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Design Phase Guidelines

Although the number of publications on underfloor technology has been on the increase in recent years, until recently there were no standardized design guidelines for use by the industry. Designers having experience with underfloor systems had largely developed guidelines and practices of their own. To address this situation, the American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) initiated a research project to develop and publish a design guide on underfloor air distribution (UFAD) and task/ambient conditioning (TAC) systems. CBE completed this project in December of 2003, and the design guide is now available from ASHRAE.

This section was developed before the completion of the Underfloor Air Distribution (UFAD) Design Guide published by ASHRAE. Please refer to the design guide for the most comprehensive and up-to-date guidelines.

UFAD vs. Conventional Overhead System Design
UFAD systems are quite similar to conventional overhead systems in terms of the types of equipment used at the cooling and heating plants and primary air handling units (AHU). Key differences arise with UFAD systems in their use of an underfloor air supply plenum, localized air distribution (with or without individual control) and the resulting floor-to-ceiling air flow pattern, and the solutions used for perimeter systems. Although the methods used to calculate cooling and heating loads for a building with an UFAD system are similar to those used for conventional overhead systems, the calculation of design cooling air supply volumes requires different considerations. This is primarily due to the fact that overhead systems assume a single well-mixed temperature throughout the space (floor-to-ceiling), while UFAD systems assume that some amount of stratification occurs. The stratified floor-to-ceiling air flow pattern in UFAD systems allows most convective heat gains from sources outside the occupied zone (up to 1.8 m [6 ft]) to be exhausted directly at ceiling level, and therefore to not be included in the air-side load (see further discussion below). In order to successfully employ an UFAD and raised access floor system, it is essential that the implications of these differences be considered, starting at an early stage in the design process. The following is a step-by-step list of issues to be considered, and decisions to be made, during the design process. Click on any step listed below for more information.

UFAD System Design Process:

1. Initial Building Design Considerations
2. Select System Configuration
3. Determine Space Cooling and Heating Loads
4. Zoning
5. Determine Ventilation Air Requirements
6. Determine Zone Supply Air Temperature and Flow Rate
7. Determine Return Air Configuration
8. Calculate Cooling Coil Load
9. Layout Ducts and Plenum Configuration
10. Select Primary HVAC Equipment
11. Select and Locate Diffusers
12. Develop a Control Strategy
13. References

1. Initial Building Design Considerations

1A. Building Section
In new construction, underfloor air distribution can achieve a 5-10% reduction in floor-to-floor heights compared to projects with ceiling-based air distribution. This is accomplished by reducing the overall height of service plenums and/or by changing from standard steel beam construction to a concrete (flat slab) structural approach (see example table and figure below). A single large overhead plenum to accommodate large supply ducts can be replaced with a smaller ceiling plenum for air return combined with a lower height underfloor plenum for unducted air flow and other building services. Concrete flat slab construction is more expensive than steel beam construction, but is preferred for underfloor systems due to thermal storage benefits as well as significantly reduced vertical height requirements. In the example shown in the figure, the underfloor/concrete configuration allows 0.25 m (10 in.) to be saved in floor-to-floor height compared to overhead/steel beam system design. Even greater savings (up to 0.56 m [22 in.]) can be realized if the ceiling plenum is completely eliminated, exposing the concrete ceilings and providing an opportunity for greater architectural creativity. In this case, proper acoustic treatment must be incorporated into the new design.

Underfloor plenums accommodating both cable/electrical distribution and an UFAD system are often deeper than those employed solely for cable management purposes. However, the additional height required for acceptable air flow performance is not large, based on recent CBE research results [6].

The following table presents a comparison of typical floor-to-floor dimensions for a midsize (5-10 stories), high-tech class A office building (assuming a 40-ft clear span between columns) [7]:

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Underfloor plenum heights are usually determined by:

(i) largest HVAC components (e.g., fan coil units, terminal boxes, ducts, dampers) located under the floor;
(ii) requirements for underfloor cabling;
(iii) clear space for underfloor air flow (usually 76 mm (3 in.) minimum).

1B. Building Plan
The modularity of all components of raised access floor systems can be an advantage in space planning, particularly over large open plan areas. 

Consider the compatibility of anticipated building plan geometries with the following typical system component dimensions.

Raised access floor panel dimensions: 610 mm (24 in.) square 
Underfloor plenum pedestal spacing: as for floor panels, e.g. 610 mm (24 in.)
Typical perimeter zone size (occupied area): 4–5 m (12–15 ft) deep, from the external wall.

The largest dimension of system components (e.g., ducts) that can reasonably fit between underfloor pedestals is 560 mm (22 in.). Where structural members/ducts cross the pedestal grid, metal bridging can be installed, spanning from one panel to another in place of a pedestal.

