Sky's the Limit

Poor air quality and high energy use are problems especially severe in educational facilities. This, of course, is due to consistently high occupant densities and high required rates of ventilation. But these twin dragons have also come home to roost because of the way school HVAC systems have historically been designed.


Poor air quality and high energy use are problems especially severe in educational facilities. This, of course, is due to consistently high occupant densities and high required rates of ventilation. But these twin dragons have also come home to roost because of the way school HVAC systems have historically been designed. In fact, nearly 70% of the nation's educational facilities report air quality problems—a clear indicator that the "tried and true" solutions of the past are not up to the challenge. Even modern educational facilities, featuring highly efficient envelopes, ironically, often use significantly more energy than facilities designed 50 years ago.

In an era of rising energy costs, this is a major concern, especially as building heating and cooling systems are responsible for approximately 40% of total energy use in the United States, and 65% of the demand on the nation's power grid. Furthermore, unlike the energy crisis of the 1970s, the rising cost of energy is unlikely to go away, as world energy production is declining and proven reserves are dropping rapidly. Soon, there will be no choice but to sharply reduce energy use.

The good news is that the technological opportunities for enhancing all aspects of the performance of educational facilities, while simultaneously correcting IAQ problems, are far greater than most designers are even willing to consider. The bad news is that the HVAC industry will have to completely rethink the basis of its current work. In other words, designers will have to return to fundamental principles and develop new solutions to eliminate the systemic inefficiencies built into designs for decades. Classical solutions will have to be abandoned in favor of methods that correctly define the challenge, identify energy conservation areas and develop new system solutions to correct performance deficiencies. Finally, engineers must learn to take advantage of the opportunities that present themselves.

Defining the challenge

How we manage ventilation is at the very heart of the challenge. It has long been viewed as expensive with respect to energy use. But before dismissing increased ventilation, an examination of typical HVAC configuration and control strategies reveals some very important distinctions:

  • Outside air is typically introduced through the mixed air path. Ventilation is either introduced in fixed amounts or controlled from temperature, which is unrelated to the need for ventilation. This practice is thermodynamically equivalent to infiltration.

  • Air introduced through the mixed path must be relieved from the structure through exhaust systems or a relief path. This results in large thermodynamic losses in the form of conditioned air exhausted or relieved from the building.

  • Energy is added to or removed from the mixed air stream to condition the air, literally a brute force strategy.

When outdoor air is introduced through the mixed air path to meet space cooling needs, we euphemistically call it an "economizer" or "free cooling" process. When cold air is introduced in this manner, and the ventilation air thermodynamically balances internally generated heat, the result is "free" cooling. However, when the amount of outdoor air exceeds what's required for free cooling, or when humidification is required—or at virtually any time during the cooling season—the outdoor air represents an additional load on the system and is the thermodynamic equivalent of infiltration.

Analyzing the total impact of ventilation and reduced cooling loads on the dynamics of this process reveals that free cooling is not occurring, and the economizer process—less energy intensive than using refrigeration in cold weather—is actually the major source of HVAC energy use. This is especially true in areas that are ventilation dominant, like educational facilities.

To illustrate this point, consider the dynamics of a typical classroom served by a rooftop VAV-reheat air-handling system. The typical classroom is usually about 900 sq. ft. in area. Lacking major solar loads or large internal equipment-generated heat gains, a generous estimate of the design cooling load from lighting and occupancy is approximately 1.0 cfm per occupied sq. ft. Class size varies, but typically ranges between 25 and 30 students. For convenience, the occupancy densities in Table 2 of ASHRAE Standard 62 serve as the benchmark, which is 20 sq. ft. per person. Cooling design conditions of 90°Fdb and 73°Fwb are also employed. Therefore, 15 cfm per person of outdoor air ventilation must be provided at a ventilation effectiveness of 0.8. As a result, AHUs would have to be designed to handle approximately 25% outdoor air in peak heating and cooling conditions. Given these assumptions, the following can be computed:

  • The cooling-based design airflow would be approximately 900 cfm, or a sensible cooling load of approximately 19,500 BTU/hr.

