The energy crisis of the 1970s led industry experts and legislators to conclude that buildings needed more insulation, infiltration required additional control, and ventilation rates had to be reduced. This had the unintended consequence of depressing indoor-air quality (IAQ). By 1989, this was recognized as a major problem, and ANSI/ASHRAE Standard 62, Ventilation for Acceptable Indoor Air Quality, was updated and reissued. The most significant change to the standard involved the priority assigned to ventilation. Before Standard 62-1989's publication, ventilation could be based on occupancy at the system level, and a system could be designed to provide the larger of the minimum outdoor-air or makeup-air requirements to offset exhaust.

Standard 62-1989 fundamentally changed ventilation requirements. In addition to substantially increasing minimum ventilation rates, it required each space to receive at least the minimum required ventilation under all operating conditions, meaning the critical space dictated the amount of outdoor air needed for all of the spaces served by a system. An equation was provided to reduce overventilation and permit a lower overall rate of outdoor-air introduction at the system level. Section 403.3.3 of the International Mechanical Code, Section 5.3 of Standard 62, and Section 6.3.2.1 of ANSI/ASHRAE/IESNA Standard 90.1, Energy Standard for Buildings Except Low-Rise Residential Buildings, explicitly require special controls to override mixed-air controls during mixing and recirculation in variable-air-volume (VAV) systems to ensure sufficient introduction of outdoor air at the system level to compensate for flow reductions.

The impact of these requirements on classic HVAC-system design was enormous. The dramatic increase of outdoor-air ventilation rates shifted the primary function of HVAC systems from thermal control to ventilation management. Conventional HVAC systems, by virtue of configuration and processes employed, simply were incapable of either efficiently or effectively accomplishing this switch.

For engineers concerned with energy conservation, Standard 62-1989 seemed to be a major step backward. Most HVAC-system strategies could not achieve required ventilation levels without major capacity and/or operating energy penalties. Compliance could not be achieved with strategies that had been employed for decades, including shutoff-VAV strategies. VAV-reheat strategies had such severe problems achieving compliance that practitioners essentially had to resort to using 100-percent outdoor air to achieve compliance or ignore the standard.

Classic VAV-system strategies derive energy conservation at the expense of ventilation. Maintaining a given ventilation rate with a VAV system requires that the amount of outdoor air introduced at the air-handling unit must increase as airflow to the space declines. The Regenerative Dual Duct (RGDD) System was developed to provide a VAV system that could process and manage ventilation efficiently. (“Regenerative Dual Duct System” is a registered trademark and service mark of Lentz Engineering Associates Inc.)

GENESIS OF A SYSTEM

Two projects with extreme air-quality problems led to the development of the RGDD System:

• An industrial application required close control of temperature and relative humidity for process purposes and large amounts of outdoor-air ventilation to dilute and dissipate diffusely generated contaminants. The facility had been designed much like a data center, with constant-volume air delivery, terminal reheat, steam humidification, and minimal ventilation. However, contaminants generated in the space created potentially serious health and safety issues for the facility's employees. The situation called for an increase in air exchange.

  The local public utility funded a study to examine the heating and cooling implications of various configurations of multiple air-to-air heat exchangers. Using a bin-hour analysis, an engineer computed a potential thermal-energy savings of 91 percent through the application of energy-recovery technology. The equipment manufacturer independently confirmed that a 93-percent thermal-energy reduction was possible. Performance expectations were so high that the utility paid to submeter the energy-recovery/air-handling process subsequent to construction of an energy-recovery air-handling system. To everyone's surprise, the system's performance exceeded expectations when actual energy-use savings were measured at 97 percent. The owner recovered the entire cost of the system in less than two years. Fundamentally, the project demonstrated that — when designed aggressively — 100-percent outdoor-air systems can outperform and compete economically with recirculating systems without subsidies.

• An educational facility had documented thermal-control and IAQ problems.¹ The study performed for the industrial facility demonstrated that different configurations of heat exchangers could produce relatively similar energy-performance levels, while providing distinctly different environmental conditions. Because the educational facility lacked the industrial facility's need for humidity control, the application was less energy-intensive. Also, because the educational facility's HVAC system emphasized cooling-load avoidance, rather than humidification, a different mix of technologies offered superior performance capabilities. Further, while the industrial facility was essentially a large single-zone system, the educational facility had hundreds of rooms with different cooling needs. This led to the educational facility's adoption of a dual-path air-distribution system that could control cooling and ventilation.

Although its immediate concern was to correct the air-quality problems, the client also wanted to reduce energy use. When shown documentation on the actual performance of the systems installed at the industrial facility, the client agreed to install an RGDD System.