2. Select System Configuration
When configuring an UFAD system, there are three basic approaches: (1) pressurized plenum with a central air handler delivering air through the plenum and into the space through passive grills/diffusers; (2) zero-pressure plenum with air delivered into the conditioned space through local fan-powered (active) supply outlets in combination with the central air handler; and (3) in some cases, ducted air supply through the plenum to terminal devices and supply outlets. In the discussion below, we will focus on the two plenum supply configurations, as guidelines for fully ducted systems are well established.

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2A. Pressurized Underfloor Plenum
Supply air that has been filtered and conditioned to the required temperature and humidity, and includes at least the minimum required volume of outside air, is delivered by a conventional air handling unit through a minimum amount of ductwork to the underfloor plenum. The number of plenum inlet locations is a function of the size of control zones, access points available in the building, amount of distribution ductwork used under the floor, and other design issues. Within the underfloor plenum, it is always desirable to the extent possible to have the supply air flow freely to the supply outlets.

Pressurized plenums are designed to deliver supply air at constant temperature and volume to all passive outlets of the same size and control setting within the conditioned space.

Because the supply air in the underfloor plenum is in direct contact with the concrete structural slab, a number of energy- and cost-saving strategies are possible with UFAD systems. Such thermal storage strategies should be implemented with great care. See further discussion under section 12 - Develop a Control Strategy.

The underfloor thermal mass also has the effect of providing a consistent cool air temperature reservoir (for cooling applications), making UFAD systems extremely stable in their operation.

Almost all available floor diffusers allow some amount of individual control (usually volume and in some cases direction) by nearby occupants.

There are several approaches to address zones with significantly different thermal loads:

  • plenum partitioning with ducted VAV devices supplying air to each zone; 
  • plenum partitioning with fan-powered terminal devices supplying air to each zone; 
  • thermostatically controlled VAV diffusers may be used in both partitioned and open plenums; 
  • local fan-driven supply outlets may be used in both partitioned and open plenums;
  • open plenums with mixing boxes and ducted outlets.

Zoning and partitioning of the underfloor plenum should be kept to the minimum necessary to optimize UFAD performance and efficiency, as this helps to maintain the plenum for it's intended purpose: to serve as a highly flexible and accessible service plenum.

There is some evidence from completed projects that uncontrolled air leakage from the pressurized plenum can impair system performance. It is important that proper attention be given to the sealing of junctions between plenum partitions, slab, access floor panels, and exterior or interior permanent walls during the construction phase of the project. Due to the relatively low pressure (12.5 - 50 Pa [0.05 - 0.2 in. H2O]) used in pressurized plenums, proponents of pressurized plenums claim that leakage into adjacent zones is minimal, and much of the leakage (between raised floor panels) will be into the same conditioned zone of the building [5]. This is a design issue that is still in need of further investigation.

If access panels are removed for long periods of time, or the distance between primary air inlets and supply diffusers is too great, control of air flow will be diminished.

Although not a requirement, some designers recommend limiting the size of underfloor zones (partitioned or otherwise) that are served by a single ducted primary air inlet, from the air handler (see below). This ensures the system's ability to avoid unacceptable variations in supply air temperature (due to heat gain from or loss to the concrete slab and raised floor structure) and quantity (due to pressure loss, obstructions, or friction) within the zone. 

In some system designs, using multiple medium or small-sized (floor-by-floor) AHU’s can minimize or totally eliminate ductwork; and improve zone control when AHU capacities correspond to the specific requirements of each plenum zone. 

Some designs have resulted in higher pressures in the plenum on the order of 125 Pa (0.5 in. H2O); these have resulted in problems with over-conditioning, lifting of carpets, and problems with diffusers.

Typical plenum pressure: 12.5 - 50 Pa (0.05 - 0.2 in. H2O)
In some cases, maximum floor area served by each
free-discharge supply air duct: 300 m2 (3,200 ft2)

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2B. Zero-Pressure Underfloor Plenum
Primary supply air from the central air handler is delivered to the underfloor plenum in much the same manner as with pressurized plenums. In this case, since the plenum is maintained at very nearly the same pressure as the conditioned space, local fan-powered supply outlets are required to supply the air into the occupied zone of the space.

Zero-pressure plenums pose no risk of uncontrolled air leakage to the conditioned space/adjacent zones.

The removal of floor panels does not disrupt overall supply-air flow.

Local fan-powered outlets under thermostatic or individual control allow supply air conditions to be controlled over a wide range as necessary. This controllability can be used to handle zones with significantly different thermal loads without underfloor partitioning. The use of partitioning for zone control can also be applied in a similar way as for pressurized plenums.

