Displacement Ventilation, Underfloor Air Distribution as Alternative HVAC Strategies

Two additional alternative HVAC strategies are displacement ventilation and underfloor air distribution. For certain building types, this approach can be a very effective cooling strategy. In delivering conditioned air at floor level, the more buoyant, warmer air is displaced and naturally floats to the ceiling level. As a result of stratified conditioning of only the lower occupied space, energy is saved.
While this sounds good in theory, the practical issue is the need to move a lot of air. Essentially, removing heat from the space requires twice as much air as a VAV system, so air velocity control is critical. Once the air velocity surpasses a certain level, the result is a well-mixed system, which means that the bulk air becomes the same temperature, as opposed to stratification, which is the key to the technology’s effectiveness.
Overall, the thermodynamics taking place within the space are complex, so a number of controls need to be in place, and the environment has to be just right.
In addition, when employing displacement ventilation, the delta T between the supply air and room temperature must be limited to 10 degrees in order to maintain comfort, as opposed to a conventional 20 degrees. It can be challenging to work with this smaller temperature range while mitigating humidity.
The other issue is application. While displacement ventilation works quite well in large volume spaces such as atriums or theatres — where it’s more practical to pull the air in low — it can be more challenging in an office environment where ceilings are lower.
Underfloor Air Distribution
While underfloor air distribution (UFAD) is often touted as having benefits similar to displacement ventilation systems, the two are different animals. UFAD is similar to displacement ventilation in that the supply air is delivered low and extracted high. But that’s where the similarity ends. Although it is sometimes said that air will be well stratified in office environments using UFAD, the reality is that the air is well-mixed. When using UFAD, the bulk of the air within a space is nearly uniform in temperature, and one of the major benefits of displacement ventilation — stratification — is nullified. The reason is that the diffusers used to supply the air to the space have some practical limitations.
First, diffusers must rely on the same temperature difference limitation as displacement, but still be flexible to allow for freedom of movement, or lack thereof, as well as changing conditions during the day. The common swirl diffuser used today is effective, but the swirl pattern induces room air to mix with the cool supply air and the result is a well-mixed room, which is exactly the same result achieved using a conventional overhead VAV system. And, while users can manually adjust their swirl diffusers, they don’t. Unfortunately, this means that many UFAD systems don’t actually operate as variable air volume systems (many systems have no automatic flow limiting devices), and act more like constant volume systems. And, while there are excellent examples of effective UFAD systems, there are enough poor performers to make it clear that they require more coordination to design and build.
The final two options in our discussion of five alternative HVAC strategies are radiant heating/cooling and demand-control ventilation. Here is a look at both.
Radiant Heating/Cooling
Water-based systems, which can be successfully coupled with DOAS, can make effective HVAC systems. Radiant systems are energy efficient in that they take care of the space-sensible load without moving a lot of air around. Heat can be moved using any one of the three modes of heat transfer: conduction (heat transfer from objects touching), convection (like your oven at home), or radiation (like sitting in front of the fireplace.) Most commercial spaces have all three modes of heat transfer occurring simultaneously.
While radiant systems offer a variety of benefits, they aren’t always the ideal choice for some U.S. climates and building types. When commercial buildings heat up, space loads can change relatively quickly, as when the number of people in a room suddenly increases or solar heat gain infiltrates one side of the building. In these cases, the HVAC system needs to react quickly.
Radiant is also not a good fit for climates where high humidity is an issue, like the South. One of the complications with using radiant cooling in the South is the constant threat of condensation on interior building surfaces. To provide sufficient cooling in high load areas, the system needs a lot of surface area and colder water. If you don’t control the interior dew point temperature, the ceiling starts to look like a glass of the South’s sweet tea in the summer. And that can lead to the dreaded “M” word — mold.
Demand-Control Ventilation
While not actually a system, but rather a good conservation strategy, demand-control ventilation brings in only as much outside air as needed. Allowing in less outdoor air means there’s less air to heat, cool, or dehumidify. Although minimum levels are mandated by building codes and standards like ASHRAE Standard 62, facilities still need to promote indoor air quality. In some cases, that may mean going beyond code minimums for outdoor air. LEED, for instance, offers points for proving the ability to exceed the minimum ventilation levels for a building.
Ventilation codes are based on occupancy and space sizes and types in commercial building. The idea with demand-controlled ventilation is to match the airflow with the actual occupancy, rather than simply pumping the maximum amount of outside air required at design condition. To determine what these ventilation levels need to be, technology like CO2 sensors or access control devices, which quantify occupancy, can be tied into a building automation system, which then controls the positioning of dampers accordingly.
Making the Choice
Ultimately, most buildings will still go with today’s standard VAV systems. The technology is affordable, the system reacts quickly to climate and occupancy changes, and economizer cycles can be used in tandem with VAV for free cooling.
However, climate, building use, and budget will largely determine whether building owners consider alternatives to typical VAV systems. Understanding how these alternative technologies work — and their pros and cons and optimal applications — will help facility managers make the ultimate decision for a facility.