPROACHES

We're counting down our Top 10 list in INNOVATIONS

#3 - Underfloor, Displacement, and Ductless 
Air Distribution Systems

A trio of related HVAC systems is available to the high-performance system designer.  All offer many performance and cost advantages over conventional ducted overhead air distribution systems in commercial and institutional buildings.

Underfloor air distribution (UFAD) systems use the plenum under a raised floor to deliver supply air to the occupied space.  Little or no ductwork is required between the air handler and the space.  A combination of active diffusers (like variable-air-volume boxes) and passive diffusers (manually-adjustable orifices) in the floor tiles injects cool supply air from the plenum into the space.  The active diffusers provide temperature-controlled zoning.  The passive diffusers provide a constant flow of air to zones that have a relatively constant cooling load.

The enhanced performance of UFAD systems results from the way air is introduced into the occupied space.  As the air moves from the floor level to the high-side-wall or ceiling return grilles, it flushes contaminates from the breathing zone.  The slow, upward velocity assists thermal plumes rise toward the ceiling from people and heat-producing office equipment.  The air temperature becomes stratified (warm at ceiling), but is comfortable in the occupied zone. 

The result is better indoor air quality, greater system efficiency, and enhanced comfort: 

• 15-20% less outdoor ventilation air is required (the clean air is in better contact with the occupants than an overhead system)

• Less supply air flow (heat from people, equipment, overhead lights becomes a return-air load, not a space load;  also, since the supply air is in closer contact with the occupants than an overhead supply air system, the same cooling effect can be achieved with less flow)

• Higher supply air temperatures, making the cooling equipment more efficient

• Fan power may be lower (low pressure drop of the plenum path)

A significant benefit of the raised floor is easier reconfiguration of wiring systems (data, phone, controls) and zoning as workplaces are moved.  In some cases, this alone justifies the cost of the raised floor system.

First costs are slightly higher for UFAD systems due to the cost of the raised floor.  The floor cost can be offset by less ductwork, no ceiling system, lower-cost wiring methods, less floor prep.  Lower floor-to-floor heights are possible since the reduced ductwork allows mechanical/electrical/data systems to be integrated into a shallower space than a conventional ceiling plenum.  Some underfloor ductwork is required since the air cannot travel long distances due to interferences with cabling and other systems.

A variation on UFAD system design is to use the UFAD system to only deliver ventilation air and moisture control (humidification/dehumidification).  The remainder of the space conditioning is done by another sensible-only system -- radiant ceiling panels, fan coils, rooftop units, heat pumps.  This combination DOAS (Dedicated Outdoor Air Systems)/UFAD  ensures that the proper amount of outdoor air is delivered to all spaces with easy integration of energy recovery.  Almost all ductwork can be eliminated since the low volume of ventilation air can travel greater distances through the underfloor plenum. 

Displacement ventilation (DV) systems are similar to UFAD systems except that a raised floor is not necessary.  Supply air is introduced to the space at very low velocities, usually from side-wall grilles near the floor.  The cool, dense supply air spreads across the floor, then rises through the breathing zone due to buoyancy effects, aided by thermal plumes from heat-producing objects.  The supply air temperature is very close to the space temperature.  The quantity of air delivered is usually low -- in some cases low enough that it is merely the required ventilation air (100% outdoor air).    This system is generally more applicable to spaces with low loads and high ceilings.

Overhead ductless systems, now available from at least one manufacturer, are like upside-down UFAD systems.  Like UFAD systems, they offer air delivery without the expense or pressure drop of a sheetmetal duct system.  Two plenums, one for supply and one for return, are stacked one above the other.  The return plenum is on top, against the slab of the next floor.  The supply plenum is directly below the return plenum, separated by an intermediate suspended ceiling.  The normal suspended ceiling forms the bottom of the supply plenum.  Special electronically-controlled diffusers that pulse open and closed every few seconds are used to modulate the flow of air to each zone in response to the load.  Other systems (wiring, plumbing, sprinkler piping) can be routed through the supply or return plenums without the usual interference from air distribution components.  In a variation of the concept, the return plenum can be omitted if air can be returned to the air handlers via side-wall or ceiling grilles near the air handler room. 

First costs are slightly lower than a conventional VAV system, particularly if the return plenum can be eliminated.  The savings in ductwork, VAV boxes, and controls is greater than the cost of the intermediate suspended ceiling.  Reconfiguration is easy, since the diffusers can be moved from tile-to-tile or added as needs change instead of modifying sheetmetal.

Summary:  While ductless systems are not the best solution in all commercial and institutional buildings, they can provide significant benefits in some cases.  Careful attention must be paid to design details such as outdoor air distribution to occupants, humidity control, local fire-protection codes, plenum cleanliness, and water removal after a sprinkler discharge.  This trio of air distribution concepts offers more ways to deliver high performance.

Project Spotlight:  St. Elizabeth Medical Center
Utility Plant Expansion

ThermalTech just completed another Engineer-Led Turnkey^TM project for St. Elizabeth, the largest private health care facility in the Northern Kentucky area. 
  

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