Zoning and partitioning of the underfloor plenum should be kept to the minimum necessary to optimize UFAD performance and efficiency, as this helps to maintain the plenum for it's intended purpose: to serve as a highly flexible and accessible service plenum.

Since the supply air in the underfloor plenum is in direct contact with the concrete structural slab, the same thermal storage strategies as with pressurized systems can be used (see section 12 - Develop a Control Strategy). Similarly, the frequency of ducted primary air inlets to the plenum must take into consideration the heat exchange between the supply air and the underfloor plenum structural mass.

By relying on both a primary air handler and local fan-powered outlets to draw air from the plenum into the space, zero-pressure configurations can more reliably maintain some amount of cooling effect if the chillers are off due to the thermal inertia of the concrete slab.

The greater ability of zero-pressure systems to provide localized cooling suggests their suitability in projects involving high and diversified heat loads.

In some system designs, using multiple medium or small-sized (floor-by-floor) AHU’s can minimize or totally eliminate ductwork; and improve zone control when AHU capacities correspond to the specific requirements of each plenum zone.

Compliance with local Fire Codes may require sprinklers and partitioning to be installed in all plenum configurations.

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3. Determine space cooling and heating loads
Cooling and heating loads for a building with an UFAD system are calculated in much the same manner as for a conventional overhead system. However, the determination of design cooling air quantities must take into account key differences between these systems.

3A. Cooling
The stratified floor-to-ceiling air flow pattern in UFAD systems allows most convective heat gains from sources outside the occupied zone (up to 1.8 m [6 ft]) to be exhausted directly at ceiling level, and therefore to not be included in the air-side load. Air supply volumes therefore only consider heat sources that enter and mix with air in the occupied zone. Heat sources must be analyzed based on their convective and radiative components, a subject addressed by [8]. Depending on the location of the heat source in the space, some amount of the convective portion can be neglected in this calculation. An overview of this design approach is described in [9].

Other issues that can affect load calculations include the heat exchange between the concrete slab and the supply air as it flows through the underfloor plenum. If the slab has absorbed heat, particularly from warm return air from the next floor down flowing along the underside of the slab, then supply temperature will increase with distance from the primary air inlet to the plenum. Supply air in the plenum will also be warmed by heat transfer from the room through the raised floor panels. Estimates of supply air temperature rise as a function of air travel distance in an underfloor plenum are given in [10]. This is also the subject of ongoing CBE research.

Another difference between UFAD design and overhead systems is consideration of heat loss through the access floor. This adds another component to the space cooling load calculation –- estimated to be as high as 0.1 W/m2 (1 W/ft2). Most of this heat transferred through the floor into the supply air stream will reenter the conditioned space, although not instantaneously due to the mass of the floor panels.

Research indicates that stratification for UFAD systems can result in overall delta T's (return-supply temperature difference) in the range of 8-11°C (15-20°F), for properly designed systems. However, these values are not what determine the air flow requirement. The heat gain to the occupied zone and the air flow required to maintain a given comfort condition in that zone is what determines the actual air flow requirements, and consequently the overall delta T that will be developed. Typically this results in air flow rates that can be equal to or less than that for overhead systems, even for higher supply air temperatures.

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3B. Heating
In most applications, heating is primarily needed only near the building envelope where cold downdrafts from perimeter glass may cause discomfort. Heating may also be needed in some top floor interior zones and during periods of low occupancy (e.g., nights and weekends). 

Effective heating systems isolate the source of warm air from the thermal lag effect of the concrete slab (which is usually slightly cooler than room temperature). This can be done, for example, by ducting from an underfloor fan coil unit, or by using baseboard radiation or convection units. Quick response on heating can be very important during morning startup.

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4. Zoning

4A. Perimeter Zones
The largest loads typically occur near the skin of the building. Since these areas are influenced by climatic variations, rapid fluctuations in heating and cooling demands can happen, with peak loads often occurring only for several hours per day and relatively few days of the year. Energy-efficient envelope design is always the first stage of defense against excessive perimeter loads.

Under cooling conditions, the warm interior surface temperatures at the perimeter (due to either highly absorptive glazing or intercepted solar radiation if blinds are present) will form a strong vertical plume that can be removed with only part of the load contributing to the air-side load.

Perimeter zone considerations often lead to hybrid system designs in which active, fan-powered supply units are used to increase the rate at which the system can respond to changes in load. Many perimeter zone solutions have been successfully applied in practice (see Plan Views for a few examples). Some manufacturers offer equipment and recommended configurations for perimeter systems [10, 11].

