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Plenum Configurations
Raised Floors
Design of UFAD Systems

Cost Considerations

How do the costs of UFAD systems compare to those of overhead air distribution systems?

Costs are usually the most important consideration in choosing a building system. The cost implications of UFAD systems may be described in terms of initial, or first-cost considerations, and in terms of life-cycle costs.

First Costs
First costs for UFAD systems utilizing raised access flooring will probably, although not necessarily, be slightly higher than those for a conventional system. The added cost of the raised floor (currently in the range of 5-7 $/ft2) can be at least partially offset by savings in material and installation costs for ductwork, installation of electrical services, as well as from downsizing of some mechanical equipment. If a raised floor system has already been selected for other reasons, such as improved cable management, underfloor air distribution can be easily shown to be cost effective. In new construction, underfloor air distribution with a complementary concrete (flat slab) structural approach can lead to reduced floor-to-floor heights. This is accomplished by reducing the overall height of both service plenums and required structural elements. 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.

Due to strong interest in this topic, a number of efforts are underway to collect reliable cost information from completed UFAD projects. Several manufacturers and system-designers can provide cost data from their respective projects. CBE is currently involved in two projects aimed at collecting and developing further cost analysis: (1) Underfloor Systems Case Studies, and (2) Underfloor Cost Comparison Study.

Life-Cycle Costs
Operating costs for UFAD systems can be reduced compared to those for a conventional system through various energy-saving strategies, including: (1) reduced fan energy use due to lower static pressures and less required air supply (stratification benefit), (2) in suitable climates, higher supply and return air temperatures in UFAD systems provide the potential for increased use of an outside-air economizer and increased chiller efficiencies, and (3) in suitable climates, a 24-hour thermal storage strategy using the concrete slab. With the improved thermal comfort and individual control provided by UFAD and TAC systems, occupant complaints requiring response by facility staff can be minimized. In addition, with most of the building services now located in the underfloor plenum, labor costs for maintenance and cleaning are reduced due to working at floor level instead of on ladders or scaffolds in the overhead plenum.

UFAD systems using raised flooring provide maximum flexibility and significantly lower costs associated with reconfiguring building services. Floor-based supply outlets can be easily relocated (by simply exchanging floor panels) using in-house personnel in response to changes in people or equipment. This flexibility can be especially important over the lifetime of buildings having high churn rates.

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Plenum Configurations

What are the advantages and disadvantages of the two primary approaches to delivering air through underfloor plenums: (1) pressurized plenum, and (2) zero-pressure plenum?

In pressurized plenums, a central air handler delivers air through the plenum and into the space through passive grills/diffusers. Typical static pressures (relative to the occupied space) are quite low, falling in the range of 12.5 - 50 Pa (0.05 - 0.2 in. H2O). In zero-pressure plenums, air is delivered into the conditioned space through local fan-powered (active) supply outlets in combination with the central air handler. In practice, plenum design solutions often represent a hybrid approach in which both passive and active diffusers are used. Underfloor plenums have usually, although not always, been designed to be pressurized.

Pressurized plenum systems have been more prevalent in office buildings because of their simplicity, lower installation costs, and ease of operation and control. Many system designers and operators prefer to avoid the large number of small fan-driven outlets that are required for zero-pressure plenum systems. Although this approach has been very successful, there is evidence from some completed projects that uncontrolled air leakage from pressurized plenums can impair system performance. For example, leakage between underfloor control zones separated by sheet metal partitions (resulting from holes made in the partitions during changes in underfloor building services) is expected to increase over the lifetime of a building. Due to the relatively low pressure used in pressurized plenums, proponents of pressurized plenums claim that leakage into adjacent zones is minimal, and much of the leakage (through cracks between raised floor panels) will be into the same conditioned zone of the building. In addition the common practice of installing carpet tiles on top of the raised floor serves to reduce leakage from the plenum into the room. Leakage from pressurized plenums is a design issue that is still in need of further investigation.

In pressurized plenums with passive diffusers, if access floor panels are removed for long periods of time during repair and maintenance work, or if the distance between primary air inlets and supply diffusers is too great, control of the air flow will diminish.

