Underground air distribution (UFAD) is a method of providing ventilation and space conditioning in offices and other commercial buildings. As part of HVAC system designs, this method is becoming a popular alternative to conventional ceiling-based air ducts. UFAD uses the open space between the concrete structural slab and a raised floor system to transport conditioned air directly to areas of the building. Air can be distributed through supply outlets strategically located on floors, which is the most common method, or as part of furniture and partitions.
UFAD systems offer several advantages over traditional ceiling systems, such as better indoor air quality, thermal comfort and energy savings. The UFAD combines HVAC systems with power and data wiring in an accessible service space beneath a raised floor. This provides greater flexibility to reconfigure construction services, along with cost savings.
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How does underground air distribution work?
To understand the advantages of underground air distribution systems, it is important to identify how they differ from conventional overhead duct systems. UFAD completely changes the airflow inside buildings, which impacts thermal comfort and indoor air quality.
Ceiling-based systems
For many decades, commercial HVAC systems distributed conditioned air through extensive networks of ducts and ceiling diffusers. The operation of these systems is characterized by the following features:
- Air conditioning is supplied and returned to ceiling level.
- Ceiling plenums are large in size to accommodate the supply ducts that must pass through them.
- Return air is configured as an unducted ceiling plenum.
Conventional HVAC systems are designed to mix supply air with ambient air to maintain the entire volume of air in the space from floor to ceiling at the desired temperature. At the same time, the system must provide an adequate supply of fresh outside air to the building occupants. This is often known as mixture-type air distribution. However, these systems have the following limitations:
- No flexibility to accommodate different thermal preferences among building occupants.
- It limits the possibility of adjusting ventilation according to the needs of different areas.
Underground Systems
When UFAD systems are used, they bring the following advantages.
- Conditioned air from air handling units (AHU) is supplied to the underground plenum and this air flows freely to the supply outlets.
- Supply outlets are smaller compared to ceiling-based systems and air is distributed closer to the building's occupants.
- Sockets can be floor diffusers, part of tables or partitions equipped with individual controls. Adjustable outlets allow users to manage temperatures locally, improving thermal comfort.
- The air returns from the rooms at ceiling level, taking advantage of the natural buoyancy of the hot air. This helps remove heat produced by office appliances and occupants, as well as remove contaminants from the air in the conditioned space more efficiently.
Unlike conventional air mixing systems, UFAD systems lead to a stratification of the indoor air. In this way, warmer air and polluting particles tend to accumulate above head height, which improves thermal comfort and air quality for occupants. Furthermore, exhaust air can be removed with lower ventilation power.
There are three main approaches to configuring the air supply side of UFAD systems:
- A central air handling unit that distributes air through a pressurized plenum under the floor and into the space through grilles and diffusers.
- Zero pressure plenum where air is supplied to the conditioned space with fan driven supply outlets.
- In some arrangements, supply air is delivered directly to the outlets using ductwork through the underground plenum. However, this configuration reduces energy savings and cost benefits compared to the first two options.
Benefits of Underground Air Distribution
A well-designed UFAD system has several advantages over a traditional ceiling-based system. The following are some benefits of UFAD systems when they are properly designed and applied:
- Better thermal comfort : Building occupants have more control over the local thermal environment and this helps accommodate individual preferences.
- Increased ventilation efficiency and indoor air quality : Indoor air quality is improved by supplying new fresh air to the space at floor level, closer to the building's occupants. This allows for more efficient airflow from floor to ceiling.
- Reduced lifecycle construction costs : The cost of reconfiguring building services is significantly reduced with raised floors as the system is more flexible and affordable.
- Reduced energy use : Thermal stratification, higher supply air temperatures and reduced pressure underground plenums help reduce energy consumption.
- Reduced Height Between Floors in New Construction : UFAD systems can help reduce overall plenum heights. A large overhead plenum for bulky supply ducts can be replaced with a smaller ceiling plenum for return air, combined with a raised floor for supply air and other services.
- Improved productivity : Occupant satisfaction is directly related to their performance and productivity. By giving occupants greater control over their thermal comfort, productivity can be increased.
Because UFAD contributes to thermal comfort, energy efficiency and indoor air quality, it can help earn points for building certifications like LEED and WELL. LEED is strongly focused on energy and environmental aspects, WELL gives more priority to well-being aspects, as its name indicates. However, indoor environmental quality is addressed in both certification systems.
