Classroom HVAC JOURNAL ASHRAE. The. By Arthur E. Wheeler, P.E. Fellow ASHRAE

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1 ASHRAE JOURNAL The following article was published in ASHRAE Journal, October Copyright 1997 American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. It is presented for educational purposes only. This article may not be copied and/or distributed electronically or in paper form without permission of ASHRAE. The V in Classroom HVAC By Arthur E. Wheeler, P.E. Fellow ASHRAE C ommonly applied minimum outdoor air ventilation rates for school classrooms have evolved during the past hundred years from 30 cfm (15 L/s) per person down to 10 cfm (5 L/s) 1 per person in the mid-1920s. The rate dropped to half that some fifty years later with the onset of the energy crunch. Most recently it rose to 15 cfm (7.5 L/s) per person with ANSI/ ASHRAE Standard : Ventilation for Acceptable Indoor Air Quality 2. These shifts are the result of changes in the understanding of appropriate objectives and the nature of pollutants and their effects upon occupants. Conclusions have always been tentative due to difficulties in evaluating quantitatively the effectiveness of ventilation rates. The invariable presence of other environmental factors, some unrelated to air quality, confounds results. As always, non-environmental considerations, especially economics and energy use, continue to influence ventilation rate choices. The quest for more enlightened understanding of the role of ventilation continues. The current draft of the successor to Standard proposes a more finely delineated approach to establishing an outdoor ventilation rate for classrooms and other occupancy classifications. More than those of its predecessors, the draft strives to relate rates to the specific contaminants of concern, their sources, and the functional use and grade of the classroom. Microbe bearing particulates are given priority as a contaminant of concern. Dilution ventilation can reduce their concentration to a degree shown to provide a healthier environment. 4,5 The proposed standard recognizes that dilution of airborne particulate concentrations can be accomplished by both outdoor air (presumed to be virtually free of the microbes produced by the occupants) and recirculated air (from which particulates are removed by filtration). Following Standard , the proposed revision retains a minimum About the Author Arthur E. Wheeler, P.E., has nearly 50 years experience designing building refrigerating and air-conditioning systems. An ASHRAE Fellow and recipient of the Society s Distinguished Service Award, he has served on numerous standing, technical and standards committees, including the project committees for ASHRAE Standard and R. He is president of the Wheeler Engineering Company in Baltimore. Greater emphasis is placed upon satisfying occupants of classroom environments. Site Outdoor Summer Design: 90 F (32 C) db, 76 F (24 C) wb Indoor Summer Design: 75 F (24 C), 50% rh Classroom Floor & Roof Area: 810ft2 (75 m2) Wall Area: 270 ft 2 (25 m 2 ); Window Area: 81ft 2 (8 m 2 ) Lighting: 2 watts/ft2 (21.6 watts/m2) Normal Attendance: 23 Persons Fig. 1: Design parameters for classroom system evaluation. rate of 15 cfm (7.5 L/s) per person of outdoor air or recirculated air that is filtered to provide a dilution equivalent to the outdoor air based upon removal efficiency for three micron particles. The draft further prescribes a minimum rate of outdoor air required for dilution of non-filterable contaminants contributed by the two principle sources: occupants (and their activities) and building components. For the general classroom, the draft proposal establishes the minimum rate of outdoor air ventilation needed to dilute contaminants from these sources that is less than the current standard. 6 Two main reasons for this reduction are (1) the proposed standard considers the odor perception of adapted occupants as opposed to unadapted visitors, and (2) recognition that the rate of contaminants produced by the building on a per person basis is partially dependent upon occupancy density (persons per unit floor area). Standard prescribes outdoor air rates for classrooms that are greater than previous standards and ventilation codes. Especially in humid climates, this increase, coupled with the inherent limitations of commonly employed air-conditioning systems, has contributed to higher room humidity. Such occurrences frequently lead to extensive fungal growth and an indoor environment highly contaminated with spores and mycotoxins. The capabilities of an HVAC system to provide acceptable thermal conditions and air quality are often overlooked. Some systems dehumidify outdoor ventilation air only as an indirect result of cooling to maintain space temperature. When inadequately dehumidified outdoor air is supplied, the latent load and the humidity of the space are increased. Although higher See Wheeler, Page ASHRAE Journal October 1997