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4B. Interior Zones
Interior zones (defined as areas located further than 5 m (15 ft) from exterior walls) are usually exposed to relatively constant and lower (compared to perimeter zones) thermal loads (almost always cooling in typical office buildings). However, with modern energy efficient office equipment and high diversity rates of personnel, these loads can fluctuate significantly; control strategies and system design need to be well thought out to accommodate these conditions.

These zones can often be well served by a constant volume, or constant pressure in a pressurized system, control strategy. The need for dynamic control of these (typically) large zones is minimized due to the ability of occupants to make small local adjustments to individual diffusers. This configuration with a minimum amount of underfloor partitioning helps to maintain flexibility in the relocation of other services (e.g., cabling).

The interaction between interior and perimeter systems needs careful consideration. For example, if plenum air is used to supply cooling for perimeter zones, reset of SAT for the core zones may militate against being able to satisfy perimeter load conditions.

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4C. Other Special Zones
Other special zones having large and rapid changes in cooling load requirements, such as conference rooms or lecture halls, should incorporate fan-powered or VAV air supply solutions. This can require underfloor partitioning for these areas. Automatic controls to these zones should be capable of meeting both peak demand and significant turndown during periods of little or no occupancy. Manual control of these zones has also been used in some installations.

Partition construction: Vertical sheet metal dividers
Perimeter zone dimensions: Typically extending 4 – 5 m (12 – 15 ft) from external wall.

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5. Determine ventilation air requirements
Minimum outside air requirements should be determined according to applicable codes (e.g., ASHRAE Standard 62-1999).

Some improvement in ventilation effectiveness is expected by delivering the fresh supply air near the occupant at floor or desktop level, allowing an overall floor-to-ceiling air flow pattern to more efficiently remove contaminants from the occupied zone of the space. An optimized strategy is to control supply outlets to allow mixing of supply air with room air up to a height no higher than head level (6 ft [1.8 m]). Above this height, stratified and more polluted air is allowed to occur. The air that the occupant breathes will have a lower percentage of pollutants compared to conventional uniformly mixed systems.

If an enhanced air change effectiveness can be shown to exist in comparison to well-mixed overhead systems (see ASHRAE Standard 129-1997) future versions of ASHRAE Standard 62 may allow some credit to be taken, thereby allowing reduced ventilation air quantities. The magnitude of this improved air change effectiveness will be largest during times of outside-air economizer use. The fact that the number of hours of economizer operation is typically greater for UFAD systems also contributes to overall increased ventilation effectiveness.

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6. Determine zone supply air temperature and flow rate
Because the air is supplied directly into the occupied zone, supply air temperatures must be warmer than that used for conventional overhead system design. For cooling applications, supply air temperatures at the diffusers should be maintained no lower than in the range of 17 – 20°C (63 – 68°F) to avoid overcooling nearby occupants. This supply temperature can be reset even higher under partial load conditions.

Mixed air temperature after the cooling coil, or plenum inlet temperature, must be determined by taking into account temperature increase (or decrease, depending on the slab temperature) as the air flows through the underfloor plenum. Current estimates for typical air flow rates in an underfloor plenum with a slab that is 3°C (5°F) warmer than the plenum inlet air temperature call for a 1°C (2°F) increase for every 10 m (33 ft) of distance traveled through the plenum.

In temperate climates, where high humidity is not a problem, these warmer supply air temperatures increase the potential for economizer use, and allow higher cooling coil temperatures to be set, if desired.

Cooling air quantities for UFAD systems should be carefully determined. Higher supply air temperatures would suggest that higher supply air volumes are required, but the higher return temperatures created by stratification reduce the required increase in volume. Finally, as previously described, a calculation of the portion of heat sources that bypass the occupied zone in the space allows cooling air quantities to be further reduced. The net effect is that for most designs, controlled stratification in the space allows cooling air quantities for UFAD systems to be equal to or less than those required under the same conditions using overhead air distribution.

As discussed further below under controls, control strategies for temperature and flow rate will vary depending on the magnitude and variability of loads in each control zone, as well as other system design issues. 

Supply air temperature  - Return air temperature

UFAD 17-20°C (63-68°F)  - 25-30°C (77-86°F)

Overhead 13°C (55°F) - 24°C (75°F)

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7. Determine return air configuration
For optimal cooling operation of a UFAD system, it is important to locate return grills at ceiling level, or at a minimum, above the occupied zone (1.8 m [6 ft]). Air is typically returned through grilles located in a suspended ceiling or through high side-wall grilles if no ceiling plenum is present. This supports an overall floor-to-ceiling air flow pattern that takes advantage of the natural buoyancy produced by heat sources in the office and more efficiently removes heat loads and contaminants from the space.

