Early air-conditioning systems utilized air-distribution systems with constant volume and variable temperature to control space temperature. In the 1970s, variable-air volume (VAV) revolutionized how commercial buildings were air-conditioned. The market drivers for VAV were increased zone control and reduction in energy consumption. Now, another revolution is under way: ductless VAV systems. By eliminating ducts while augmenting the benefits of VAV, these systems have the potential to further improve comfort and reduce energy costs, while providing a more sustainable and flexible HVAC system. Additional benefits include easier design, improved air quality, reduced life-cycle cost, and, potentially, reduced capital cost.
Description of the Systems
There are two main types of ductless VAV systems: underfloor and overhead. Both systems replace branch ductwork with broad airways, so they are referred to as airway systems. (Main shaft ductwork is still required to deliver air to the airways). Both systems operate at low static pressure, which reduces air leakage and saves energy. Static pressure of 0.05 inches w.g. has been proven effective in today’s underfloor-airway systems, and is now being applied to the new overhead-airway systems.
An underfloor-airway system is used in conjunction with an access floor. The space beneath the floor is used as the supply airway (it also houses power, data, and telecomm cabling). Diffusers discharge conditioned air at floor level into the occupied zone, which is defined as the vertical space from four to 67 inches above the floor. After induced mixing, the air picks up heat and pollutants from the space. Natural convection carries the warm, polluted air to the ceiling where it is returned to the air-handler via a ceiling plenum. First-generation underfloor systems were constant-volume, and less energy-efficient than ducted VAV systems. Second-generation underfloor systems are now VAV.
Newly developed overhead-airway VAV systems deliver many of the same benefits as underfloor-airway systems. The overhead system utilizes a two-level ceiling. The lower space, between the aesthetic ceiling and the median ceiling, functions as the supply airway. The upper space, between the median ceiling and the slab above, functions as the return airway. Air is discharged into the occupied space via specialized VAV diffusers. Return grilles (which can be an integral part of the supply diffusers) carry air through the supply airway into the return airway.
Overhead-airway systems utilize an innovative diffuser and control system to accomplish VAV operation. The supply airway operates at low static pressure (0.05 inches w.g.), which lowers energy consumption and air leakage but does not work well with traditional “volume” modulation (i.e., it does not maintain consistent airflow velocity). So, a new approach was developed to bring the benefits of an airway system to overhead air-distribution.
Instead of “volume” modulation, “time” modulation is used. When a diffuser is “on,” it supplies 100-percent airflow at a consistent velocity. When it is “off,” it supplies zero-percent airflow. At 100-percent load, each diffuser will be on 100 percent of the time. At 50-percent load, each diffuser will be on only 50 percent of the time, providing for 50 percent of the cooling. The cycle rate between “on” and “off” is very short, as low as six seconds, and occupants are not aware of volume changes. In a given zone, the control system decides how long and in what sequence each diffuser should be on as the load changes. In this manner, VAV operation is accomplished while maintaining consistent-velocity air distribution.
Benefits Offered by Both Systems
There are a number of benefits that both the underfloor- and overhead-airway systems offer when compared to overhead-ducted systems.
Flexibility. Duct systems require careful design. The challenge is to maintain relatively straight runs while accommodating all building structural elements and aesthetic requirements. Changes in the floorplan during construction can impact duct-system layout and increase installation cost. Once installed, changes to a duct system are impractical, meaning comfort and energy costs may be compromised if office space is rearranged or building-use patterns change.
In airway systems, diffusers can be located anywhere in the floor or ceiling grid, simplifying air-distribution design and layout. The airway system is clearly easier and faster to design. If the facility utilizes rooftop HVAC units, the airway system also provides greater flexibility for their placement, because ductwork constraints are eliminated. The number and size of rooftops can change, even at the last minute, without extensive drawing and design revisions.
Lower energy consumption. Airway systems reduce supply-fan energy consumption. Ducted systems typically require 1.5 inches w.g. or more of static pressure in the branch ducts. By comparison, both airway systems require only 0.05 inches w.g. of static pressure to distribute air to the space. This can reduce supply-fan energy by as much as 30 percent.
An underfloor system can also save energy by reducing the airflow requirement in a number of ways. The first way is by taking advantage of temperature stratification. In a properly designed underfloor system, only the occupied zone (OZ) is conditioned. Above the OZ, the temperature will be higher (by design), unlike an overhead-ducted system where the temperature from the floor to the ceiling stays within a narrow band. So the volume of conditioned space is reduced, which may allow the airflow to be reduced. Airflow can also be reduced because of the cooling load at the building skin. Ideally, in the perimeter zone of an underfloor system, portions of the skin-cooling load never make it into the OZ. Instead, the warmed air flows up the skin directly into the stratified zone. Because the supply air does not have to handle this load, the supply airflow can be reduced. The amount of reduction will depend on the geometry of the facility and the construction of its skin.
An underfloor system can also reduce the mechanical-cooling energy. Because it supplies 60 to 65 degrees F. air – vs. the 55 degrees F. air typically required by an overhead system – outside-air free cooling can be used for more hours during the cooling season. The reduction in mechanical-cooling operating hours may be as much as 30 percent annually, depending upon the local climate.