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3 Wheeler, From Page 48 outdoor air ventilation rates may exacerbate humidity problems, lower rates may not cause them to disappear. Basis of Study This study examines and compares the performance capabilities of several HVAC systems popularly applied to general classrooms with respect to their ability to maintain space relative humidity and to control risk of microbial infection transmission. It also evaluates the effect of different outdoor ventilation rates upon classroom humidity during warm weather. The study examines a typical public school classroom located in central Maryland. Design parameters are given in Figure 1. Outdoor design conditions are selected for June or September, 2.5% frequency of occurrence. 7 Classroom orientation is a composite averaging north, east, south and west. Maximum attendance of 25 is discounted by two to account for normal absenteeism. The period of continuous occupancy is considered longer than three hours, therefore the variable occupancy provision contained in Standard is not applicable. Both occupied and unoccupied periods are analyzed to more fully evaluate system relative humidity control capabilities. Thermal loads are assumed to be steady state varying with outdoor air temperature as depicted in Figure 2, in which room sensible cooling load is plotted against outdoor air temperature. The room sensible load is the determinant of either the supply air temperature for a constant volume (CV) HVAC system or supply air flow rate, assuming a constant supply air temperature, for a variable air volume (VAV) system. The variable supply air temperature or flow is controlled to offset heat gain or loss. The study accounts for the effect of water vapor produced by the occupants upon space relative humidity; however, moisture entry due to infiltration, permeation and migration is not considered. These sources should be, but are not always, minimized by the building envelope design and construction. The systems compared are: Unit ventilators (UV) still probably the most popular throughout the country. VAV single duct for cooling, with controls that ensure a constant outdoor air ventilation supply even as total supply air flow changes as required by Standard VAV systems are more commonly selected for larger schools and those with both interior and perimeter classrooms. Single zone rooftop units (SZRT) with direct expansion cooling coils; one for each classroom. This system is often selected for single story buildings due to favorable capital cost. Performance with respect to ventilation and humidity control is similar to many self-contained and heat pump unit ventilators. Conditioned outdoor air supply system (COA+) supplemented by a room unit, such as a fan coil or heat pump. As well as being applied to new schools, the outdoor air supply system is being added as a remedial or upgrade measure employing existing unit ventilators with sealed outdoor air intakes. 8 Dual fan, dual duct (DFDD) supplying primary air for outdoor air ventilation and cooling through one duct system and secondary recirculated air, filtered and optionally heated through another duct system. The airstreams are blended in a terminal for each classroom to vary the supply temperature while holding the room supply rate constant. 9 While application of this system has been limited, it is receiving growing consideration for new schools. The systems are evaluated and compared for their capabilities to maintain classroom relative humidity, provide outdoor air ventilation, and to limit concentrations of airborne infectious particulates. Space relative humidity is analyzed coincident with three outdoor ambient conditions: 90 F (32 C) db 76 F (24 C) wb 112 grains/lb (16 g/kg) 52% rh 75 F (24 C) db 69 F (20 C) wb 98 grains/lb (14 g/kg) 75% rh 20 F (-6 C) db 17 F (-8 C) wb 10 grains/lb (1.4g/kg) 50% rh Outdoor design conditions do not always depict the seasonal humidity extremes encountered in many parts of the country, including Maryland. Partial load conditions, as represented by the second summer ambient, are not only more commonly encountered than those selected for design, but may test the systems humidity control more severely. Both occupied and unoccupied situations are examined during summer. To better understand the effects of different outdoor air ventilation rates, the rates of 5, 10 and 15 cfm (2.5, 5, 7.5 L/s) per person are compared for the unit ventilator system. Some school districts have adopted lower rates than the 15 cfm (7.5 L/s) per person recommended by Standard to ameliorate humidity conditions within the classrooms. The outdoor air rates are held constant during system operation except when an economizer control provides more outdoor air for the purpose of free cooling. Fig. 2: Cooling load plotted against outdoor air temperature. See Wheeler, Page ASHRAE Journal October 1997