In contrast to pressurized plenums, zero-pressure plenums pose no risk of uncontrolled air leakage to the conditioned space or adjacent zones. The removal of floor panels will not disrupt overall supply-air flow. Local fan-powered outlets typically allow individuals to control supply air conditions over a wider range than that allowed by a passive diffuser. Under thermostatic control, this increased controllability can also be used to handle zones with significantly different and rapidly changing thermal loads (e.g., perimeter zones). The major disadvantage of zero-pressure plenum systems is the perceived added cost, energy use, and complexity of using a large number of small fan-powered units. As more experience with zero-pressure systems is obtained, the tradeoff between improved performance vs. increased costs can be more accurately evaluated.

Both pressurized and zero-pressure systems offer the same level of flexibility (due to the raised floor) and most of the same energy-savings benefits associated with underfloor air distribution.

What fire safety precautions must be taken in UFAD systems?

The combustibility of cabling (power, data, communication) contained in supply air plenums in underfloor air distribution systems is an important consideration. In gen­eral, applicable codes state that placing wires and cables in an air supply plenum is not a problem as long as they are contained in conduit, or are rated to be non-com­bustible. If not, the installation of sprinklers in the underfloor plenum may be required.

Local fire codes often place restrictions on the size of open supply air plenums without any smoke breaks in the form of partitions separating the plenum into smaller zones. Typically, these fire codes limit the total area (e.g., less than 280 m2 [3,000 ft2]) and horizontal dimension in one direction (e.g., less than 9 m [30 ft]) of an unobstructed underfloor air supply plenum. During installation, it is important to ensure plenums are adequately sealed at junctions where a raised access floor meets a standard floor finish.

Although advisable to avoid the use of sprinklers if at all possible, where sprinklers have been installed, the slope of the underfloor slab should be sufficient to drain water at a rate at least equivalent to that of sprinkler and fire-hose delivery. Most access floor panels currently on the market meet fire-rating requirements. With respect to core materials, lightweight concrete filled panels have good non-combustible characteristics and many high-density particleboards typically carry a Class A fire rating.

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How do diffusers differ in their performance as air supply outlets?

The primary difference between various diffuser models lies in the characteristics (direction and potential for adjustment) of the supply air emitted into the conditioned space, and the location of the diffuser. Diffusers are typically installed in raised floor panels, although other configurations include desk-based and partition-based. The discussion below focuses on floor diffusers.

Swirl floor diffuser: 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. Occupants do not have control over the direction of incoming air as it is always in the same swirling pattern. They do 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 diffuser: 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. 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 grill: 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.

How close to an occupant’s workstation can a diffuser be located?

Although local thermal conditions in the direct path of the incoming air flow from a floor diffuser may be acceptable for short-term occupancy, and when under individual control by the occupant, air velocities may be too high and temperatures too low (under cooling conditions) to satisfy the thermal comfort preferences of a large majority of occupants.

For this reason, during the placement of diffusers a ‘clear zone’ is typically defined that is not recommended for long-term occupancy. This zone is defined as an imaginary cylinder, diameter 0.9-1.8 m (3-6 ft), centered on each diffuser –- actual dimensions may vary according to manufacturer's data. Therefore, office furniture layouts should take this into consideration and maintain a distance of at least half the diameter of the clear zone between occupants’ seating and any diffusers in the proximity of their workspace.

Will occupants seated at workstations adjacent to floor diffusers suffer from cold feet?

A common misconception about UFAD systems is the risk of a cold floor and, due to the proximity of occupants to diffusers, thermal discomfort due to unwanted drafts. Should such problems arise they are more indicative of a poorly operated system than the status quo of typical installations. A number of features unique to UFAD help maintain occupant thermal comfort:

Typical UFAD supply air temperatures of 17°C – 20°C (63°F – 68°F) are higher than those for overhead HVAC systems, which typically operate at 10°C – 16°C (50°F - 60°F), reducing the risk of uncomfortably cool temperatures in the conditioned space.

