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Regenerative Dual Duct: An Overview

May 1, 2009
System helps ensure VAV systems provide adequate ventilation

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 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.)


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.

Actual IAQ and energy-conservation benefits were monitored by multiple state agencies. This project demonstrated that a single properly designed 100-percent-outdoor-air system not only can meet the heating and cooling needs of any space efficiently, it can process, manage, and monitor the ventilation needs of any mix of occupancies efficiently using variable-volume ventilation down to the individual-room level.


Outdoor-air ventilation is the largest heating and cooling load component in most facilities. In Wisconsin, envelope heating losses of Standard 90.1-compliant schools with classic VAV-reheat systems comprise only 6 to 8 percent of total heating-plant-capacity requirements. The remaining 92 to 94 percent is needed to offset the energy used to process and deliver air. This remaining capacity tends to be larger than that of outdoor-air loads because of the presence of other systemic inefficiencies, such as reheat and excessive reliance on refrigeration.

Outdoor-air economizers generally are regarded as inefficient because delivered air must be cooled to satisfy the space requiring the coldest air, then reheated for other areas. If outdoor air is introduced through a mixed-air path, the mixed-air temperature may need to approach outdoor ambient design conditions at minimum system flows to satisfy ventilation requirements. This occurs because VAV systems are thermally biased, designed to cool and deliver air at temperatures substantially lower than the space temperature. Therefore, because traditional VAV systems cannot utilize low-grade heat resources from people, lights, and equipment, they are treated as waste products, preventing energy from being recycled.

Lastly, HVAC systems rarely are designed to operate from enthalpy. They are controlled by temperature, a measure of sensible heat. Enthalpy is used only to initiate a switch between minimum and 100-percent outdoor air to minimize refrigeration energy use. HVAC systems typically do not have a mechanism to take advantage of latent energy reduction for cooling or humidification purposes. The only way to accomplish that is with the adiabatic phase change between liquid and vapor.


RGDD systems address the major causes of inefficiency in classic HVAC systems, such as difficulty processing and managing outdoor air. RGDD systems employ only 100-percent outside air, using no “return” air. Because all of the air is exhausted, the “return” path becomes a large central exhaust system, permitting exhaust from individual zones to be funneled through a common path in which contaminants are diluted to harmless levels before the air is stripped of its energy assets.

RGDD systems are designed around a two-stage air-to-air energy-recovery process employing three air paths (Figure 1). First, in the supply-air stream, a primary heat exchanger employs an indirect evaporative-cooling process. A second heat exchanger can be a sensible, only, or total energy air-to-air unit. Located between the two heat exchangers is a direct evaporative process that chills the water passing directly over the evaporative media in lieu of using a cooling coil. During the cooling season, the air leaving the direct evaporative cooler is kept at approximately 55°F to maintain humidity using an apparatus dew-point control process.

Used by Willis H. Carrier,2 this technique serves multiple purposes. First, the direct evaporative cooler acts as a wet-air scrubber, removing particulate and water-soluble gases from the air. Second, microbial contaminants are removed from the air as particulate, captured in the sump, treated with a non-chemical biological control system, and automatically flushed out of the system. Third, by eliminating the need for a cooling coil, parasitic fan energy losses are reduced substantially. Fourth, the “soft” nature of evaporative media acts as an effective noise-reduction system, typically eliminating the need for sound attenuators. Finally, the air is delivered at a constant dew-point temperature, automatically maximizing the thermodynamic impact of the adiabatic processes and minimizing the need to expend energy for the mechanical-refrigeration process, relegating refrigeration to the role of “cooling resource of last resort.”

Downstream of the fan, the supply-air stream is split into cold and ventilation decks, which deliver air at two temperatures. The ventilation deck is tempered through the secondary air-to-air heat exchanger using low-grade heat energy stripped from the building's exhaust. Supplemental heat is introduced into the exhaust path where a single source of heat provides the necessary energy for the primary heat exchanger's preheat, supplemental-heat, and frost-prevention features.

This energy exchange provides multiple benefits. First, it eliminates the need to expend virgin energy assets (in the form of energy-wasting reheat) to provide precision temperature and humidity control during the cooling season. Second, by depressing the dry- and wet-bulb temperature of the exhaust air, the exchange of energy amplifies the recoverability of latent cooling potential in the indirect evaporative cooler. This can reduce outdoor-air cooling loads up to 80 percent during hot and humid weather and eliminate the need for refrigeration in less extreme weather.

The RGDD System's design carefully considers how airflow is delivered to a space. It is suited for displacement strategies in which heat gain to a space is deferred to the exhaust-air path and can be displaced out of the building, rather than seen as a refrigeration load. Because both decks provide 100-percent outdoor air, actual ventilation rates provided through dual-duct VAV air-terminal units can be monitored.

Flow reduction is maximized when airflow is measured at the cold-deck and total-flow positions. Using 100-percent outdoor air allows VAV systems to control individual-space airflows over a large range of outdoor-air ventilation rates. Occupancy sensors can be used to reduce minimum ventilation rates to zero when spaces are unoccupied, reducing the control of the cold deck to shut off VAV. Total-airflow sensors can be used to monitor and trend actual air-delivery rates to individual spaces to document compliance with Standard 62 requirements. Local heat typically is supplied and controlled locally via radiant floors, ceilings, or baseboard radiation.