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5 Wheeler, From Page 50 System Oa Vent/ Person Occupied UV 15 57% 68% Rh Range 1 Rh Ir 3 Unoccupied 63% F, 10 GR/# System Supply Air Flow CFM Cor V Db F Humidity Gr/# Fig. 3: Characteristics of each system studied. Filter Efficiency Dust Spot % Uv 1000 C <10 30 Vav 1000 V Cond. Oa+ 1345* C <10** 30** Single Zone 1000 C <10 30 Dfdd 1000 C *Oa + Recirculated Air ** Recirculated Air-thru Room Unit The study focuses upon particles three microns in size. Such particles are typical and roughly central to the size range of droplet nuclei carrying bacteria and viruses. These desiccated droplets produced by coughing or sneezing are able to remain in suspension because of their small size, generally ranging from one to five microns. 10 They are capable of being recirculated through air distribution systems unless removed by filtration or disinfection. To evaluate the effectiveness of filtration several levels of efficiency are compared. The comparison relies upon the use of the Infection Risk Index (I r ), based upon the Wells-Riley equation, both previously described in the ASHRAE Journal. 11 This empirically derived equation considers the strength of a discrete infection source or sources, the ventilation rate, and period of exposure. The dilution ventilation rate is considered to be outdoor air (assumed to be free of the occupant-produced droplet nuclei) plus the equivalent in filtered recirculated air, i.e. recirculated airflow times the three micron particulate efficiency. Source strength for inclusion to the Wells-Riley equation is chosen to be in the range of 20 to 40 quanta per hour. The risk comparison has been checked by mass balance equations (Standard 62:1989, p.23) based upon a generation rate of 425 colony forming units (CFU) per minute per person. 12 Only filters normally applicable to the systems are compared. These are identified by the ASHRAE Dust Spot Efficiency. 13 As they relate to a typical classroom, characteristics of each system studied are shown in Figure 3. The unit ventilator selected for the classroom is rated at 1000 cfm (500L/s), equipped with a four-row coil, supplied with chilled water for cooling and warm water for heating, with proportional water flow control by a room thermostat combined with Cycle II Control. 14 The 1 in. (0.025m) deep coarse fiber filter commonly employed is considered to have a dust spot efficiency less than 10% and neglible effectiveness for removal of three micron particles. The VAV system supply air temperature is slightly higher than for UV due to greater fan heat. 30% extended surface filters are assigned 60% removal efficiency for three micron particles 15 and for 85% filters, 98% removal efficiency for three micron particles. 16 The COA+ system supply air is close to temperature neutral. Following dehumidification of the airstream the temperature is increased to the level indicated by reheat or heat recovery to avoid overcooling. The filter efficiency indicated applies to the air recirculated through the room unit for determination of outdoor air equivalency. The SZRT direct expansion coil is controlled on/off by the classroom thermostat. Ventilation is continuous. Filters selected for both primary and secondary air paths of the DFDD system are the same as for the VAV system. Unlike the VAV system, the supply airflow to the classroom is held constant. Results For each of the five systems, the resultant space thermal conditions are calculated by psychrometric analysis based upon the selected parameters described in Figure 4. For all systems, room humidity is in some measure controlled indirectly as a consequence of room temperature control. The room humidity, during system operation, varies depending upon the magnitude of the room sensible load. For occupied perimeter classrooms this will vary largely depending upon the solar heat gain. When the space is unoccupied < % % 66% 60% 70% 36% % 63% 58% 65% >63% VAV 15 50% 54% 43% 4 24% Cond OA % 58% 47% 24% Single Zone Duel F & D 15 58% 75% 5 63% 75% 5 24% % 54% 43% 6 24% indoors, based upon 90 F db, 52%rh & 75 F, 75%rh outdoors; 2 indoors, occupied, no humidification; 3 infection risk index, based upon infectious particle removal from recirculated air; cfm min. supply air, reheat req d; 5 approximate; cfm min. primary air; Fig. 4: Psychrometric analysis of each system studied. with the system operating and the lights turned off, room sensible load is minimal. For some systems, the room humidity reaches its maximum during unoccupied periods. Humidity levels above 60% are conducive to mold growth. High relative humidity within the space is often indicative of the presence of moisture at the surfaces or substrates of the space envelope. Such moisture is a condition prone to cause rapid propagation of molds. Under such circumstances the effectiveness afforded by ventilation and filtration in reducing the space concentrations of fungal contaminants is likely to be quite limited. Spores and mycotoxins generated by molds can be odorous and cause adverse health effects. 17 The winter time room humidity is a consequence of both the moisture content of the See Wheeler, Page ASHRAE Journal October 1997

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8 VENTILATION Wheeler, From Page 52 outdoor air and the water vapor emanating from the occupants. An infection risk index of 1.0 is a value based solely on the dilution effect of 15 cfm (7.5 L/s) of outdoor air. Values other than 1.0 are comparative to this base condition. Mass balance analysis yielded higher changes in the risk indices for reductions in outdoor air ventilation than by Wells-Riley. This alternate approach indicates the risk for UV with <10% filters increases proportionally to the decrease in outdoor air, but with 30% filters the two methods of analysis were in close agreement. Discussion Unit ventilator system. Since the dew point of the supply air is regulated by the thermostat, reductions in room sensible load can result in high humidity conditions even when the occupant latent load is removed. Though not studied, face and bypass control in place of water flow control is likely to produce somewhat lower maximums. Room humidity would be even higher for humid outdoor ambient conditions in southern climates or during rainy summer periods elsewhere. 