  • The number of people for which the system would need to be designed would be 45 (900 sq. ft.

  • The minimum amount of required outdoor air is approximately 845 cfm (45 people x 15 cfm/person

VAV systems are typically designed to meet cooling needs and permitted to deliver less than design air delivery rates when cooling loads permit. They presume a cooling dominant design. This is where things get interesting—ASHRAE Standard 90.1 requires that minimum air delivery rates be set to no greater than 30% unless greater rates are spelled out under ASHRAE Standard 62, which of course, requires minimum rates of ventilation be provided whenever a space is occupied.

  • At 30% design cooling flow and 25% outside air at the AHU, the system provides 270 cfm total air with 68 cfm of that being outdoor air—only 8% of the minimum classroom ventilation rate, but in compliance with Standard 62. Remember the minimum rate of ventilation required for a classroom at a 25% AHU outdoor air setting is computed by dividing the outdoor air requirements of the space by the outside air fraction on the unit. This computes to a minimum air delivery rate of 3,380 cfm to the space or 375% of the rate required to meet space cooling requirements. The significance of this observation is that the air delivery rates are not determined by cooling loads, but by ventilation requirements.

  • The minimum computed air delivery rate requires approximately 2,480 cfm of air in excess of that required for the space cooling loads to meet minimum ventilation requirements. This requires a total cooling load of approximately 124,000 BTU/hr—approximately four times that of the basic cooling load—and a reheat load of another 55,000 BTU/hr., due to the fact that the space must be over-ventilated. This produces a combined net ventilation energy penalty of approximately 146,000 BTU/hr. above and beyond what is required to meet a basic cooling load of 33,000 BTU/hr.

Considering this waste, note how just a few simple changes would fix these problems:

  • If the system were providing 100% outdoor air without energy recovery, the total cooling energy requirements for the space would only be 64,000 BTU/hr.

  • If the system were providing 100% outdoor air with an effective energy recovery strategy, this load could be reduced to approximately 40,000 BTU/hr., or approximately 22% of the recirculating VAV reheat strategy.

This analysis only considers the load characteristics of the system at design cooling conditions. Under part-load conditions, the scheme will use progressively more energy, because it is necessary to false load the space with reheat to provide the necessary thermal control. But the example demonstrates at least three engineering facts that run completely counter to prevailing engineering wisdom:

  • Cooling loads do not necessarily determine air delivery rates, especially in high density occupancies. When proper IAQ calculations are performed for educational facilities, ventilation is often found to be the dominant variable for most spaces served.

  • Recirculation of air does not reduce either energy use or the capacity required in ventilation dominated spaces. In fact, when the needs of ventilation are considered, recirculation actually dramatically increases energy use and system capacity requirements by forcing the system to over-ventilate the space.

  • It is not hard to justify the economic viability of energy recovery. When properly applied and analyzed, it is actually hard not to. This is especially true in educational facilities because of the ventilation rates required throughout.

Again, because conventional wisdom does not consider the implications of ventilation, many other examples of engineering platitudes can be developed:

  • Terminal reheat is not necessary for humidity control. Terminal reheat is required when the cooling effect associated with ventilation exceeds that required to meet space cooling requirements. Find a way to eliminate the need for terminal reheat, and every unit of energy saved in the form of terminal reheat will save between one and five units of cooling energy, depending on whether the cooling effect is sensible, latent or a combination. There are at least four different ways to accomplish this objective, three of which employ dual-path ventilation strategies.

  • The dominant control variable for almost all HVAC systems is temperature, a direct measurement of sensible heat. Yet sensible heat represents only half of the thermodynamic definition of enthalpy. Most HVAC systems are completely open loop on latent energy: Humidity is usually removed through the expenditure of refrigeration energy—and often reheat—and added through the expenditure of new energy resources for the generation of steam. The rest of the time, no effort is made to exchange sensible for latent heat energy to manipulate the psychrometric conditions of the air provided. To close the loop, the system must be able to take advantage of enthalpy throughout its entire range of operation.