A certain portion of return air is mixed with primary air from the AHU to achieve desired air temperatures and humidity, and enable reduced energy costs. In many climates to achieve proper humidity control, conventional cooling coil temperatures must be used (producing a coil leaving temperature of 12.8°C (55°F)). In this situation, a return air bypass control strategy can be employed in which a portion of the return air is bypassed around the cooling coil and then mixed with the air leaving the coil to produce the desired warmer supply air temperature (17-20°C [63-68°F]).

In some cases, a percentage of return air can be re-circulated directly back into the underfloor plenum via return shafts near the ceiling or from the ceiling plenum. Room air flowing back into the plenum through open floor grills can also serve as make-up air for zero pressure plenum designs when local fan-powered outlets require more air than that being supplied from the central AHU.

If re-circulation takes place directly in the underfloor plenum, the supply and return air streams must be well mixed within the underfloor plenum before delivery to the conditioned space. This can usually be achieved by distributing the primary air at regularly spaced intervals throughout the plenum, and/or employing fan-powered local supply units to aid mixing of primary supply air with the return air.

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8. Calculate cooling coil load
The calculation of cooling coil load is done in much the same manner as for a conventional mixing-type air distribution system. All of the loads in the space (some of which have bypassed the occupied zone, allowing a reduction in cooling air-side loads) must be handled at the coil.

Humidity control considerations require that, in more humid climates the cooling coil be operated to produce the necessary dehumidification of outside air resulting in a coil leaving air temperature of 12.8°C (55°F). This air is then mixed with warmer bypassed return air to produce the desired plenum inlet air temperature. It is important that these systems use enthalpy based economizer control to ensure proper supply air humidity control.

In temperate climates, the warmer supply air temperatures of UFAD systems increase the potential for economizer use and may also allow higher cooling coil temperatures than for conventional overhead systems, thereby increasing chiller efficiencies.

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9. Layout ducts and plenum configuration
Underfloor ducts serving specific zones should be sized to accommodate peak cooling loads. The capacity of the central chiller plant, air handlers and main duct risers can generally be reduced by accounting for time variations and load diversity (up to 50%).

The design and layout of main ducts from the central plant to plenum inlet locations is similar to that of conventional overhead systems except that access must be provided for the ducts to reach the underfloor plenum. The amount of main ductwork can be reduced in designs using medium to small-sized air handlers (floor-by-floor units) that are located closer to the point of use. However, ductwork for ventilation air is still required and must be sized accordingly if the use of an outside-air economizer will be an important operating strategy.

The amount of re-circulation ductwork can be reduced by taking some of the return air at ceiling level directly back into the underfloor plenum without returning to the air handler. For example, return air can be brought down induction shafts formed with furring spaces along structural columns. This alternative configuration of bypass control can be used as long as proper dehumidification is maintained back at the air handler and complete blending of return and supply air is achieved within the underfloor plenum.

The height of underfloor plenums is generally determined by the largest HVAC components located under the floor. These components can typically be distribution ductwork, fire dampers at plenum inlets, fan coil units, and terminal boxes. Commercially available HVAC products allow plenum configurations as low as 0.2 m (8 in.), although heights in the range of 0.31-0.46 m (12-18 in.) are more common.

In pressurized plenums as low as 0.2 m (8 in.), full-scale experiments have demonstrated that static pressure remains very uniform across the entire plenum, making balancing of individual diffusers unnecessary [6]. These same experiments also showed that solid obstructions, even with only 38 mm (1.5 in.) of clear space above them, may be located in a 0.2-m (8-in.) or higher plenum and have very little impact on the uniformity of the overall pressure and air flow distribution.

In both zero-pressure and pressurized plenums, the delivery of air through fan-powered outlets is even more reliable than that through passive diffusers in pressurized plenums. Active diffusers are less susceptible to pressure variations (such as when access floor panels are removed) and other flow restrictions.

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10. Select primary HVAC equipment
Due to the low operational static pressures in underfloor air supply plenums (typical pressures are around 25 Pa (0.1 in. H2O)), central fan energy use and sizing can potentially be reduced relative to traditional ducted overhead air distribution systems depending on the design strategy adopted (see [12] for a more complete discussion of this issue).

As discussed earlier, cooling air quantities, and therefore air handler capacities, for UFAD systems should be carefully determined and may be equal to or less than those required under the same conditions using overhead systems.

Although the total space cooling load is similar for both UFAD and conventional overhead systems, higher supply and return air temperatures in UFAD systems allow higher efficiency chillers to be used, when suitable weather conditions, and in particular humidity conditions, prevail.

