Above or Below?

Experts on air distribution weigh the advantages and disadvantages of overhead vs. underfloor air distribution. Consulting-Specifying Engineer: As HVAC technology and systems continue to evolve, are you seeing more movement toward either overhead or underfloor air systems? ABNEY: We're seeing growth in the use of underfloor HVAC systems.

By Barbara Horwitz-Bennett, Contributing Editor October 1, 2004

Experts on air distribution weigh the advantages and disadvantages of overhead vs. underfloor air distribution.

Consulting-Specifying Engineer: As HVAC technology and systems continue to evolve, are you seeing more movement toward either overhead or underfloor air systems?

ABNEY: We’re seeing growth in the use of underfloor HVAC systems. Office buildings today favor large, open-plan cubicle areas, which are perfect applications for underfloor air distribution. We are also seeing a lot of interest in underfloor systems for the U.S. Green Building Council’s LEED certification. As a matter of fact, underfloor systems are specifically mentioned in several LEED credits.

KALLSTROM: But overhead air systems are, of course, still much more prevalent. One reason that UFAD has not been more widely accepted is that, until recently, there were no standardized design guidelines. Engineers had to develop guidelines on their own. However, the situation is changing. For example, ASHRAE’s new Underfloor Air Distribution Design Guide will have a positive impact on the acceptance and use of UFAD systems.

MUMMA: The move to UFAD is not evolution but devolution, in my opinion. There is nothing about UFAD that is either cutting edge or new. In fact, Thomas Jefferson’s Monticello had it in the late 1700s

In more recent times, computer rooms and data processing facilities have been served via ducted or unducted raised-floor plenums. The trend toward underfloor network cabling and electrical service has renewed interest in UFAD, based on the idea that with the floor plenum space already created, air could be delivered through it at little or no additional cost.

INT-HOUT: We see UFAD as just another way of solving a common problem. I wouldn’t go so far as to characterize it as a whole new movement.

CSE: What are the advantages and disadvantages of each approach?

ABNEY: Modularity and flexibility are two of the biggest advantages of underfloor designs. And underfloor air should also reduce a building’s HVAC energy use, because lower supply-air temperatures and lower-horsepower fans can be used. This, of course, depends on building design and location.

Underfloor systems also tend to be very quiet. This can be an advantage or disadvantage, depending on building needs. Many buildings with UFAD systems also use white noise generators, which add cost to the design.

Overhead systems are better for tenant buildings and buildings with many private offices, because overhead designs allow for better zone control. The underfloor plenum is typically one large pressurized plenum. What this means is that occupants can control their individual diffuser in an underfloor system, but the whole open plan area has a consistent climate throughout.

KALLSTROM: Those are all good points, but you can’t forget the importance of cost; overhead systems typically have the advantage of lower first cost, where underfloor systems have lower life-cycle costs. The life-cycle savings are primarily attributable to lower space-reconfiguration costs, along with reduced energy costs. Because ductwork is greatly reduced with underfloor distribution, fan energy usage is reduced due to lower system static pressures. In certain climates where high humidity is not a concern, UFAD’s 63°F to 65°F supply-air temperature means additional energy savings by increasing economizer usage.

Overhead systems, though, are much more advantageous in retrofit applications. The number of changes required to the space, contending with doorways, elevators, etc., make retrofitting with UFAD, in most cases, cost-prohibitive.

CSE: Beyond first cost, do you see other issues standing in the way of greater UFAD acceptance?

KALLSTROM: One of the biggest problems for UFAD is that it is new and unfamiliar technology. Overhead systems have the advantage of having been used for years. Over time, overhead systems have been refined and design guidelines and tools have been developed, making their application much easier. On the other hand, UFAD systems are new to the scene, designs have been developed through trial and error, and until recently, no standardized guidelines existed.