  • The heat from lights and people are resources long considered by most practitioners to be too low grade to be economically viable. While true for central station air handling strategies—which are biased toward cooling like VAV and terminal reheat systems—the air relieved through the use of an economizer process contains valuable energy assets which are largely recoverable for multiple purposes using appropriate processes. In fact, it is possible to employ these energy assets throughout the entire range of system operation in virtually any climatic region. This, in turn, can be used to make 100% outside air systems not only viable, but substantially more efficient than recirculation strategies.

Part 2: High performance

To review, these arguments, hopefully, convey the benefits of employing 100% outside air systems. And obviously, a 100% outside air system helps resolve the IAQ part of the school problem, but the high performance issue remains. To make 100% outside air systems economically viable, HVAC systems must be constructed around heat exchanger technologies. This completely changes the whole design dynamic because heat exchanger technologies create a vastly different set of challenges and opportunities. However, 100% outside air systems offer impressive advantages. Specifically, they:

  • Take the whole IAQ issue off the table, protecting the design professional and his or her client from litigation.

  • Have enormous flexibility. All categories of space can be served by a 100% outside air system.

  • Permits an almost infinite variety of spaces to be served from the same system regardless of the activities supported in adjacent spaces.

  • Allow future modifications to require little change. This is important, as it can be used to reduce the total number of HVAC systems required for a project, as well as produce future cost avoidance when space uses are modified.

  • Permit ventilation to be effectively managed down to individual rooms.

  • Have greater turndown characteristics—when used with variable volume—than would be possible with recirculating strategies.

  • Permit energy use reduction through air distribution and processing techniques not possible with recirculating systems.

  • Can be thermal bias neutral, yielding operational economies unachievable with recirculating strategies.

Another key technology for making 100% outside air schemes work is the adiabatic change of latent heat to sensible heat, and sensible heat to latent heat. The latter is often referred to as the direct evaporative cooling process. The former occurs with the indirect evaporative process and also when condensation occurs in a heat exchanger. These processes are, at best, very poorly understood in the HVAC engineering community. Yet these processes—and these processes only—permit engineers to close the thermodynamic loop on latent energy. While usually dismissed out-of-hand by most engineers, they are extraordinarily powerful when properly employed. They are also so versatile that they know no climatic limitations, be it an arctic or a rain forest environment.

How to get there

The HVAC industry is far from mature. The potential for performance enhancement is not measurable in percentages, but in orders of magnitude. The HVAC industry has been told for years by the scientific community that it could do much better. The engineering community responded with the attitude of, "If it is possible, why hasn't anybody done it?"

All of the technologies necessary to construct high performance HVAC systems are—and have been—available for many years. The problem lies with designers being either reluctant or incapable of conceiving such solutions. The industry typically uses only a relatively small proportion of the tools available and does not even do this very well. As difficult as it may seem, all the noted feats are not only possible, they are actually being done, and being done at construction costs directly competitive with most traditional solutions (see "Sample Sites," opposite page).

Specific answers to questions regarding high performance design can be found in the fundamentals of thermodynamics, heat transfer and psychrometrics. They can be found in the very definition of the term enthalpy. The skills necessary to create new solutions are developed by attempting to continuously expand our technological horizons with every job we do. There is no single answer or correct solution to the challenges of eliminating high energy use and poor air quality—there is only knowledge and an engineer's willingness to use it to solve today's challenges to the best of their ability. The keys to a high performance HVAC system are understanding the fundamental design challenges to be addressed in an application and devising a solution specifically conceived to meet those challenges. Finally, pay close attention to the details. High performance design is a knowledge-based design philosophy that is more difficult to execute than the typically employed cookie-cutter practices, but it produces significantly better results when executed correctly.

Furthermore, high performance HVAC systems are radically different from traditional HVAC system solutions. For example:

  • High performance solutions will be expected to achieve the desired results in the most efficient manner. This means the elimination of systemic thermodynamic inefficiencies becomes a primary design objective, which sharply increases the engineering challenge and forces the HVAC industry to raise the bar of expectations. Examples of systemic inefficiencies include elimination of terminal reheat energy, failure to recycle low-grade energy resources, closing the loop on latent energy, and the elimination of excess ventilation. These are common weaknesses in traditional HVAC solutions.