Thermal storage strategies using nighttime precooling of the concrete slab in the plenum can reduce peak demand for mechanical cooling during the following day, thereby allowing refrigeration equipment to be downsized.

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11. Select and locate diffusers
The flexibility of mounting supply diffusers in movable raised access floor panels is a major advantage for UFAD systems. The inherent ability to easily move diffusers to more closely match the distribution of loads in the space makes the placement of diffusers a much easier task. In fact, initial layout can be done quite crudely. Final placement can take place after the location of furniture and loads, as well as the preferences of individual occupants, are more accurately determined.

11A. Passive Diffusers
Passive diffusers are defined as air supply outlets that rely on a pressurized underfloor plenum to deliver air from the plenum through the diffuser into the conditioned space of the building. Swirl floor diffusers, constant velocity floor diffusers, and linear floor grills are the primary types of floor diffusers (see below).

Diffuser Types:

Airflow pattern - Swirling upwards, rapid mixing.
Adjustability - Rotate grill or bucket to adjust air flow volume.
Ideal Location - Interior and perimeter zones.

Constant Velocity
Airflow pattern - Multi directional, air jet.
Adjustability - Adjust grill for changes to air flow direction, thermostat for air flow volume.
Ideal Location - Interior and perimeter zones.

Airflow pattern - Planar sheet, air jet.
Adjustability - Multi-blade damper is used to adjust air flow volume.
Ideal Location - Perimeter zones.

swirl floor diffuser       constant velocity floor                 linear floor grill

Swirl Floor Diffusers. This is the most commonly installed type of diffuser in UFAD systems; more models are commercially available than any other design. The swirling air flow pattern of air discharged from this round floor diffuser provides rapid mixing of supply air with the room air in the occupied zone. Swirl diffusers are generally installed as passive diffusers, requiring a pressurized underfloor plenum, although fan-driven models are available. Occupants have limited control of the amount of air being delivered by rotating the face of the diffuser, or by opening the diffuser and adjusting a volume control damper. 

Constant Velocity Floor Diffusers. This recently introduced diffuser is designed for variable-air-volume operation. It uses an automatic internal damper to maintain a constant discharge velocity, even at reduced supply air volumes. This is a passive diffuser (power is needed for the damper motor only), requiring a pressurized plenum, but fan-driven configurations are also available. Air is supplied through a slotted square floor grill in a jet-type air flow pattern. Occupants can adjust the direction of the supply jets by changing the orientation of the grill. Supply volume is controlled by a thermostat on a zone basis, or if available, as adjusted by an individual user. 

Linear Floor Grills. Linear grills have been used for many years, particularly in computer room applications. Air is supplied in a jet-type planar sheet making them well matched for placement in perimeter zones adjacent to exterior windows. Although linear grills often have multi-blade dampers, they are not designed for frequent adjustment by individuals, and are therefore not typically used in densely occupied office space. 

It is recommended that you contact the diffuser manufacturers directly to obtain the most up-to-date product information on passive diffusers. 

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11B. Active Diffusers
Active diffusers are defined as air supply outlets that rely on a local fan to deliver air from the plenum through the diffuser into the conditioned space of the building. Passive diffusers can generally be converted to an active diffuser by simply attaching a fan-powered outlet box to the underside of the diffuser or grill. Most manufacturers provide both passive and active diffusers.

In addition to the three types of diffusers described above for passive diffusers, several different designs for active diffusers are available. Fan-powered diffusers include:

Floor supply module, consisting of four round discharge grills (jet-type) mounted in a single raised floor panel. Fixed vanes in the grills are inclined at 40°, so that air flow direction can be adjusted by rotating the grills. A rotary speed control knob recessed into one grill allows air supply volume (fan speed) to be controlled [13-19]. Please check on the availability of this unit with the manufacturer. 

Desktop air supply pedestals (typically two), which are fully adjustable for air flow direction, as well as air supply volume by adjusting a control panel on the desk. Air is supplied from a mixing box that is typically hung in the back or corner of the knee space of the desk and connected by flexible duct to the two desktop supply nozzles. The mixing box uses a small variable-speed fan to pull air from the underfloor plenum and deliver a free-jet-type air flow from the nozzles [15, 18-22]. 

Under-desk diffusers, which consist of one or more fully adjustable (for air flow direction) grills, similar to a car's dashboard. A fan unit located either adjacent to the desk or in the underfloor plenum delivers air through flexible duct to the grills (jet-type) mounted just below and even with the front edge of the desk surface (other positions are possible). Two primary models are available. One allows adjustment of the fan speed to change air supply volume [18, 19], and the other uses a damper to adjust air supply volume towards the occupant while maintaining a thermostatically controlled total volume (through other supply outlets) into the space [23]. 

