It happens all the time. In any commercial environment with an open-plan office space, some people just need more cool air and others in the same space bundle up with winter wear, or battle for temperature control. Besides taking care of this age-old struggle, the underfloor air distribution method of HVAC (heating, ventilation, air conditioning) design, within the larger MEP (mechanical, electrical and plumbing) engineering design system, has a few other advantages to offer.

Underfloor air distribution, also known as UFAD, is increasingly being welcomed as an alternative to conventional ceiling-based air distribution systems. Here’s why:

The open space between the structural concrete slab on the floor and the raised access floor system, or underfloor plenum, is used to supply conditioned air to the occupied area of the building directly. It is possible to supply this air through a choice of outlets at the floor level, as part of the furniture in the space or embedded in partition walls. Some of the advantages to this method of air distribution compared to the traditional overhead system are: improved thermal comfort on an individual scale (less office disruptions), improved air quality and reduced use of energy.

When the HVAC system and major power, voice and data cabling are combined into a service plenum, which is easily accessible under a raised floor, then reconfiguring these building services in the future can be more flexible and less expensive. This system works particularly well for office spaces.

For more than a decade, Europe, South Africa and Japan have seen increasing use of UFAD systems. Some of the reasons why these systems are preferred over conventional ceiling-based air distribution systems are linked to cooling conditions.

Typically, HVAC design has been devised to supply air to an evenly spaced line of diffusers in the ceiling through a network of ducts. Large supply ducts can easily fit into large ceiling plenums created specifically to accommodate the ducts. Air, that is conditioned, is supplied and returned at the level of the ceiling. The HVAC systems were designed to completely mix supply air and room air, so that the total air volume in the space would remain constant from floor to ceiling. This air would be maintained at a set temperature and the system would ensure that the building occupants receive an adequate supply of fresh air from the exterior of the building. This system does not provide the capability to cater to individual thermal preferences of building occupants.

When UFAD systems are used, the air handling unit (AHU) sends conditioned air through ducts in the underfloor plenum to flow without constraints and barriers to supply outlets. Many small supply outlets, such as floor diffusers, desktop or partition outlets, are placed near occupants in the underfloor system. These individual outlets may have their own controls, enabling occupants nearby to have a certain amount of control of thermal conditions in their immediate vicinity.

The air that is returned from the room to the ceiling produces an air pattern from the floor to the ceiling that uses the natural buoyancy of heat sources in the office to its advantage. Heat loads are efficiently removed, and air contaminants are removed to aid cooling. Stratification above head height is encouraged so that occupants are least affected by increased temperatures and high levels of pollutants.

The underfloor air distribution system is increasingly used in office buildings, particularly offices with open plans with raised floors and where the layouts are easy to reconfigure. The UFAD system is also used in command centres, IT data centres and server rooms with large cooling loads due to electronic equipment and routing power needs and data cables.

While installing UFAD systems in laboratories, special care must be taken to prevent chemical spillage into the access floor plenum and in maintaining critical room air pressures. Certain wet spaces, such as restrooms, pool areas, kitchens, dining areas and gymnasiums should avoid installing UFAD systems. For improved performance, UFAD systems can be used with other HVAC systems, such as displacement ventilation systems, overhead air distribution systems, radiant ceiling systems or chilled beam systems.

Three basic approaches to configure the supply-air side of an UFAD system are:

1. A central AHU that delivers air through a pressurised underfloor plenum and into the space through grills/diffusers

2. Local fan-driven supply outlets and a central AHU that delivers air to the space from a zero-pressure plenum; and

3. Supply air delivered via ducts through the underfloor plenum to supply outlets.

Benefits of the UFAD System

Well-engineered UFAD systems offer several advantages, such as flexibility in layouts, for example. They also provide:

It is important to ensure that UFAD systems are precisely designed and installed. A well-sealed plenum is critical during construction. All openings or gaps through the floor either to the space or into the walls, where plumbing or electrical equipment penetrates the space, must be carefully sealed as well. Attentive and regular inspection during construction is mandatory.

Another advantage to UFAD systems is that the floor slab does not store too much heat with it floor being raised. This results in higher peak cooling loads compared to conventional systems. Traditionally, during the day, incoming solar heat gain is stored in the floor slab, resulting in a reduction in peak zone cooling loads. With a UFAD system, the raised floor changes the large floor slab absorbing solar heat to a lightweight material, resulting in higher peak zone cooling loads.

UFAD Tools and Research

Research projects, such as the ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers) Research Project, have devised a simple tool to predict the temperature difference vertically between occupants’ heads and ankles, the supply air flow rate for a plenum zone, the total number of diffusers and the effectiveness of air distribution. The zone cooling load and the percentage of the cooling load assigned to the underfloor plenum must be specified. Data on the supply air temperature at the diffuser or duct, plenum flow rate and zonal supply flow rate must also be included. Then, one of three type of diffusers can be selected and applied to different building types, such as offices, classrooms, workshops, restaurants, retail shops, conference rooms and auditoriums.

The Center for Built Environment’s (CBE) UFAD design tool can predict the cooling load for UFAD systems using input from the design cooling load. It can predict air flow rates, temperature stratification and the plenum temperature gains for interior and perimeter zones.