  • High performance solutions will have to be designed to achieve different types of objectives. Design objectives and priorities will vary from application to application, and high performance systems will have to be more precisely engineered to achieve those objectives. Individual applications will also display more unique characteristics regarding process selection and physical configuration. For educational facilities, one of these objectives becomes the efficient management of ventilation energy.

  • The engineering process will have to change radically in response to the increased use of unique solutions. While computerized calculation procedures will still be used for basic purposes, the use of mass-produced cookie-cutter solutions will disappear for lack of applicability.

  • Systems and equipment must have much longer service lives. Two of the major benefits of high performance solutions are their resistance to energy conservation measures and the fact that they tend to have much longer service lives. This, however, places a premium on equipment quality as the devices installed will be expected to have much longer effective service lives.

Room for improvement

In the book, Natural Capitalism , the authors state that the potential for energy use reduction is on the order of 50 to 1 or better. While the true potential of any solution depends on the application—and what solution any high performance solution is compared to—LEA's experience with almost 2,000,000 sq. ft. of high performance educational facilities completed or currently under construction, is that these numbers are not that far off.

However, achieving high performance is not possible with the classical solutions provided by the HVAC industry. Classic HVAC solutions are fundamentally inefficient. They are dependent on energy intensive processes. Worse yet, they simply do not address the underlying technical challenges. In short, traditional system solutions are the problem. This is where the opportunities for meeting society's needs for the future can be found.

Some 20 years ago, a senior staff engineer at a firm I once worked for said to me, "Nothing has changed in this industry in 25 years except the economics." In the context in which this statement was made, it would be every bit as valid today as it was at that time. While it is true that classic solutions have more capable controls today, complete with new bells and whistles, I agree with my former colleague in that basic solutions—and the presumptions on which they are based—have not evolved at all. However, when taken in context of what is possible with today's available technologies, my former colleague could not have been more wrong.

Be a leader, be patient

Without question, it is the proper role of the HVAC design professional to provide technological leadership. Addressing the challenges we will all be facing in the next decade and beyond requires solutions that not only provide adequate levels of ventilation where and when needed—and with good thermal control—but solutions that operate at levels of efficiency far greater than current design practices ever anticipated.

Keep in mind dramatic performance gains tend not to come in giant leaps, but small steps. Large gains usually result from the realizations of overlooked opportunities. And when a technological paradigm shift occurs, the risks to the designers and to technologies—both old and new—are amplified exponentially.

But risk is not the biggest concern for the engineer. The hard truth is that HVAC designers typically lack the fundamental skills necessary to think outside of the box. More so, they really need to operate in a bigger box and too often are afraid to expand their technological horizons because they associate innovation with risk. But those who subscribe to this philosophy need to be reminded that there is even greater risk in failing to innovate.


MODEL A1.a A2.a A2.b A3.a A3.b A4.c
Alternate System Definitions:
A1 - Regenerative Dual Duct as per LEA design documents
A2 - Built-up VAV with Water-Cooled Rotary Screw Chillers rated at 0.68 kW/ton
A3 - Built-up VAV with Air-Cooled Rotary Screw Chillers rated at 1.3 kW/ton
A4 - Ground Coupled Geothermal Heat Pumps, ISO Performance - 15.0 EER / 3.5 COP, with Variable Speed Pumping
a - Occupancy sensors in classrooms to close supply air dampers upon vacancy and CO2 sensors in cafetorium, gymnasium to reduce ventilation based on occupancy
b - Only CO2 sensors in cafetorium, gymnasium to reduce ventilation based on occupancy
c - Make-up Air Units with Sensible Heat Recovery (80% efficiency), Gas Furnace Heating
Energy Use (Therms)000006417
Demand (Mbh)00000997
Energy Savings (Therms)0000-6417
Demand Savings (Mbh)0000-997
Energy Use (Therms)12326270793757127079375710
Demand (Mbh)199031164379311643790
Energy Savings (Therms)-14753-25245-14753-2524512326
Demand Savings(Mbh)-1126-2389-1126-23891990
Energy Use (Therms)12326270793757127079375716417
Demand (Mbh)19903116437931164379997
Energy Savings (Therms)-14753-25245-14753-252455909
Demand Savings (Mbh)-1126-2389-1126-2389993
Annual Bill ($)11093243713381424371338145775
Annual Savings ($)-13278-22721-13278-227215318
Annual Bill ($)86702111950134972123309149126153231
Annual Savings ($)-25248-48270-36607-62424-66529