Partition-based diffusers, mounted in the partitions immediately adjacent to the desk. Air is delivered through passageways that are integrated into the partition design to controllable supply grills (jet-type) that may be located just above desk level or just below the top of the panel. Although uncommon in the U.S., some of these systems have been installed in Japan [24]. 

It is recommended that you contact the diffuser manufacturers directly to obtain the most up-to-date product information on active diffusers.

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12. Develop control strategy

12a Temperature, humidity, and volume control
As described earlier, since air is supplied directly into the occupied zone near floor level, supply outlet temperatures should be maintained no cooler than in the range of 17-20°C (63-68°F) to avoid overcooling nearby occupants.

To achieve the required higher supply air temperatures while still maintaining humidity control the following approach, called side-stream bypass, is often used. Cooling coil temperatures are typically in the range of 10-13°C (50-55°F) for dehumidification purposes. Only the incoming outside air and a portion of the return air is dehumidified (minimum amount needed for humidity control). The remaining return air is bypassed around the coil, if done at the air handler, and mixed with the cool primary air to produce supply air of the proper temperature and humidity before being delivered directly into the underfloor plenum. In this configuration, a range of coil temperatures can be produced, including low temperature air systems with or without ice storage.

An alternative bypass configuration involves re-circulating some of the return air directly back into the underfloor plenum where it is mixed with the cool primary air from the air handler. In this case, it is very important to ensure thorough mixing of primary and return air in the plenum.

In less humid climates, the warmer supply air temperatures increase the potential for economizer use and may also allow higher cooling coil temperatures to be used, thereby increasing chiller efficiencies.

A variety of operating strategies have been used in existing installations. A preferred method for interior zones is constant air volume, variable temperature (CAV-VT). Supply temperature is controlled based on zone thermostats. Occupants can make minor changes to local comfort conditions by adjusting a diffuser. 

Due to temperature stratification that naturally occurs with UFAD systems, thermostats should be carefully placed and their readings correlated with acceptable comfort conditions in the occupied zone. For example, if stratification is large, an acceptable temperature at the five-foot level may in fact coincide with uncomfortably cool temperatures at the ankle level. 

While the floor supply system is very well matched to the stable load profiles in most interior building zones, it is incompatible with the more rapidly varying loads found in perimeter and other special zones. These zones require a separate system, or at least a hybrid solution in which fan-powered outlets and/or additional equipment are used to address the extra heating and cooling requirements of these zones. 

Perimeter zone operation can be either variable air volume (VAV) or CAV-VT, but in either case must be able to respond to the special cooling and heating demands of these zones. Perimeter zone solutions have not been standardized in existing UFAD installations; many different approaches have been used. 

12B. Thermal storage control strategies
Since supply air flowing through the underfloor plenum is in direct contact with the concrete floor slab of the building, control strategies must consider thermal storage in the slab as well as other mass in the plenum (e.g., floor panels). UFAD systems are very stable in operation with only gradual (usually unnoticeable to occupants) changes in supply temperature over time, unless the supply air is isolated from the mass.

Energy and operating cost savings can be achieved using a thermal storage strategy in the concrete slab. In temperate climates, cool nighttime air can be brought into the underfloor plenum where it effectively cools the slab overnight. During the following day's cooling operation, higher supply air temperatures can be used to meet the cooling demand, thereby reducing refrigeration loads for at least part of the day. This 24-hour thermal storage strategy benefits from lower off-peak utility rates and extends the hours of economizer operation. For this strategy to be successful, the following issues must be addressed:

  • Heating night setback must be used.
  • Since a precooled slab will be at its coolest temperature in the morning, the design, capacity, and response rate of the heating system, if needed, will be particularly important under morning startup conditions.
  • Enthalpy-based economizer control must be used to maintain proper humidity levels of the incoming nighttime air and to protect against condensation in the plenum.
  • Lower limit control switches for both slab and space temperatures must be activated to prevent overcooling.
  • Preliminary estimates indicate that a precooled slab is most effective at reducing daytime cooling loads during morning hours only.
  • If the slab is not pre-cooled at night, then supply outlet temperatures will likely increase with distance from the primary air inlet to the plenum due to the effects of stored heat in the slab (particularly from warm return air from the next floor down flowing along the underside of the slab).

CBE is currently conducting research to investigate the thermal storage performance of underfloor air supply plenums.

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13. References
[1] Bauman, F.S., and E.A. Arens. 1996. “Task/ambient conditioning systems: Engineering and application guidelines.” Center for Environmental Design Research, University of California, Berkeley.

[2] McCarry, B.T. 1995. “Underfloor air distribution systems: Benefits and when to use the system in building design.” ASHRAE Transactions, Vol. 101 (2).

