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Sample Plans and Sections

The use of partitions to divide the underfloor plenum into separate zones (e.g., interior and perimeter) is comparable to the conventional approach to zoning with overhead air distribution systems. It enables supply air of different temperatures or volumes to be delivered to different spaces within the overall plan. Diffusers within the same zone typically receive supply air of similar temperature and pressure.

In addition to being either partitioned or open-plan, plenums can operate as pressurized or zero-pressure. Pressurized plenums have a small positive static pressure applied, created by a central fan in the air-handling unit (AHU), that drives the air up through the diffusers to the lower pressure surroundings of the conditioned office space. Standard methods of control at the central AHU are used to maintain the desired overall supply volume and temperature under thermostatic control, and a pre-set pressure level, at which the diffusers achieve optimum performance. The low operational pressure of underfloor plenums, compared to ceiling-based systems, potentially reduces fan energy consumption and thus operating costs.

In zero-pressure plenums, small local fan units draw air from the underfloor plenum and deliver it to the space through floor diffusers or outlets located in the furniture or partitions. Compared to passive diffusers, used with pressurized-plenum systems, fan-powered outlets are generally more easily and effectively controlled by nearby occupants. However, for reasons of reduced maintenance, installation and equipment costs, pressurized-plenums are the most commonly installed systems at present.

Air is typically extracted from the room through return grilles located at ceiling level using a conventional ceiling-return plenum. If no plenum is present, air can be returned through high side-wall grilles. The return air is then ducted back to the AHU where it is either exhausted to the outdoors, or filtered and dehumidified as necessary in the AHU before mixing with the supply air and re-entering the underfloor plenum. In some configurations, a portion of the return air is re-circulated directly back into the underfloor plenum where it is thoroughly mixed with incoming primary air from the AHU.

Some portion of each floor in a typical building will be devoted to accommodating the mechanical plant risers, equipment (e.g. the AHU) and circulation shafts such as stairs and elevators. In the plans indicated this area is placed as centralized at the core of the floor plate, however this is not always the case. Other possibilities include a distributed or perimeter service area, for example.

Configuration Examples

Shown below, in plan view and corresponding floor-to-floor section, are three different pressurized-plenum configurations. The first two examples represent earlier UFAD designs in which the more extensive use of ductwork and partitions within the plenum was a more common practice.  It is now recommended to minimize to the extent possible, the amount of installed ducts, air highways, and other HVAC-related components in the underfloor plenum so that it can serve as a highly flexible and accessible service plenum.  For any particular building project, the most suitable configuration will be determined by a number of parameters unique to that project - such as floor plate area, site-specific climate, or anticipated HVAC loads and their distribution, for example. For this reason it is important that decisions as to the plenum configuration be made with respect to the project's characteristics. In addition, please note that the drawings shown below represent only three of any number of possible underfloor plenum designs.

  1. Multi-zoned with extensive perimeter ductwork
  2. Multi-zoned with limited perimeter ductwork
  3. Open-plan plenum

1. Multi-zoned with extensive perimeter ductwork
This approach has been used in many existing underfloor installations and uses a similar control strategy to overhead variable-air-volume (VAV) systems. Conditioned air is supplied to interior and perimeter zones through an underfloor supply duct encircling the interior/perimeter boundary. In addition to defining the interior and perimeter, underfloor sheet metal partitioning is used to create smaller zones around the perimeter of the floor plan, as well as other zones as necessary. Using a conventional variable-air-volume (VAV) control strategy, air flow to each zone is controlled by VAV terminal units in response to zone thermostats. Supply air is delivered to all zones through passive (swirl or other non fan-powered) diffusers. Nearby occupants have some amount of control over these floor diffusers. In this example, heating in the perimeter zones is accomplished with reheat coils (electric or water-based) in the terminal units. The large amount of perimeter ductwork may restrict the flexibility to reconfigure plenum services in the future.

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2. Multi-zoned, limited perimeter ductwork
This approach also uses underfloor partitioning to divide interior and perimeter zones. However, the amount of underfloor ductwork is reduced by relying on a single feeder duct to supply conditioned air to each of the four large perimeter zones, as shown. Standard methods of control at the central AHU are used to deliver a constant volume, variable temperature supply of air to the interior zone. Air flow from individual diffusers in the interior zone may be controlled by nearby occupants. Air flow to the perimeter zones is delivered through linear grills and controlled by VAV terminal units in response to zone thermostats. Perimeter finned tube heating coils, located directly under the linear grills in the underfloor plenum, are used during the heating mode.


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3. Open-plan plenum
Conditioned air from a central source is distributed to diffusers across the entire floor area through a single, undivided pressurized plenum. Standard methods of control at the central AHU are used to maintain the static pressure in the plenum at a constant level. Supply air is delivered to the interior zone through passive diffusers that can be controlled by nearby occupants. During cooling mode in the perimeter zone, supply volume through the diffusers is automatically controlled in response to zone thermostats. During heating mode, a fan coil unit is activated, drawing inlet air through ducts from designated floor diffusers, and discharging the supply air through ducted diffusers near the building facade. These inlet and discharge diffusers are typically closed off to the underfloor plenum to isolate the heating operation from the cooler plenum air. When called for by the zone thermostat, a reheat coil in the fan coil unit provides additional heating capacity.


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