INT-HOUT: I agree. Overhead systems have been in use for many years, and there are many equipment and system-design options. Both the design community and the building operators understand these systems. When designed, installed as intended and maintained, overhead systems provide comfortable, energy-efficient spaces. The fact is that engineers don’t have as many equipment options yet with UFAD. And, because this is a new technology, construction details are different and building personnel need training on different equipment.

ABNEY: Engineers aren’t alone on the learning curve. Until recently, there wasn’t an applicable NFPA 90A test method for raised-floor diffusers, so some local authorities wouldn’t allow polymer diffusers. The 2002 Edition of NFPA 90A was updated to provide a test for discrete devices used in the underfloor plenum. It added UL 2043 as the test method for air diffusers designed for raised-floor applications, so now polymers can be tested and UL listed to meet an NFPA test.

CSE: What other trends are affecting the specification of overhead and underfloor air systems?

ABNEY: The increase in green buildings is the biggest driver for underfloor. The GSA and many state and local municipalities are requiring LEED certification for all federal, state and city-funded buildings. Some municipalities are even giving tax credits for green buildings. If there is a financial benefit, like a tax credit or reduced energy costs, we will see a growth in underfloor systems, too.

KALLSTROM: Studies have shown that occupancy comfort levels are higher with an underfloor system; in an overhead system, occupants have little or no control over their individual workspace. Also, by introducing the fresh supply air at the floor, closer to the occupant, and allowing a floor-to-ceiling airflow, indoor air quality is improved by removing contaminants from the occupied space.

MUMMA: On the other hand, the growing interest in ceiling radiant cooling panel (CRCP) systems is helping push overhead systems, because they offer a couple of significant benefits. First, low-temperature ventilation air—44°F to 50°F—can be delivered by a dedicated outside-air system at ceiling level via high-induction diffusers, which removes about 6 Btu/hr. per sq. ft..of sensible load from the radiant panels. This can help reduce CRCP first costs significantly and cut associated pump operating costs. And second, better air delivery to the CRCPs can boost the amount of heat the units remove from the building by up to 40%. This helps cut CCRP first costs—and pump operating costs—even more.

CSE: What factors affect how much outdoor air is used?

ABNEY: ASHRAE 62-1999, Addendum n, Table 6.2, defines the zone air effectiveness of overhead and underfloor systems. In a typical overhead system, with a ceiling supply of cool air, the air distribution effectiveness (Ez) is 1.0, which is the same as for a typical underfloor system.

MUMMA: More specifically, the ASHRAE Addendum states: Floor supply of cool air and ceiling return provided that the 0.8 m/s (150 fpm) supply jet reaches at least 1.4 m (4.5 ft.) above the floor. Most underfloor air distribution systems comply with this proviso, where Ez = 1.0.

Also, it allows for floor supply of cool air and ceiling return, provided low velocity displacement ventilation achieves unidirectional flow and thermal stratification where Ez = 1.2. With ceiling supply of cool air, Ez = 1.0. Ez, the zone air distribution effectiveness, is used in the denominator to compute the corrected outside airflow per zone. In general, ceiling and UFAD systems have Ez = 1.0, indicating no difference in the quantity of outside air required. But if low velocity displacement ventilation—not normally the case with UFAD—achieves unidirectional flow and thermal stratification, the corrected outside air is about 17% less than that for UFAD or ceiling delivery approaches.

INT-HOUT: Humidity is a major factor in outside air management. Increased awareness of mold and fungus, which are directly related to moisture in the building, is a driving force in minimizing outdoor air. But, at the same time, new systems are being introduced allowing a higher percentage of outside air to be properly and more economically conditioned, i.e. dehumidified. Productivity studies now show that there is an economic benefit for increasing the quantity of outside air, so this will eventually drive ventilation rates above the minimum levels required in codes and Standard 62.

CSE: Are there any recent code changes that could affect the design of these systems?

MUMMA: Yes, as code jurisdictions adopt ASHRAE Standard 62, Addendum n.