[3] Shute, R.W. 1995. “Integrated access floor HVAC: Lessons learned.” ASHRAE Transactions, Vol. 101 (2). 
[4] Houghton, D. 1995. “Turning air conditioning on its head: Underfloor air distribution offers flexibility, comfort, and efficiency.” E Source Tech Update TU-95-8, E Source, Inc., Boulder, Colo., August, 16 pp.

[5] Sodec, F., and R. Craig. 1991. Underfloor air supply system: Guidelines for the mechanical engineer. Report No. 3787A. Aachen, West Germany: Krantz GmbH & Co., January.

[6] Bauman, F., P. Pecora, and T. Webster. 1999. How low can you go? Air flow performance of low-height underfloor plenums. Center for the Built Environment, University of California, Berkeley, October.

[7] John Kerley. 2000. Personal communication. Webcor Builders, San Mateo, CA.

[8] Hosni, M.H., B.W. Jones, and H. Xu. 1999. “Experimental results for heat gain and radiant/convective split from equipment in buildings.” ASHRAE Transactions, 105 (1).

[9] Loudermilk, K. 1999. "Underfloor air distribution solutions for open office applications." ASHRAE Transactions, Vol. 105 (1).

[10] York International. 1999. York Modular Integrated Terminals: Convection Enhanced Ventilation – Technical Manual. York International, York, PA.

[11] Trox International. 2000. Trox Underfloor Air Design Manual. Trox USA, Alpharetta, GA.

[12] Webster, T., E. Ring, and F. Bauman. 2000. "Supply fan energy use in pressurized underfloor plenum systems." Center for the Built Environment, University of California, Berkeley, October.

[13] Bauman, F.S., L. Johnston, H. Zhang, and E. Arens. 1991. "Performance testing of a floor-based, occupant-controlled office ventilation system." ASHRAE Transactions, Vol. 97, Pt. 1.

[14] Fisk, W.J., D. Faulkner, D. Pih, P. McNeel, F. Bauman, and E. Arens. 1991. "Indoor air flow and pollutant removal in a room with task ventilation." Indoor Air, No. 3, pp. 247-262.

[15] Fountain, M., E. Arens, R. de Dear, F. Bauman, and K. Miura. 1994. "Locally controlled air movement preferred in warm isothermal environments." ASHRAE Transactions, Vol. 100, Pt. 2, 14 pp.

[16] Bauman, F.S., E.A. Arens, S. Tanabe, H. Zhang, and A. Baharlo. 1995. "Testing and optimizing the performance of a floor-based task conditioning system." Energy and Buildings, Vol. 22, No. 3, pp. 173-186.

[17] Faulkner, D., W.J. Fisk, and D.P. Sullivan. 1995. "Indoor air flow and pollutant removal in a room with floor-based task ventilation: results of additional experiments." Building and Environment, Vol. 30, No. 3, pp. 323-332.
[18] Tsuzuki, K., E.A. Arens, F.S. Bauman, and D.P. Wyon. 1999. "Individual thermal comfort control with desk-mounted and floor-mounted task/ambient conditioning (TAC) systems." Proceedings of Indoor Air 99, Edinburgh, Scotland, 8-13 August.

[19] Bauman, F., K. Tsuzuki, H. Zhang, T. Stockwell, C. Huizenga, E. Arens, and A. Smart. 1999. "Experimental Comparison of Three Individual Control Devices: Thermal Manikin Tests." Final Report. Center for Environmental Design Research, University of California, Berkeley.

[20] Bauman, F.S., H. Zhang, E. Arens, and C. Benton. 1993. "Localized comfort control with a desktop task conditioning system: laboratory and field measurements." ASHRAE Transactions, Vol. 99, Pt. 2.

[21] Bauman, F., G. Carter, A. Baughman, and E. Arens. 1998. "Field Study of the Impact of a Desktop Task/Ambient Conditioning System in Office Buildings." ASHRAE Transactions, Vol. 104, Pt. 1.

[22] Faulkner, D., W.J. Fisk, and D.P. Sullivan. 1993. "Indoor air flow and pollutant removal in a room with desktop ventilation." ASHRAE Transactions, Vol. 99, Pt. 2.

[23] Bauman, F., V. Inkarojrit, and H. Zhang. 2000. "Laboratory test of the Argon personal air-conditioning system (APACS)." Center for Environmental Design Research, University of California, Berkeley, April.

[24] Matsunawa, K., H. Iizuka, and S. Tanabe. 1995. "Development and application of an underfloor air conditioning system with improved outlets for a smart building in Tokyo." ASHRAE Transactions, Vol. 101, Pt. 2. 

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