What is Underfloor Air Distribution?

Economics

The basic cost of an access floor system is approximately €4.7 per square foot. Savings from reduced wiring and HVAC costs during the first tenant improvement bring the net cost down to about €3 per square foot. Access floors reduce ongoing operating costs in spaces and a high churn rate, where people, furniture, and equipment are frequently moved to accommodate changing project or team requirements.  Savings can also be made since changes in wiring and HVAC services are simple and uncompromising.

The system can also afford the building owner an overall decrease in the life cycle cost. Estimates of savings in ongoing operations, maintenance, and remodelling are shown in table 1.  

Table 1:        Energy and non-energy-related cost savings for access floor systems          

                      with under floor air distribution          

Category                                                                Estimated Savings          

HVAC Energy                                                                15-30%

Operations and facility staff reduction                                                 25-50%

Cost of telecommunications moves, adds, and changes                        40-70%

Floor plate modifications cost                                                  40-70%

Power distribution changes to individual workstations                                50-80%

Moving computers and peripherals                                                80-90%

Space unavailable during remodelling                                        30-70%

Absenteeism                                                                5-10%

Fig 2. Section through Underfloor air displacement system

Design

As mentioned earlier, Underfloor Air Distribution Systems (UFAD) are a `class of air distribution system that deliver air through diffusers in the raised access floor of a building, with the intent of maintaining comfort and indoor air quality levels only in the occupied lower portion of space.

Design Specification of UFAD

• The fresh air enters through a ventilation duct allowing the air to travel into the room and underneath the access floor.
• Mixing diffusers transfer the fresh air above the access floor
• Typical diffuser designs include:

1. Manually adjustable 200mm dia., circular ‘swirl’ type models;
2. Manually or automatically adjusted 250mm X 250mm constant velocity models.

See Figure 3 below on ‘swirl’ type diffusers.

Figure 3 Swirl type floor diffusers

                                

• The ‘swirl’ type models can be installed in 300mm high raised floors while ‘constant velocity models’ require 400mm pedestals.
• Most manufacturers designs allow occupants to manually regulate the airflow through diffusers without removing the diffuser cover.
• Floor diffusers can be easily repositioned to suit the floor plan and workstation layouts.
• A system known as the ‘Task Ambient Conditioning System’ actually extends the underfloor air through the floor in flexible ducts housed in the furniture allowing arm’s length control of the direction and flow of air e.g. overhead air conditioning on a bus or plane.
• The diffusers are placed on what are known as ‘pedestals’, which also support the access floor panels.

See Figure 4 below.

• ‘Pedestal’ heights will as previously discussed depend on the type of diffuser used i.e. 300mm or 400mm high pedestals depending on the type of diffuser.
• Exhaust/return grills are positioned at ceiling level in the most adequate positions to suit the design.
• The stale air is then exhausted through the return outlet.

Design Considerations

• Temperature gradient: In order to maintain occupant comfort, underfloor air systems must limit the temperature gradient in the occupied space. In order to limit the gradient, supply air temperatures on the order of 65°F to 68°F are used with a recommended minimum ceiling height of 8ft (2.88m).
• Airflow rate: The airflow rate must be balanced against temperature gradient considerations. Lower temperature gradients imply higher flow rates and excessive fan energy. If the air volume is too high, occupants are more likely to be exposed to drafty conditions resulting from high velocity currents discharged from the diffusers to the lower portion of space.

Design Methods

• The following should be calculated when considering the method of design:

1. Space load;
2. Supply flow rate;
3. The temperature;
4. Size and number of diffusers.

• A variety of methods are available but the most complex and rigorous methods usually involve the use of computerised computational fluid dynamics (CFD) analysis, which is generally reserved for research applications or very large projects with unique conditions.

Maintenance and Cleaning

        

• Where high velocity air supply points are used, dust and particles can be entrained from the floor plenum into the airflow and distributed into the

• occupied zone in poorly maintained systems. This would leave an unpleasant work environment. However, with regular housekeeping of the floor plenum this is less of an issue.
• HVAC diffusers have a removable bucket trap that captures the dirt. These traps can be emptied during the janitorial service.

Characteristics of Underfloor Air Distribution

• Raised floor panels supported on vertical supports approximately 30 – 40 cms above the floor slab.
• All wiring, cables etc are located in the underfloor space.
• Floor-mounted diffusers deliver conditioned air to the individual workspaces.
• The surface of the floor panels can be made from carpet tiles, decorative concrete, metal or wood.
• Swirl diffusers quickly mix the supply air as it enters the occupied zone.
• Air is circulated in an upward motion and is released through return grilles in the ceiling.

Advantages of Underfloor Air Distribution

• Reduced life-cycle building costs by improving flexibility in reconfiguring building services and office layout.
• Diffusers can easily be rearranged if office layout changes.
• Improved productivity in the workplace because diffusers can be adjusted for individual preference.
• Improved ventilation efficiency, indoor air quality and health by delivering fresh air to the building occupants.
• Reduced energy use from thermal stratification, reduced static pressures, and increased economizer operation.
• Reduced floor-to-floor height in new construction by lowering the height of service plenums.
• Increased chiller efficiency and reduced cooling load requirements.

Disadvantages of Underfloor Air Distribution

• The higher initial cost of the system.
• Concerns about the problem of dust stirring from the floor.
• Discomfort caused by draft and vertical temperature differences.
• Possible obstructions to the air-flow in the access floor.