Individually controlled supply diffusers allow occupants to adjust the local airflow to match their personal preferences and avoid undesirable drafts.
Although the floor temperature in a UFAD system will be slightly cooler than that of a conventional overhead system, excessive cooling can be avoided and controlled through the use of carpet tiles for increased insulation and/or by adjusting the room air setpoint temperature upwards to compensate for the radiant cooling effect of the floor

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Raised Floors

What is the typical height of a raised floor?

The height is defined as the distance from the top surface of the structural slab in an underfloor plenum, to the top surface of the raised floor panels. Typical heights for UFAD applications are 200 mm – 460 mm (8 in. – 18 in.).

What is the minimum height requirement for acceptable air flow performance of UFAD systems?

Underfloor plenums serving UFAD systems are flexible in terms of accommodating components from other distribution systems. Experiments have been carried out by CBE, in a 300 m2 (3,200 ft2) underfloor zone, into the variables that affect performance of UFAD systems. These have shown that pressurized plenums with at least 75 mm (3 in.) of clear space for air flow, in addition to the space required for other elements such as cable runs, are capable of maintaining a uniform distribution of airflow to diffusers. In situations where obstructions leave as little as 38 mm (1.5 in.) clear space above them, the risk of disrupting airflow is minimal providing the overall plenum height is at least 205 mm (8 in.).

How are the pedestals affixed to the structural slab, and floor panels affixed to the pedestals?

Once the concrete floor slab has been sealed to reduce dust, the pedestals are laid out according to the predetermined grid and glued (sometimes bolted for seismic purposes) to the slab. After components for the UFAD and/or cable management systems have been installed and positioned, the floor panels can be laid. Four pedestals typically support each square panel at its corners. Ideally, pedestal-to-panel connections would comprise a mechanism enabling both a firm securing and easy removal of the panel. Currently available devices include screw fasteners or bolts at each corner of the panel, where a pedestal head is located; and for panels held by their self weight alone pedestals may have flanges designed for maximum support and gravity-lock collars to ensure levelness. In areas regulated by seismic building codes, pedestals can be braced and fitted with special seismic-bases. 

Are raised floor systems capable of supporting the typical loading of an office; will concentrated loads pose a problem?

The majority of access floor systems currently on the market are engineered to meet the concentrated, uniform and rolling loads experienced in a typical workplace environment. For example, galvanized steel-encased lightweight concrete panels combine the tensile strength of steel with the compressive strength of concrete to offer a high degree of rigidity. The lower self weight of both lightweight concrete, or steel-encased high density particle board panels, reduces deflection even further and makes removing the panels an easier process than that for equivalent standard concrete filled panels.

High quality manufacturing processes enable panels made to very low dimensional tolerances such as ±3.8 mm (0.15 in.). Together with a uniform panel thickness, good edge sealing, and flush-mounted floor diffusers it is possible to achieve a homogeneous floor surface over which loads can be distributed.

Of course any systems is only as strong as its weakest link so attention must be paid to the pedestal-to-panel connection, described in the previous question.

How significant is the self-weight of the access floor?

For the purpose of load calculations, the self-weight of raised floors should be included and considered a dead load. The development of lightweight concrete- or particleboard–core panels has reduced this self-weight without compromising structural strength or rigidity.

When an office floor has areas with raised floors adjacent to those without, how is the change in level reconciled?

In situations where raised flooring meets conventional flooring, suitable steps and/or a short ramp can be installed to make the change in level. Attention should be given to detailing such junctions:

a) At floor level, in terms of maintaining the integrity of the floor plane and complying with ADA and other health and safety codes.

b) Under the floor level, in terms of sealing the plenum to avoid air leakage or creating a fire hazard.

What are the advantages and disadvantages of different carpet tile systems that are commonly installed on raised floors?

Currently the vast majority of UFAD installations comprise floor panels and carpet tiles from different manufacturers, and in different modular sizes – floor panels are typically 600 mm (24 in.) square; carpet tiles are typically 450 mm (18 in.), although they are also available in 600 and 900 mm (24 and 36 in.). This brings up the issue of joint alignment versus overlap, which is complicated by the method of securing tiles to panels.