KALLSTROM: The same codes and standards apply to both overhead and underfloor systems. But building officials are getting more concerned about UFAD system compliance to existing codes and standards, as these designs become more common. Engineers need to consult local officials throughout design to reduce the potential for problems. As additional research results are obtained and existing UFAD applications are evaluated, we will see changes and exceptions to the codes and standards for UFAD systems.

CSE: What kinds of technological improvements are being made with underfloor and overhead air systems?

INT-HOUT: Equipment is continually being introduced for both concepts, to meet competitive needs, and new specification software is improving product selection for both types of systems.

KALLSTROM: The two approaches use many of the same components, but some new products are being introduced specifically for UFAD. Early UFAD applications required adapting overhead products under the floor, which possibly resulted in design compromises. A number of manufacturers now have developed products ideally suited to UFAD systems, including products designed to fit within the 2-ft. by 2-ft. raised access panels, products with top access to aid in the maintenance from above, increased controllability and feedback of UFAD components.

ABNEY: Use of ultra high-efficiency ECM motors for both overhead and underfloor systems has increased over the last couple years. ECM motors are 70% to 80% efficient across the operating range, providing energy savings to the building owner.

Alternate liner options for both overhead and underfloor systems also have become available recently as an option to fiberglass liners. In addition, the International Agency for Research on Cancer has upgraded its assessment of the danger of formaldehyde, declaring that it is “carcinogenic to humans.” This will increase the demand for non-formaldehyde liner materials.


Jenny Abney , Senior Project Manager, Titus, Richardson, Texas

Stanley A. Mumma , P.E., Ph.D. , Architectural Engineering, Penn State University, University Park, Pa.

Dave Kallstrom , Sales & Marketing Manager, Fans & Ventilators, Greenheck, Schofield, Wis.

Dan Int-Hout , Chief Engineer, Krueger, Richardson, Texas

UFAD: Pluses and Minuses

UFAD Advantages:

If 100% outside air, slightly improved IAQ at breathing zone.

For interior spaces that do not have dropped ceiling, the floor above acts as radiant panel at perhaps 70°F. If the air is delivered in the plenum unducted and is able to extract part of the sensible load from the space below, the required supply air flow rate is reduced.

Generates LEED rating points.

UFAD disadvantages are:

If not 100% outside air, the typical application where 80% of the supply air is recirculated air is already loaded with problems.

66°F air at floor can cause cold feet.

Leakage in the plenum and the floor.

If the leakage problem is fixed, then the plenum creates a drainage problem, particularly in a sprinkled building.

IAQ issues in 90% of applications.

Predicted energy savings not always realized.

Lacks field verification.

Humidity control problems.

Not generally suitable for serving perimeter zones.

Elimination of acoustical ceiling causes noise problems.

Floor dirt can become airborne.

Air distribution problems.

Code requires wiring and cabling to be in conduit, greatly reducing the flexibility.

Floor plenum greater than 18 in. must be sprinkled.

Non-occupant sources of contaminants originating below the breathing zone create a higher concentration than conventional air delivery systems.

High-Induction Diffusers

High-induction diffusers entrain large quantities of room air, greater than 10:1. High entrainment is achieved by:

Introducing the injected air with high momentum.

Providing maximum contact surface between the injected and room air.

Establishing nonturbulent linear flow within the nozzles.

Spacing the nozzles for maximum contact between the jets and the room air.

Directing the jets at a specific angle away from the ceiling, to create a negative-pressure region, and enhancing room air entrainment above the jet.

Diffusers with these capabilities were originally designed for low-temperature applications and are readily available today. Though these systems can make it more difficult for owners to accommodate office churn, they offer a number of advantages, including:

High ADPI, near 100%.

Virtually no short circuiting, supply to return.

Air can be supplied cold, reducing supply-air quantity.

Can increase the heat transfer to terminal heating and cooling equipment such as radiant ceilings.

Puts heat removal at the upper body, not the feet, resulting in improved thermal comfort.

Provides needed air motion without drafts.

Superior air distribution.