Mold prevention. With the growing recognition that supply-air ductwork can be a breeding ground for mold, building owners are concerned with preventing the conditions conducive to mold growth. The conditions that can promote mold growth in supply ductwork are nutrient-providing dust and relative humidities of 90 percent or greater. Better filtering, and access for cleaning the ductwork, can reduce dust, but at higher energy and capital costs. A less costly solution is to maintain the relative humidity of the supply air under 80 percent, which rids the air system of enough moisture for mold formation.
An underfloor-airway system can easily meet this requirement. As stated above, it is designed to supply air to the space at 60 to 65 degrees F., instead of the 55 degrees F. air supplied by an overhead system. And the method by which the system creates 60 to 65 degrees F. supply air is the key to reducing its relative humidity.
When the return air from the space reaches the air-handler, it is split into two separate airstreams. One stream mixes with fresh air and flows through the filters and cooling coil, emerging at approximately 55 degrees F. and 90-percent relative humidity. The other stream passes through separate filters and bypasses the coil, so its conditions might be 85 degrees F. and 50-percent relative humidity. Downstream of the coil, the two airstreams mix and the resultant air is warm enough to be introduced into the space from the floor, and dry enough to prevent mold formation, even under high-humidity outside-air conditions.
Overhead-airway systems can also be designed to prevent mold formation. These systems typically supply 55 degrees F. near-saturated air directly from the cooling coil, just as ducted systems do. So the airway system must be modified to lower the relative humidity of the supply air. One technique is to place bypass fans between the supply and return airways, which bypass some return air into the supply air. This raises the temperature and lowers the relative humidity of the supply air. Heat from light fixtures can perform the same function.
Reduced façade costs. A typical overhead-ducted system requires 36 to 48 inches between the ceiling and the structural slab above. The access floor in an underfloor-airway system adds 12 inches to the slab-to-slab height, but the elimination of branch ducts removes as much as 24 inches above the ceiling. The net result can be a reduction in slab-to-slab height of as much as 12 inches. The overhead-airway system can also reduce slab-to-slab height because each airway functions like a very wide, and therefore low-height, duct. In this case, the reduction can be as much as 24 inches. In either case, the cost savings can be appreciable.
Unique Benefits of the Underfloor-airway SystemIn addition to the shared benefits, an underfloor-airway system offers unique benefits when compared to an overhead-ducted system:
Improved air quality. Any overhead system provides air quality by dilution: The fresh air mixes with the contaminated air in the space, and a portion of the mixture is removed. Because fresh air only dilutes the contaminated air, air quality is compromised. In short, the contaminant level in the occupied zone is much closer to the level of the exhaust air than to that of the supply air.
Underfloor-airway systems introduce fresh air directly into the occupants’ breathing zone. Heat, pollutants, and stale air rise to the ceiling level, where they are exhausted. Fresh air replaces the old air rather than diluting it. The net effect is to improve the ventilation effectiveness, which improves the air quality. Figure 4 demonstrates this process.
Lower life-cycle costs. On average, the capital cost of equipping a building with access flooring, underfloor HVAC, and underfloor power/data/telecomm cabling is roughly the same as equipping it with overhead-ducted HVAC and poke-through cabling.
However, underfloor-airway systems can offer major savings in life-cycle costs, and these cost savings can be enjoyed every year of the building’s operation. Overall, operating cost savings include reduced workstation churn costs, reduced HVAC energy costs, and accelerated depreciation costs (the access floor, underfloor HVAC, and cabling components can often be depreciated over six years instead of 39). Occupancy cost savings may also include better productivity due to the improved air quality.
Better comfort. An overhead-ducted VAV system will always pose air-distribution challenges that may compromise comfort because the ducted system uses volume modulation. The diffusers are designed to discharge the air at high velocity at their design volume. The high velocity induces good mixing with the air in the space.
The challenge occurs at low loads, when the airflow and discharge velocity are reduced. Under these conditions, the throw from fixed-area diffusers will diminish, and the airflow in areas between diffusers will stagnate.
Overhead-airway systems avoid these problems because they use time modulation instead of volume modulation, as described earlier. Time modulation ensures that whenever a diffuser is distributing air, it is at 100-percent flow and throw. So, there is no stagnation in the space.
Lower capital cost. Ductwork can be a considerable portion of the cost of an overhead-ducted system. Overhead-airway systems offer an attractive alternative. The ductwork portion of the overhead-ducted system will usually cost $3/square foot or more in materials and labor. In addition to the ductwork costs, there are the costs of the VAV boxes, controls, and diffusers. In contrast, an overhead-airway system will usually cost $2/square foot for the portion of the system that replaces the ductwork. Added to this price is the cost of the airway diffusers and controls, which are of comparable value to the VAV boxes, controls, and diffusers of a ducted system. Installing the overhead airways is easier than installing ductwork, and it is not much more difficult than installing a conventional suspended ceiling, despite the two levels.
In general, airway-based air-distribution systems offer a number of potential benefits when compared to overhead-ducted systems. Both underfloor- and overhead-airway systems provide easier design, lower energy consumption, mold abatement, and reduced building-façade costs. Underfloor systems offer the additional benefits of improved air quality and lower life-cycle costs, while overhead systems offer better comfort and lower capital costs. Air distribution may never be the same.
Joseph M. Halza is a product marketing manager for York International Corp. (www.york.com), York, PA, where he has been employed for 15 years in various engineering and marketing positions related to air-handling and air-distribution systems.