Although there is at least one manufacturer who offers an indexed and adhesive-free carpet tile system, most tiles are affixed to the floor panels with adhesive. This creates a number of drawbacks:

The use of adhesive on the underside of carpet tiles risks bonding adjacent floor panels to each other, and gluing the panel screws into their corner holes; both of which reduces any benefits in terms of accessibility and flexibility in removing floor panels.
Adhesive may also seep into the underfloor plenum and cause problems by damaging cable management components, and negatively affecting supply-air quality with chemicals and particulates.Should the initial carpet tile type need to be replaced, building owners are left with an adhesive residue that must be removed before installation of an alternative tile system.

All of the above have the potential to override the cost savings that raised floor systems offer as they impede flexibility, ease of installation and maintenance.

A second major issue is the question of overlap or alignment between carpet tile seams and joints between floor panels. Since the majority of installed carpet tile does not match the size of typical floor panels (see above), the convention is to overlap the floor panel joints with carpet tile. The benefits of this approach are claimed to be (1) a finished look of the carpet, (2) the carpet serves as a gasket or seal for the underfloor plenum, and (3) carpet seams are not damaged (potential to be caught in floor panel joints when seams are aligned) during reconfiguration. From a true flexibility point of view, however, the best solution is a one-to-one matching of carpet tile to floor panel. This provides the potential to simply exchange panels (along with their matching carpet tile) when, for example, a floor diffuser needs to be moved to another location. In addition, by using a carpet tile that is the same size as the floor panel and does not overlap, the amount of carpet waste generated during moves and changes is minimized. In buildings with high churn rates, this can be a significant cost saving.

The appearance of a carpet tile installation is largely influenced by the quality and precision of the access floor installation. For a typical configuration of 450 mm (18 in.) carpet tiles laid over 600 mm (24 in.) floor panels, each panel is covered in part by a minimum of four carpet tiles. If this panel is not perfectly aligned with those adjoining, the profile of each of the four carpet tiles will be disturbed and uplift may occur. Thus in order to maintain the integrity of the finished-floor surface, it is important to specify a well engineered access floor system, with low dimensional tolerances, to be installed by experienced contractors.

For the above reasons, tiles that are either adhesive-free or exactly sized to the floor panel module are recommended.

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UFAD Systems Design

How do floor-to-floor heights in buildings with UFAD compare to those in buildings with ceiling-based distribution?

Underfloor plenums accommodating an UFAD system are often deeper than those employed solely for cable management purposes. Yet, by placing the air distribution system in the underfloor zone and reducing the need for deep ceiling ductwork associated with conventional ceiling-based systems, new build projects can achieve a 5-10% reduction in floor-to-floor heights compared to projects with ceiling-based air distribution. Further height reduction can be achieved by changing from standard steel beam construction to a concrete (flat slab) structural approach.

Can UFAD systems be installed in existing buildings, during retrofit projects for example? 

Due to the tremendous size of the existing building stock, retrofit construction will play a dominant role in the future for the building industry. Projects requiring the addition of an HVAC system often encounter the problem of having limited space for accommodating ducts and other components. By eliminating large overhead ceiling ducts, the total plenum height required in UFAD installations is less than that for ceiling based systems. Therefore, UFAD is feasible for existing buildings where, due to restricted floor-to-floor/floor-to-ceiling heights, it is necessary to minimize the vertical space occupied by ductwork.

In addition, the intervention of a raised floor system is less disruptive than that of ducting for overhead systems as the floor can be easily installed, and removed, as an independent platform leaving relatively few structural scars. This issue is important in buildings where maintaining the integrity of the existing building structure is important for heritage/cultural/structural reasons. Furthermore, installation can be a relatively dry process, once the concrete structural slab has been adequately sealed, minimizing damage to other building elements.

Can the installation of an UFAD system eliminate the need for a hung ceiling?

By locating the main air distribution components within an underfloor plenum, through which supply air can flow freely, UFAD systems eliminate the need for sizeable ceiling ducts and plenums. In terms of return air, the same principle applies as that for supply –- air can either flow within an open plenum above a hung ceiling, or if desired, the ceiling can be eliminated altogether when an alternative means of removing return air from the space is provided. Options include high side-wall return or exposed return ducts below the ceiling plane.

