The Perils of 'Progress'
Seeking simplicity helps bring IAQ solutions to grade schools
Fred Schultz, P.E., EnerVation, Inc., Milwaukee, Wis.
Progress has been defined as "man's ability to complicate simplicity," and the events of more than a half century in the heating, ventilation and air-conditioning (HVAC) industry make this definition particularly apropos in one area: mechanical systems that are being recommended for U.S. educational facilities at the primary and secondary levels.
Technical publications expound on the merits of variable-air-volume (VAV) systems, heat-recovery equipment of every type, demand-control ventilation (DCV) through the use of CO 2 sensors, direct-digital control (DDC) systems and central building-management systems. The technological advances in systems and equipment can come with severe cost penalties, and the vast majority of public and private school districts are unable to take advantage of these advances without exceeding budget limitations or incurring the wrath of the taxpayer. A favorable payback on the investment is immaterial if the district cannot afford the initial cost.
The subject of indoor-air quality (IAQ) is receiving top billing despite the fact that there is no positive solution, even from the American Society of Heating, Refrigerating and Air-conditioning Engineers (ASHRAE). "Increase the amount of outside air," is the battle cry; one can fully expect that mechanical systems will ultimately require 100-percent outside air (OA) to satisfy ASHRAE, the codes and environmentalists. Scholastic HVAC traditions
Over the past 50 years, primary and secondary schools have predominantly used constant-volume air-distribution systems, steam or hydronic heating systems and pneumatic temperature-control systems. There generally has been no problem with IAQ, nor with maintaining satisfactory temperatures in the vast majority of facilities. In addition, maintenance staffs have been fully capable of repairing and calibrating the equipment and, in fact, stocking all vital components.
Why was IAQ a nonentity in these schools? Was it because the necessary instrumentation for contaminant detection was nonexistent? Was it because manufacturers used nontoxic materials in floor coverings, furniture, adhesives and cleaning solutions? Was students' personal hygiene better? Was it because there was no mechanical cooling—and hence no problem with condensation or mold development in system components?
In fact, the problem of IAQ—the infamous "sick building syndrome"—occurred almost simultaneously with the introduction of VAV air distribution. Rather than modulating the constant-volume air temperature to a room, VAV maintains the supply-air temperature, but varies the air quantity.
Great idea! This would save fan power, improve temperature control in each room and eliminate any reheat ... especially in the summer. Of course, by reducing the supply-air volume, the designs also reduce the amount of fresh OA ventilation, ofttimes to near full closure of OA dampers. Beyond the "stuffiness" complaints from building occupants, the severe reduction of OA places the entire building in a high negative-pressure condition, creating an entirely new—and quite lengthy—list of problems.
ASHRAE and state and local governments quickly surmised that the only solution would be a mandatory increase in the amount of outside air per building occupant. For schools, ASHRAE recommends 15 cubic feet per minute (cfm) of outside air per classroom student. The Wisconsin State Ventilation Code, as another example, requires 7.5 cfm per occupant. Prior to this code, Wisconsin required 5 cfm per occupant; during the "energy crisis" of the 1970s, there was virtually no requirement for OA.
This prescriptive ventilation-rate procedure specifies explicit ventilation rates as a function of occupancy. However, engineers know that there are many sources of indoor pollutants other than those emanating from people, and it has been suggested that the ventilation rate per person may be an incomplete characterization of ventilation requirements. Outside-air overkill?
As it applies to the HVAC industry, "progress" in school IAQ—and environmental problems in general—has resulted in excessive installation costs due primarily to OA "overkill" requirements. How many school districts in this country are financially able to: 1) install heat-recovery equipment at $3 per cfm above normal system costs, or 2) install a double mechanical system—one for 100-percent OA and the other for maintaining comfort levels?
Also, most maintenance staffs are now relegated to the role of custodian, and high-priced electronics technicians must perform adjustments to the control of monitoring equipment. Major mechanical equipment now requires outside service contracts for equipment longevity and warranties.