The RGDD System should not be employed in facilities smaller than 20,000 sq ft. It specifically supports ventilation-dominant variable-volume applications in which outdoor-air ventilation rates exceed 21 percent of peak system design airflow. Able to support almost any mix of occupancies, the RGDD System can be designed to deliver airflow of up to 140,000 cfm per unit. It is applicable in laboratories, hospitals/clinics, casinos, convention centers, educational facilities, and other high-density-occupancy structures. Viable in hot/humid and arid environments, the system is appropriate in multifunction applications in which multiple individual HVAC systems can be replaced by a single system and air quality is or has been a problem.


Physically, the RGDD System is straightforward. By efficiently processing ventilation and eliminating systemic inefficiencies, it can significantly reduce primary-plant heating-capacity requirements. Cooling capacity also can be reduced significantly. For example, in the RGDD System prototype installation, the educational facility's required boiler-plant capacity was reduced by 75 percent; actual primary-cooling-plant capacity was reduced by a factor of 8-to-1.

Psychrometrically, the RGDD System is different from other types of HVAC systems. With six sets of design conditions, the RGDD System's airflows are only partially based on cooling loads. Psychrometric calculations must be prepared for heating, cooling, and intermediate operating conditions. Most elements in the RGDD System serve multiple functions that vary depending on ambient conditions and thermal loads. Its processes are selected, configured, and designed to work synergistically by converting energy back and forth between latent and sensible forms and exchanging energy between airflow streams.

The system creates and relies on the interaction of its various components, permitting it to amplify energy availability for recovery. However, this interaction makes the system difficult to design and commission. Because the RGDD System requires commissioning, engineers and commissioning agents will need training in its application.

The system can be used for thermal storage. Because smaller chillers consume less energy and reduce electricity demand, the implications for primary-cooling-plant size and energy and cost savings are significant.

A designer also must keep in mind that the RGDD System strategy is new, and contractors and temperature-control specialists probably will not have had previous experience with it. Because the system employs 100-percent outdoor air, proper testing and balancing, system-control setup, and commissioning are critical to a successful installation. A system designer must have command of the system's engineering and control issues and be prepared to provide substantial field supervision and troubleshooting services to help ensure a successful installation. The system's design is unusual enough that a first-time designer probably will not be able to provide a successful application without proper instruction and guidance. Designers and commissioning agents must be trained and licensed, and projects must be registered for the protection of the system, those involved in its installation, and public health and safety.


  1. Lentz, M.S. (2009, January). Regenerative Dual Duct: A case study. HPAC Engineering, pp. 20-26.

  2. Carrier, W.H. (1937). The contact mixture analogy applied to heat transfer with mixtures of air and water vapor. Transactions, 59, 49-53.

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The president of Lentz Engineering Associates Inc. ( and a member of HPAC Engineering's Editorial Advisory Board, Mark S. Lentz, PE, is nationally recognized for having successfully developed, tested, and proven several advanced HVAC-system strategies designed to exceed the performance requirements of ANSI/ASHRAE/IESNA Standard 90.1, Energy Standard for Buildings Except Low-Rise Residential Buildings, while meeting or exceeding the requirements of ANSI/ASHRAE Standard 62.1, Ventilation for Acceptable Indoor Air Quality. He is the recipient of numerous national engineering awards, including an American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) Energy Award, and an ASHRAE Distinguished Service Award.

Evaporative Cooling and the Risk of Legionnaires' Disease

Evaporation is the “greenest” of all HVAC processes. It has the ability to alter psychrometric state points without expending new energy assets. It is 100-percent efficient and has zero carbon footprint. When properly applied, evaporative processes have the potential to save more energy and better enhance air quality than other green solutions, such as geothermal, solar, and wind power. Evaporation alone can close the thermodynamic loop on latent energy.

However, the use of evaporative cooling often elicits concern for Legionnaires' disease. Legionnaires' disease requires operating temperatures between 85°F and 125°F to flourish. Evaporative-cooling systems never approach these temperatures and, therefore, Legionnaire's disease has never been traced to this kind of equipment. Additionally, research has demonstrated that corrugated evaporative media not only do not aerosolize bio-aerosols into the air downstream of direct evaporative equipment, they actually remove particulate from air. An 8-in.-deep bed of media will remove particulate and many gaseous contaminants from an air stream, as well as approximately 70 percent of bioaerosols when it is irrigated with water. Therefore, it is not surprising to find microbes in sump water.

The real concern is the formation of bio-slimes, which can cause “swamp-cooler odor” when they die. The trick is to prevent them from setting up. ASHRAE Guideline 12, Minimizing the Risk of Legionellosis Associated With Building Water Systems, recommends the use of low-level photochemical ozone generators. These devices, in combination with appropriate purge and dump cycles, can control the level of microbes in a sump and prevent the scale formation in the media.