Eliminating drop ceilings have the benefit of increasing architectural opportunities for creative internal design, enhancing daylighting and artificial lighting effects. However, if the conventional suspended acoustic tile ceiling is eliminated, leaving an exposed concrete ceiling or other configuration, careful consideration must be made of the acoustic quality of the space.

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UFAD Systems Operation and Maintenance

What measures are taken to avoid the supply air becoming contaminated by dirt and particulates collecting in the underfloor plenum and floor diffusers?

The typical velocity of supply air in pressurized plenums is too low for dirt particles to be carried into the air stream and emitted through floor diffusers. Tests have shown that floor diffusers do not blow more dirt into a conditioned space than any other air distribution systems, and in fact, contaminant levels are generally lower in UFAD systems with a floor-to-ceiling air flow pattern. However, further research is still needed to fully understand the health and air quality impacts of UFAD systems. With proper filtration, the amount of dirt or dust entering the underfloor plenum can be minimized. If cleaning is required, the ease with which the underfloor plenum is accessed (compared to extensive distribution ductwork in the ceiling) allows for a regular maintenance schedule to be carried out, avoiding a build up of dirt or dust.

What are the acoustic characteristics of UFAD systems?

Due to the elimination or minimal use of ductwork in underfloor plenums, the noise generated from the operation of a UFAD system can be substantially less than that from a conventional ducted overhead system. This reduction in commonly found levels of background "HVAC" noise may create a situation where active sound masking or other acoustic design measures may be required. The small volumetric fans used for active diffusers typically have low noise levels. Noise generated by fan terminal units, however, may be problematic and attention should be paid to these devices in terms of manufacturer’s specifications, correct installation to avoid unnecessary vibrations, and other acoustic measures that can be taken.

Raised floor panels comprise a metal casing –typically galvanized steel- and core material. By introducing a solid core, such as lightweight concrete or high-density particleboard, problems with hollow sounding floors are avoided and acoustic insulation performance is improved. In terms of reaching a compromise between a sound-tight installation and the flexibility of frequent and easy removal of floor panels, any seals/mechanical-locking configurations must be durable enough to avoid degeneration from wear and tear. In addition to the panel itself, carpet-finished systems offer a good level of acoustic insulation by damping vibrations.

Sound transmission between adjacent rooms with floor diffusers served by the same plenum is sometimes cited as a concern. More performance data are needed to fully resolve this issue, but in most configurations this should not be a problem.

What is the potential for economizer use with UFAD systems?

During cooling periods, when outside air temperatures are several degrees lower than that of the HVAC system’s return air, a building’s mechanical ventilation system can run on 100% outside air. This reduces the energy required in the process of cooling the supply, and re-circulated, air to the required temperature under normal operating conditions. This method of cooling, often described as ‘free cooling’, is only feasible in temperate climates where outside air temperatures are in the range of 21-24°C (70-75°F) by day and will periodically be less than the supply air temperatures (nighttime economizer cycles are frequently employed, for example). Enthalpy-based control is always recommended to ensure proper humidity control of the supply air entering the underfloor plenum.

In comparison to ceiling-based systems, UFAD supply air is distributed at a higher temperature, typically 18°C (65°F) for UFAD, 13°C (55°F) for ceiling HVAC. In most temperate climates the increased number of hours during which the outside air temperature is less than 18°C (65°F) compared to the hours when it is less than 13°C (55°F) results in a greater potential for UFAD systems to achieve significant energy savings due to economizer use. However, climate characteristics will ultimately determine the feasibility of economizer cycles. In climate zones experiencing high humidity, the process of dehumidifying the outside air will override other energy savings and may pose problems with condensation risks.

Is there a risk of condensation as cool supply air flows over the thermal mass of the structural slab?

In humid climates, outside air must be properly dehumidified before delivering supply air to the underfloor plenum where condensation may occur on the cool structural slab surfaces. While humidity control of this sort is not difficult, due to the large surface area of the structural slab in the underfloor plenum it is important that it be done correctly. If a higher cooling coil temperature is used (allowing an increased chiller efficiency) to produce the warmer supply air temperatures needed in UFAD systems, the cooling coil’s capacity to dehumidify will be reduced.

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.

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