As to DCV using CO 2 sensors, there is no advantage when the "minimum" OA for the building is dictated by exhaust-fan requirements or occupancy requirements. To install a sensor in this system to reduce OA cfm is to assure negative pressures throughout the building. Measurement of building static pressures must be superimposed on the CO 2 control system as a low-limit for OA quantities. The CO 2 sensor is primarily an attempt to reduce the energy consumption required to treat incoming OA, not to improve IAQ, since CO 2 has the least effect on air quality. Further, it would tend to reduce the free-cooling benefits of the economizer control system.
The use of VAV air distribution in schools invariably leads to IAQ problems due to the difficulty in maintaining minimum OA requirements through the full range of supply-air reduction. This problem, of course, can be successfully overcome by way of complex control systems, increased capacity of HVAC equipment or a totally separate constant-volume system for 100-percent OA employing heat-recovery devices to lessen the impact on operating costs. Cost barriers
Unfortunately, these IAQ solutions can have a considerable effect on the initial cost of a project; thus, they are invariably omitted by the majority of school districts, favorable payback periods notwithstanding. Those schools with VAV regulators should seriously consider "fixing" all regulators in the 100-percent-open position during the heating season, and resetting the primary mixed-air temperature at the air-handling unit (AHU). In the cooling season, the VAV could be allowed to control space temperatures through variations in air volume— provided that the minimum position of the VAV system does not reduce OA quantities below the exhaust requirements of the building. Frankly, there are too many problems associated with this concept, not only in initial cost expenditures, but also in long-range service and maintenance.
Direct-digital control is now the preferred approach to environmental control in public buildings ... by the myriad of DDC manufacturers, that is, not the majority of building-maintenance personnel, who have a strong liking for pneumatics. Are the increased installation costs and the mandatory reliance on high-cost outside service contractors worth the small percentage of improved control accuracy? It is, but only if the data obtained is gathered on a weekly or monthly basis and put to practical use as a watchdog for excessive energy consumption and improved maintenance procedures. As to the selection of a DDC system, the key up-front decision by a school district is to specify a manufacturer whose equipment can be readily serviced by several qualified contractors in the area, not just one. The reduction in service costs is remarkable.
The use of unit ventilators in the classroom is slowly but surely disappearing from the scene, primarily because of the excessive maintenance of OA dampers, filters and controls; the constant potential for frozen coils; disturbing noise levels; and poor air distribution throughout the room. "Progress" in this area has now produced totally self-contained unit ventilators complete with gas/electric/hot-water heating and direct-expansion (DX) cooling. Unfortunately, the installed cost can be three to four times that of conventional unit ventilators and, as is often the case, the budget may not allow. Floor space sets trends
Because indoor floor space is nonexistent and already at a premium for classroom functions, the trend today in retrofitting existing schools is to totally remove all unit ventilators and replace them with overhead air-distribution systems served primarily by rooftop units. Heating is provided by 80- to 85-percent-efficient hot-water boilers serving automatically controlled booster coils or radiation for each occupied space. Cooling is provided at the central units, not only to provide satisfactory comfort levels in the spring and fall, but also to contribute a vital component of IAQ improvement: dehumidification.
Recent experience with this system concept at the Edgewood Elementary School in Oak Creek, Wis., indicated an installed cost of $11 per square foot, including: complete demolition of all HVAC; installation of new heating and air-conditioning overhead equipment and piping; and a central DDC energy-monitoring and -control system. The Oak Creek School District is now proceeding with a similar retrofit at Meadowview Elementary School.
In general, cooling equipment for school conditioning is vastly oversized, resulting in excessive on/off cycling of the compressors and an increase in humidity levels. Refrigeration equipment is most efficient when it is hard at work; good designs should routinely size equipment at least 10 percent under the calculated load, to maintain temperatures of 75°F to 80°F and humidities in the 40 to 50 percent range.
In designing new or retrofit HVAC systems for primary and secondary schools, engineers should consider the benefits of:
The economizer-control cycle for all air-handling units.
The sizing of all heating and cooling equipment on the basis of 7.5 cfm minimum OA per student—if not superseded by the minimum exhaust-fan requirements.
Sizing cooling equipment for 10 percent less than the calculated total heat requirement.
Installing overhead air distribution with slot diffusion at perimeter walls.
Primary/secondary hot-water piping systems to prevent thermal shock at the boilers and to permit automatic reset of the secondary hot-water system with outdoor temperatures.