MODEL A1.a A2.a A2.b A3.a A3.b A4.c
Energy Use (Kwh)351357351357351357351357351357351357
Demand (Kw)10310210210381102
Energy Savings (Kwh)00000
Demand Savings (Kw)110221
Energy Use (Kwh)820278202782027820278202782027
Demand (Kw)262424261524
Energy Savings (Kwh)00000
Demand Savings (Kw)220112
Energy Use (Kwh)631155902313488759023134887177365
Demand (Kw)583348485057
Energy Savings (Kwh)4092-717724092-71772-114250
Demand Savings (Kw)25101081
Energy Use (Kwh)00000138342
Demand (Kw)000000
Energy Savings (Kwh)0000-138342
Demand Savings (Kw)00000
Energy Use (Kwh)0000076359
Demand (Kw)00000180
Energy Savings (Kwh)0000-76359
Demand Savings (Kw)0000-180
Energy Use (Kwh)271040704368407043680
Demand (Kw)111110
Energy Savings (Kwh)-1360-1658-1360-16582710
Demand Savings (Kw)00001
Energy Use (Kwh)599001111621375361765002168000
Demand (Kw)701732112653830
Energy Savings (Kwh)-51262-77636-11660015690059900
Demand Savings (Kw)-103-141-195-31370
Energy Use (Kwh)11507973510536000
Demand (Kw)111011000
Energy Savings (Kwh)1772971115071150711507
Demand Savings (Kw)10111111
Energy Use (Kwh)1371635334386122349824999192288
Demand (Kw)112834152438
Energy Savings (Kwh)-21618-24896-9782-11283-178572
Demand Savings (Kw)17-23-4-13-27
Energy Use (Kwh)5843326527087593246964748144391017739
Demand (Kw)280371431458554400
Energy Savings (Kwh)-68376-174992-112142-230107-433407
Demand Savings (Kw)-91-151-178-274-120
Annual Bill ($)756098757910115898938115312126835
Annual Savings ($)-11970-25549-23329-39703-51226

Sample Sites

Wausau West High School : A 323,000-sq.-ft. facility in central Wisconsin was first modified in 1998, then recently renovated to a high performance HVAC system to correct IAQ issues and problems associated with long-term maintenance neglect. The system was converted to 100% outside air; the boiler plant was reduced by 60%, yet retained 40% redundant heating capacity; the chiller plant was reduced from768 tons to a 100% redundant 150 tons. Cost: $14.03/sq. ft.

Old River Road School : A 100,000-sq.-ft. middle school in Rockton, Ill., is served by a single 100% outside air main air-handling system. It also includes three furnaces with a total fuel input capacity of 3,026 MBH, and a chiller of 75-tons nominal capacity. Cost: $10.22/sq. ft.

Harper Creek High School : A 247,000-sq.-ft. high school in Battle Creek, Mich., included pool systems served by two 100% outside air main air-handling systems and one, 100% outside air, specialized pool ventilation system. Total heating capacity, including pool water heating, is provided by three 2,000 MBH boilers, including one redundant unit and two 80-ton chillers with 100% redundancy. Cost: $19.63/sq. ft., including plumbing.

Howell Township, N.J. : Three schools—one 115,000-sq.-ft. middle school and two 70,000-sq.-ft. elementary schools are served by 100% outside air handling systems. The elementary schools are equipped with 50 kW photo-voltaic solar arrays. The middle school is served by a pair of 2,000 MBH boilers (one redundant unit) and two 50-ton chillers with 100% redundancy. The elementary schools are each served by two 2,000 MBH boilers (one redundant unit) and two 40-ton chillers with 100% redundancy. Total energy costs, including lighting were projected to be 57% of the costs for a geothermal heat pump system at less than half the installed cost (See table p.33).

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