18 Reduced rates of outdoor air ventilation do not fully eliminate excursions of relative humidity in warm weather. It is also evident that in winter too little outdoor air can result in excessive humidity within the classroom as may be evidenced by condensation on the windows. The reduced infection risk attributed to 30% efficiency filters may understate the benefit of reducing buildup of nutrient (dirt) on the internals of the unit and the propagation of microbials from these locations. VAV system. The minimum supply air flow to the classroom will usually be greater than the minimum outdoor ventilation rate due to the turndown limitations of the volume control or the designers desire to maintain a greater supply of air for dilution and room air distribution. Airflow is projected to be at the minimum for many winter days. Both preheat and reheat may be necessary to avoid overcooling the space. Even with 85% efficiency filters, the infection risk index remains relatively high due to the reduced supply airflow occurring throughout most of the occupied period. Conditioned outdoor air supply plus recirculating room unit. The space is dehumidified both by the low dew point air supply and the room unit. Reduction in I r is solely the result of improved filtration of air recirculated through the room unit. Single zone rooftop. The high humidity conditions are due to humid outdoor air introduced through the unit with the cooling coil control indexed off in response to the space thermostat. Some of the moisture condensed on the coil during the cooling cycle is re-evaporated adding to the water vapor content of the supply air. Due to the variable periodicity of the cooling cycles, the upper end of the space humidity range as stated is approximate. Dual fan, dual duct. Analysis demonstrates equal or superior performance than the others in humidity control and the lowest infection risk index (using 85% efficiency filters). The mass balance analysis indicated even greater effectiveness at reducing the concentration of colony forming units (65% reduction) than that yielded by Wells-Riley. Concern has been voiced that ASHRAE recommended ventilation rates have contributed to indoor air quality problems by increasing room humidity. The comparisons demonstrate that HVAC system capabilities bear more significantly upon room humidity than outdoor air ventilation rates. Conclusions Too often the consequences of HVAC system selection upon both thermal conditions and air quality are overlooked. System selection is usually determined by the design engineer in collaboration with the architect and the school district staff whose chief concerns may be capital cost or past practice. Currently, greater emphasis is being placed upon satisfying occupants with the classroom environment. The capabilities of applicable systems to satisfy that goal deserves close attention. It s the engineer s responsibility to carefully examine the choices. Rather than recommending one system over others, this article offers a tool for thorough evaluation and provides designers with a means to influence others to select the provides HVAC system most likely to contribute to a suitable learning environment. References 1. Waters, T.J., Heating and ventilation of school buildings. Transactions of ASHVE, Vol 1, pp ANSI/ASHRAE Standard , Ventilation for Acceptable Indoor Air Quality. p Ibid., p Burge, H.A., Bioaerosols. CRC Press Inc., Boca Raton, FL. p Kuehn T.H Matching filtration to health requirements. ASHRAE Transactions 97(2). 6. BSR/ASHRAE Standard R. Public Review Draft August, p ASHRAE Handbook Fundamentals p Scofield, C.M., DesChamps, N.H. May HVAC design for classrooms: divide and conquer. Heating Piping & Air-Conditioning. Penton Publishing. Vol 65, No.5 9. Warden, D., January Dual fan dual duct: better performance and lower costs. ASHRAE Journal. Vol.38, No.1. p Riley R.L., Nardell, E.A Controlling transmission of tuberculosis in health care facilities: ventilation, filtration and ultraviolet air disinfection. Plant Technology and Safety Management Series. Oakbrook Terrace, IL. JCAHC Organization. Series No. 1. p Wheeler, A.E. June Better filtration, a prescription for healthier buildings. ASHRAE Journal. Vol 36, No.6. p Wheeler, A.E., Abend, A.C. Meeting the objectives of energy conservation & ventilation for acceptable IAQ in the classroom. Proceedings of IAQ 91 Healthy Buildings, ASHRAE, Atlanta,GA. 13. ANSI/ASHRAE Standard Gravimetric and Dust-Spot Procedures for Testing Air Cleaning Devices Used in General Ventilation for Removing Particulate Matter. 14. ASHRAE Handbook HVAC Application p Ibid., p Farr Co., Los Angeles, CA Bulletin B Miller, J.D. January, Fungi and indoor air quality. INvironment Professional. Vol. 3, No.1. p Bayer, C.W., Downing, C.C. Indoor Conditions in Schools with Insufficient Humidity Controls. Proceedings of IAQ 92, Environments for Buildings, ASHRAE. Please circle the appropriate number on the Reader Service Card at the back of the publication. Extremely Helpful Helpful Somewhat Helpful Not Helpful October 1997 ASHRAE Journal 55