Providing filters with 35-percent minimum efficiency.
(See "Schools and Simplicity," below, for an example of this.) Not just HVAC
The tide may be turning, and HVAC system concepts at schools may be returning to more simplistic designs. Even cast-iron radiators are now back in production.
Outside of the realm of HVAC, however, it's a different story, and our society may be turning into one of the most boring in history: people no longer seem capable of verbal communication at a nontechnical level or of producing meaningful correspondence. First-grade students become computer experts before they can read their names. When engineers need quick help with HVAC problems, they invariably get "voice mail"; for help selecting equipment, they get an Internet address.
The fact is, no one has ever known a computer by its first name; similarly, one can never draw on a machine's field of experience in building design.
While these comments are made with tongue in cheek, it seems fair to be concerned about the impact of the electronics industry on the younger generation. Very few students hope to enter the hands-on fields of steamfitting, sheet-metal work or plumbing, which are not glamorous and cannot approach current salaries offered to computer programmers and electronics technicians.
This is yet another alarming example of "progress."
Remember, neither the electronics industry nor schools can survive without HVAC, and HVAC cannot survive without the "hands-on" skills of all trades.
Area: 1,000 square feet.
Cubic feet per minute (cfm) requirement: 1,200
Number of students: 30
Total minimum outside air (OA) at 7.5 cfm per occupant: 225 cfm
Winter design temperature: -10°F
Return-air (RA) temperature: 70°F
In Wisconsin, the code requirement is 7.5 cfm of OA per student. The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) recommends 15 cfm per student, and in any court of law this is considered the standard and supersedes state code. The state, of course, does not wish to burden school districts with the excessive installation and operating costs associated with 15 cfm per occupant; ASHRAE, on the other hand, appears to be much less concerned about installation and operational costs or, for that matter, the impact on national energy consumption. Contrary to the early 1970s, when the energy crisis demanded conservation, today's crisis is seen more as a matter of increasing production, not using less—at least according to newly elected national leaders.
The graph (shown in the Figure on page 20) is based on an economizer-control cycle that maintains a discharge temperature of 55°F by mixing OA temperatures with 70°F return air. It is noteworthy that at -5°F, the system is already introducing 8.0 cfm per occupant. At 15°F, the spaces receive more than 10 cfm per occupant. At an average winter temperature of 32°F, the ASHRAE recommendation of 15 cfm per occupant is already exceeded.
This system performance should be more than sufficient to maintain a high level of indoor-air quality, and thousands of installations have proven this to be true. In addition, designing for 7.5 cfm per occupant results in the following benefits:
The rooftop units require no heating for ventilation purposes, because the mixed-air temperature is already at 55°F at -10°F.
Because 15 cfm per occupant requires a 37-percent mix of -10°F OA and 70°F RA, heating must be provided at the central unit to boost the mix from 40°F to 55°F. To avoid the corrosive effects of condensation, the heat exchanger must be constructed of noncorrosive metals, increasing the unit cost by approximately 25 percent. This is not a requirement with the 7.5-cfm design.
If the central unit also provides cooling, the refrigeration equipment must be increased by 20 percent per classroom to accommodate the 15-cfm design, with a resultant 20-percent increase in electrical operating costs.
The 15-cfm design results in escalated gas operating costs of approximately 2.5 million Btus per classroom each winter, and it requires a complete gas-piping distribution system to the rooftop units.
It is safe to say that systems incorporating the recommendation of 15 cfm per occupant can easily have installation costs that increase from $11 per square foot to $14 per square foot.
And for most schools, this can be the difference between a project proceeding—or a project being shelved.
Schools and Simplicity: Balancing IAQ, Cost and Energy
The following example helps illustrate how to ensure good indoor-air quality (IAQ) at elementary schools without the burdens of excessive costs and systems that are hard to maintain or operate. Similar design principles were incorporated recently at Edgewood Elementary School in Oak Creek, Wis., at an installed cost of $11 per square foot. A similar system is being considered for Oak Creek (Wis.) School District, at Meadowview Elementary School.
The information in the Figure (page 20) is based on a typical classroom with these design parameters: