UNIVERSITY OF CALIFORNIA COLLEGE OF AGRICULTURE AGRICULTURAL EXPERIMENT STATION BERKELEY, CALIFORNIA CIRCULAR 351 May, 1942 AIR CONDITIONING FOR HOUSES IN CALIFORNIA^ HAYDEN S. G0ED0N2 and R. L. PERRY« Mechanical equipment for cooling houses in summer is widely distrib- uted, and air conditioning as an all-year operation is becoming common- place. Many interested houseowners desire, however, to know more about the results obtainable by the various methods and about the approximate cost. This circular is designed to furnish information that may be helpful. Air conditioning is usually defined as the simultaneous control of the temperature, movement, purity, and humidity of the air, whether applied to heating a house in winter or to cooling it in summer. Also of major importance for the comfort of a person are the effects of radiation. This circular will deal principally with the problems presented by summer conditions in California, although the houseowner is normally more concerned with the winter conditions. The emphasis on the problems of air conditioning and cooling encountered during the summer should not, however, obscure the fact that most permanent installations combine the equipment for both heating and cooling. Many components of the ap- paratus function under both sets of conditions, and certain items, notably insulation, improve summer comfort as well as reduce winter heating costs. The subject matter has been separated into four general divisions, which concern (1) the physiological principles of air conditioning and the optimum conditions desired, that is, the methods and rates at which ^ This publication is the sixteenth of a series reporting results of investigations con- ducted by the California Agricultural Experiment Station in cooperation with the California Committee on the Relation of Electricity to Agriculture. It supersedes Bul- letin 589, Air Conditioning for California Homes, by Baldwin M. Woods and Benedict F. Eaber, released as the thirteenth of this series in 1935. 2 Instructor in Agricultural Engineering and Junior Agricultural Engineer in the Experiment Station. ^ Associate Professor of Agricultural Engineering and Associate Agricultural En- gineer in the Experiment Station. [1] 2 University of California — Experiment Station body heat is generated and dissipated under different conditions and the optimum conditions for comfort and health ; (2) the loads and require- ments to be satisfied, that is, the heating and cooling loads which will be required for different climatic and internal conditions for various types of building construction and the requirements of air movement, purity, and humidity; (3) various methods of completely or partially achieving the desired conditions — how to minimize the demands upon the equip- ment; and (4) how to select equipment that will satisfy as many of the remaining demands as the houseowner wishes. Clearly, the design, the details of computation, and the selection of equipment for a given house are complex technical problems, whose solu- tions require careful analysis. Many installations excellent in principle may give difficulty in practice because of errors in fitting them to the houses. A glossary of the more technical terms employed in this circular has been given at the back, in the hope of clarifying the text. PHYSIOLOGICAL PRINCIPLES OF AIR CONDITIONING The chief purpose of air conditioning, aside from the special require- ments of certain industrial and pathological applications, is to increase the health and comfort of human beings. According to studies made dur- ing the past thirty years, the effects of climate and weather upon the individual are largely due to the method and magnitude of the heat ex- change between the human body and its environment. GENERATION OF BODY HEAT Basal Metaholism. — Bodily activity is accomplished by energy trans- formations within the body cells, in which food reserves are oxidized with the release of muscular energy and heat. The transformations are governed almost entirely by the cell temperatures ; and the physiological superiority of warm-blooded animals and birds is due to the presence of a mechanism for maintaining a constant body temperature. When a person is at rest, the amount of heat released as a by-product of the energy transformations required for operating the internal organs is only about 25 per cent of the heat required for maintaining the body at a constant temperature under comfortable external conditions. Ac- cording to experiments, the remaining 75 per cent is supplied by direct oxidation in the inactive muscles." The minimum rate of heat generation to maintain a constant body * Keeton, E. W., F. K. Hick, N. Glickman, and M. M. Montgomery. The influence of physiological research on comfort requirements. American Society of Heating and Ventilating Engineers Journal 13(3) : 188-95. Mar., 1941. Cm. 351] Am Conditioning 3 temperature in a warm-blooded animal is termed the "basal metabolism." This rate, suprisingiy constant for any person, is not affected by daily variations of the environment, although it is lowered by continued warm weather or a tropical climate.^ The basal metabolism may also be affected by certain diseases, notably those of the thyroid gland. As many measure- ments have shown, the basal metabolism is related to the weight of a person, but is not directly proportional to the weight.*' Thus an adult weighing 180 pounds would not have twice the basal metabolism of a child weighing 90 pounds, but only about 68 per cent more. The basal metabolism for an average adult is about 290 British thermal units per hour. Muscular Activity. — Since the efficiency of the body in converting food energy into muscular energy is 25 per cent or less, the waste heat accompanying the production of muscular energy is at least three times the useful output. The energy for digesting a meal will increase the heat rate to about 10 per cent above the basal metabolism, and physical work may increase it by 500 per cent or more. The total heat generated by a person seated and doing" light work is around 400 British thermal units per hour, whereas housework will increase the total to a value between 600 and 850, according to the arduousness of the tasks. DISSIPATION OF BODY HEAT Internal Transport of Heat. — Heat is generated in the interior of the body and dissipated from the surface. Before dissipation, however, it must be transported from the interior to the surface by conduction and convection. The body surface consists of a layer of epidermis, dermis, and subcutaneous tissue, this last being rich in capillary vessels. Blood flows from the interior to these capillaries and returns after being cooled. Below a body-surface temperature of about 82° F for a person at rest, this transfer by convection becomes very small because the capillaries contract, and the flow of heat from the interior of the body to the surface follows the laws of conduction. The conductivity of the surface layer of the body is about the same as that of paper, leather, or cork.^ An increase in the thickness of the layer of subcutaneous fat decreases the conduc- tivity of the surface layer and thus renders a person better able to con- serve body heat. As the body-surface temperature rises above this minimal value of 82°, the capillaries dilate, blood circulates faster through the surface laj^er, and there is a decrease in the temperature ^ Petersen, W. F. "What weather does to man. Heating, Piping and Air Conditioning 10(9):595-98. Sept., 1938. ® Kleiber, Max. Body size and metabolism. Hilgardia 6(11) : 315-53. Jan., 1932. "^ Hardy, J. D. The physical laws of heat loss from the human body. National Acad- emy of Sciences Proceedings 23(12) :635. Dec, 1937. 4 University of California — Experiment Station difference between the surface of the skin and the interior of the body. The amount of heat transferred through the surface layer by conduction is reduced as that by convection is increased.^ When the body-surface temperature equals that of the interior, practically all the heat is trans- ported by convection from the interior to the surface. The abundance of capillary vessels in subcutaneous tissues differs in the various areas of the body, being at a maximum in the extremities. At high temperatures of the surrounding air, clothing, and radiating surfaces, the rate of capillary circulation in the extremities may be as great as five to eight times the values obtaining belov^ 82°, v^hile the average for the entire body will rise to two and a half times this value. Incidentally, this fact indicates the relatively high effectiveness of the bodily extremities in dissipating heat under adverse conditions. Dissipation of Sensible Heat. — Both sensible heat and latent heat may be dissipated from the surface of the body. Sensible heat is detectable by a rise in temperature of the fluid or body that absorbs it. When, for ex- ample, a draft of air is directed against a hot surface, the air is warmed, and heat is carried away from the hot surface in the form of sensible heat. The amount may be calculated for a mass of fluid or for a body if the physical properties of the substance and the temperatures are known. Sensible heat is lost from the human body by radiation, conduction, and convection. Heat radiation, sometimes termed infrared radiation, is identical with light in its mechanism, but differs in being below the range of the frequencies sensed as light. Radiation may occur independently of any physical transmitting medium. Heat flows by conduction through a substance, the rate depending upon the physical properties of the sub- stance. Heat loss from the human body by conduction is negligible unless the body is in contact with a colder surface. Convective heat loss occurs when a cold fluid — for example, air — comes in contact with the body, is warmed by it, and is then replaced by other cold fluid. The circulation of the fluid may be due either to density differences caused by a change of temperature (free convection) or to some method of mechanical circu- lation such as an electric fan (forced convection). Dissipation of Latent Heat. — Latent heat, being the energy necessary to cause a change of state, is not measured by a change of temperature. For a pound of water at 98.6° F, the latent heat of evaporation is 1,038 British thermal units, although the vapor remains at the same tempera- ture as the liquid. Free water surfaces from which vapor is escaping are « Hick, F. K., E. W. Keeton, N. GHckman, and H. C. Wall. Cardiac output, periph- eral blood flow, and blood volume changes in normal individuals subjected to varying environmental temperatures. American Society of Heating and Ventilating Engi- neers Journal 11(1) :50-53. Jan., 1939. CiR. 351] Air Conditioning 5 generally cooler than their surroundings because the water furnishes the heat of vaporization. At body -surf ace temperatures above 85° F, in the temperature range known as the "zone of evaporative regulation," most of the heat is carried away from the body by the evaporation of the perspiration secreted by the skin. If the air temperature is above the skin temperature, heat is transferred to the clothing and the exposed skin by convection even though it is being carried from the skin by evaporation. If the room sur- faces and furnishings are above the skin temperature, heat will also be transferred to the clothing and exposed skin surfaces by radiation. Ac- tually, heat is transferred through clothing or other insulating material by the three processes of conduction, convection, and radiation. At air temperatures below 78° F, in the zone of body cooling, the tem- perature of the skin is below that of the interior of the body, the secretion of perspiration is at a minimum, and the loss of heat is due largely to radiation and convection. Between the temperatures of 78° and 85°, the lightly clothed body can easily maintain a thermal equilibrium with the surroundings. In this zone about one third of the heat that the body gen- erates is dissipated as latent heat of evaporation, and two thirds as sensi- ble heat by radiation and convection.'' Under all conditions in all zones, heat is carried away from the body as latent heat by the moisture that evaporates into the exhaled air from the surfaces of the respiratory tract. Some animals, notably dogs, depend largely upon this type of latent heat dissipation to maintain uniform body temperature. OPTIMUM CONDITIONS FOR COMFORT AND HEALTH Because of individual variations in health, age, clothing, activity, and adaptation, no single set of conditions will give every person the same sensation of comfort. Comfort is a subjective experience, not arbitrarily defined, but evaluated by the individual. The American Society of Heating and Ventilating Engineers has de- termined^" the various combinations and values of the factors affecting heat loss from the body that will give a feeling of comfort. Because there are seasonal variations in metabolism, there are corresponding changes in the feeling of comfort for given conditions. Limits of Temperature and Humidity for Comfort. — In summer weather, according to tests of the American Society of Heating and Ven- " DuBois, Eugene F. Heat loss from the human body. New York Academy of Medi- cine Bulletin 15(3) : 143-73. Mar., 1939. ^" American Society of Heating and Ventilating Engineers. Heating, ventilating and air conditioning guide. 19th ed. p. 53-60. New York, N. Y. 1941. 6 University of California — Experiment Station tilating Engineers, 98 per cent of the lightly clad subjects observed felt comfortable at conditions between 74° F with 70 per cent relative hu- midity and 78° with 30 per cent relative humidity. In winter a com- parable number of similarly clad subjects were comfortable between temperatures of 68° and 72° with the same respective humidity limits. These studies were made with the subjects in the room for periods of 3 hours or more, with air velocities of 15 to 25 feet per minute, and with the surroundings at substantially the air temperatures. As has been implied, some individuals will feel comfortable at lower temperatures than those mentioned above ; others at higher temperatures. Furthermore, an increase in the velocity of air movement permits com- fort at a higher temperature because the convective and evaporative heat loss from the body will be increased for a given temperature difference. If the temperatures of the surrounding walls are lowered substantially below the air temperature, the amount of heat lost by radiation is in- creased while the amount lost by convection and evaporation is reduced. In winter, accordingly, higher air tem^peratures must be maintained in a room with cold walls to offset the increased radiation loss. In summer, conversely, the air within a room must be at a lower temperature for a given sensation of comfort if the walls, ceiling, and floor are at a high temperature than if they are at a low temperature. Comfort Zone. — Under the comfort conditions outlined above, the heat production and heat dissipation of the body balance with great exactness and ease. There is no cumulative heating or cooling of the body, and the heat produced is dissipated without noticeable secretion of perspiration. Figure 1 presents the temperature and humidity limits of the comfort zone in graphical form for summer and winter conditions, as determined by the American Society of Heating and Ventilating Engineers. This graph shows the temperature range over which different percentages of those tested felt comfortable for a given condition of relative humidity. Figure 1 also shows the interrelation between relative humidity and tem- perature for equal conditions of comfort, indicating that as the humidity is decreased, the temperature may be increased to give equivalent condi- tions of comfort. Since, however, the tests were not carried beyond the limits of 70 and 30 per cent relative humidity, they may not present the comfort limitations throughout the possible range of relative humidity. They did not include, furthermore, the effects of radiation for conditions under which the temperature of the ceilings or other surfaces differed substantially from the temperature of the air. As the following sections will indicate, the temperature limits of the comfort zone as defined here are not those most desirable for the average air-conditioning installation. CiR. 351] Air Conditioning Under conditions of high temperature, the sensation of comfort seems closely related to the wetted area of the skin. If the relative humidity is increased, this wetted area increases to maintain the evaporative heat dis- sipation at the level necessary to balance heat generation against heat Ni ,./ oU!^ i'-f^^. h 0% JOr. 9d% 50%. 0% V\ ' ' \ , \ \ \\ \ \ \ \ \ \ \ \ \ A \ \ 07, 50% 33'/. SO'/. 0% _ 1 T 1 1 1 1 1 90 % "fO 30 \ H^/'/j^er Comfort Zorze > 0% £0% 97% ^OVo 0% go so 40 30 20 70 SO A/'r te/r?perc7^i/re , c/egrees f Fig. 1. — Comfort zone in terms of relative humidity and air temperature, as determined by the American Society of Heating and Ventilating Engineers. Under the conditions of the test, the subjects were at rest and exposed for 3 hours or more, and the air movement was 15 to 25 feet per minute. The upper chart is for summer, the lower for win- ter. Each line shows the various combinations of tempera- ture and humidity at which a given percentage of subjects expressed themselves as comfortable. dissipation, and the subject is less comfortable. The use of conditioned air of very low relative humidity facilitates, therefore, the evaporation of moisture from the body, and thus aids the mechanism of evaporative regulation. On the other hand, lower temperatures of the air and sur- roundings increase the radiative and convective heat loss and thereby Moisture content, lbs. water vapor /lb. dry air ill t3Q '^ CiR. 351] Air Conditioning 9 reduce the evaporation requirement and permit higher humidities. The particular conditions depend largely upon the type of equipment se- lected. Psychronietric Chart. — The psychrometric chart presented in figure 2 shows the relations among dry-bulb temperature, moisture content, wet- bulb temperature, relative humidity, and dew point. (Dew point is not .^/2S /OO' ■ 74'/. / _J>1 a ^Oja^ c ^^ /OO' Fig. 3. — Explanation of the uses of the psychrometric chart : A, With air at 100° F dry-bulb and 74° wet-bulb, the intersection of the wet-bulb and dry-bulb lines gives 30 per cent relative humidity. B, with air at 100° F dry-bulb and 74° wet-bulb, a horizontal line to saturation line (100 per cent relative humidity) gives 63° dew point. On the scale of ordinates, the moisture content is 0.0123 pound of water vapor per pound of dry air. The hori- zontal line also represents simple cooling and heating with no change of moisture content. C, Cooling and dehumidifying with air initially in the condition represented by point a and with heat-exchange surface at a § (50° F). Air from the system at point b warms to c before discharge. D, Direct evaporative cooling of air from point a along wet-bulb line to point &. Air warms from Z) to c before discharge. shown on the chart but may be determined from it as demonstrated in figure 3, 5.) If any two of these quantities are known, the other three may be determined. The changes occurring in certain thermodynamic processes may also be determined, as illustrated in the explanatory dia- grams (fig. 3). 10 University of California — Experiment Station Period of Occupancy. — The problem of delimiting the most desirable conditions for different periods of occupancy deserves careful considera- tion. Under the conditions previously defined as optimum for summer- time, anyone entering the air-conditioned space from outdoors will be subjected to a sudden drop in temperature — about 20 degrees for an afternoon temperature of 100° F. This sudden change v^ill chill the body, which had previously adjusted itself to the outdoor conditions of tem- perature and humidity. Conversely, the sudden transition from the cool indoor condition to the hot outdoors will again make a sudden and stren- uous demand upon the temperature-controlling mechanism. This effect is shocklike : as in shock, the vessels of the circulatory system are sud- denly dilated to their maximum size, and since there is no corresponding increase in blood volume, the heart cannot maintain normal circulation. This change will be less severe if the temperature difference is reduced or if a period of transition is permitted. To reduce the temperature difference, a convenient rule of thumb specifies that the temperature of the conditioned space be maintained at a value halfway between 72° F and the outdoor temperature. For an out- door temperature of 100° F, accordingly, 86° F would be maintained in- doors. Under these temperature conditions the air-cooling equipment would reduce the severity of summer temperatures without losing the beneficial effects of climatic variability. Many users of the more elaborate types of air-conditioning equipment have insisted on maintaining a defi- nite and relatively low temperature in the cooled rooms. But the adop- tion of less stringent standards would, as has been emphasized, be more healthful and obviously less expensive. To realize in practice the operat- ing conditions necessary to permit a gradual transition from hot to cool surroundings would be difficult. The undesirable effects of rapid changes may be relieved, however, by the use of an extra item of clothing. A coat or jacket, put on when a person first enters a cooled area and then re- moved in 15 or 20 minutes, minimizes the ill effects. Effects of Weather and Acclimatization. — The effects of weather and climate upon the well-being and progressiveness of people of different geographical areas have been extensively documented." Biological studies of the effects of environmental variability also reveal the stimulating effects of changing weather conditions."^ Of course, if the extremes of 1^ Mills, Clarence A. Medical climatology, vii + 296 p. 90 figs. Charles C. Thomas, Springfield, 111., and Baltimore, Md. 1939. Huntington, Ellsworth. Civilization and climate, xliii + 333 p. 38 figs. Yale Univer- sity Press, New Haven, Conn. 1922. " Petersen, W. F. What weather does to man. Heating, Piping and Air Condition- ing 10(9) :595-98. Sept., 1938. Huntington, Ellsworth. Weather and health. National Eesearch Council Bulletin 75:123-24. 1930. CiR. 351] Air Conditioning 11 variability are too great, or if the individual is biologically inadequate, the changes may promote the onset of disease. Persons sensitive to the weather may show symptoms annoying, but not in themselves fatal — for example, migraine (sick headache), arthritis, or asthma. The prevalence of a particular kind of weather brings about physio- logical readjustments that reduce the demands upon the temperature- adaptive mechanism of the body. Thus, as mentioned by Petersen and Huntington (see footnotes 11 and 12) , a person who spends some time in the tropics has a lower basal metabolism and a lower pulse rate, and usually becomes much less active than he was in a cooler climate. All these adaptive changes tend toward reduced heat production, which in turn makes a smaller demand upon the heat-dissipating mechanism of the body. Conversely, a person traveling to cool climates from the tropics will have difficulty in keeping warm and, until acclimated, will be more sus- ceptible to colds and infections than his neighbors. Two effects of air conditioning must be guarded against. A person should not spend all his time in monotonously uniform surroundings and thus lose the beneficial effects of climatic variability, which maintain the physical vigor of the adaptive mechanisms and contribute to physical well-being; but neither should he subject himself to the extremes en- countered in going through a temperature change of 20 or 30 degrees when passing into or out of an air-conditioned space. The Spartan belief in the virtue of enduring extremes is hardly justified in the light of pres- ent knowledge of the physiological principles of air conditioning. AIR-CONDITIONING LOADS AND REQUIREMENTS The essential functions of air-conditioning equipment are to maintain the desired conditions of air temperature, movement, purity, and hu- midity within a house. Though all these elements can be controlled with the more elaborate types of equipment, economy frequently involves con- centration upon the outstanding requirement — the maintenance of a desirable temperature. TEMPERATURE REQUIREMENTS If the air within a house is to remain at a specified temperature during the summer, the equipment must dispose of all the components of heat gain. These comprise the cooling load and depend upon climatic condi- tions, building construction, and internal heat sources. The design problem consists of determining the temperature difference to be maintained, the ventilation requirements, the number of occupants, and the maximum number of appliances that may be in use. The designer 12 University of California — Experiment Station can compute the maximum load when he knows these requirements, to- gether with the characteristics of the residence construction ; and he can select the air-conditioning equipment accordingly. Fig. 4. — Climatic zones of California. Climatic Conditions. — The general climatic zones of California are given in figure 4 ; this division of zones and the following description of summer conditions in the zones are based upon summaries of the United States Weather Bureau and upon information obtained from companies making installations in different areas of the state. The lines of demar- cation among the various zones are not distinct ; they actually represent CiR. 351] Air Conditioning 13 areas of transition from conditions in one zone to conditions in another. The temperature and humidity conditions for these zones are as follows : Zone I, comprising most of the Sacramento and San Joaquin valleys, and extending northward around Mount Shasta, is marked by high summer temperatures with maxi- mums occasionally exceeding 110° F and with a difference between night and day temperatures frequently amounting to more than 40 degrees. The summer humidities are generally low. The outside design temperatures are 100° to 105° dry-bulb, and 68° to 72° wet-bulb, according to the locality within the zone. Zone II, the Imperial Valley, has long, hot summers, with high humidities during the latter part because of moist air moving northward from the Gulf of California. Design dry-bulb and wet-bulb temperatures are 110° and 78° F, respectively. Zone III, Death Valley and the vicinity, is extremely hot and extremely dry. A shade temperature of 134° F has been registered at Greenland Eanch. Zone IV, Owens Valley, the Mojave Desert, and the areas adjoining them, is an area where the summer temperatures are high, the humidities low. Zone V, the coastal region of southern California, has one of the most equable climates in the United States, although the humidity is fairly high. Zone VI, the coastal section south of San Francisco, enjoys relatively cool summers and moderate humidities. Zone VII, the northern coastal section, has relatively cool summers. On the coast the mean monthly temperatures have an annual range of only about 10 degrees. Back from the coast, among the hills and valleys, the climate is entirely different, with afternoon temperatures above 100° F frequently recorded, but with low humidities and cool nights. Conditions are influenced by elevation and by the topography of the vicinity. Zone VIII, the section eastward from San Francisco Bay, comprising the Santa Clara Valley and the interior valleys to the neighborhood of Stockton, is characterized by more moderate temperatures than the interior valleys, with moderate humidities. Near the Bay the daily and annual variations are small ; but they increase with dis- tance from the Bay. Zone IX, the northeastern section of the state, is fairly hot, with low to moderate humidities. Zone X, the section around Mt. Shasta, is cold in summer and, because of the heavy snowfall on the mountain, only moderately dry. Zone XI, the southern Sierra Nevada, running south to Mt. Whitney, is cool, with moderate humidities. The outside design dry-bulb temperatures given for figure 4 are the values used at present in calculating the cooling load for a house in any given zone. For a condition of 100° F outside design dry -bulb tempera- ture and 80° inside design dry-bulb temperature, the equipment must maintain a differential of 20 degrees. If the equipment is designed for these conditions, a day in which the temperature rises to 105° will in- crease the inside temperature to 85°. This condition is generally desira- ble, however, since too great a temperature difference between indoor and outdoor conditions may be harmful. Building Construction. — The flow of heat through the outside walls of 14 University of California — Experiment Station a house may be computed, if the outside and inside surface temperatures are known, by determining an equivalent conductivity for the composite wall. The transfer of heat across the spaces within a wall occurs by radia- tion, convection, and conduction ; but when the temperature differences are small, the rate is approximately proportional to the temperature differences for all three modes. Hence, the assumption of an equivalent conductivity will give valid results. The addition of a porous material to the air spaces within the walls will reduce the heat transferred across them by reducing the convection and radiation fractions that constitute the major modes of transfer. The temperatures of the outside wall surfaces will depend partly on the radiant energy incident upon them and partly on the outside air tempera- ture adjacent to the walls. If the surfaces are exposed to direct sunlight or to radiation from a reflecting or radiating surface, such as paving or an adjacent wall, the surface temperatures will be higher than the air temperatures. At night, conversely, the surfaces exposed to the cold sky will be several degrees cooler than the immediately adjacent air because of radiation to the sky, A substantial proportion of the solar radiation falling upon a window is transmitted through the glass. For a double-strength window glass per- pendicular to the direction of the sun's rays, the amount transmitted is about 89 per cent, which diminishes as the angle between the rays and the surface becomes acute. The energy absorbed by the glass is lost by convection to the air on each side in proportion to the temperature differ- ence existing in each case. Glass, being opaque to the infrared rays (long- wave radiation), radiates energy as does any other opaque surface. Because sunshine (short-wave radiation) is readily transmitted, the win- dow acts as a one-way heat valve. The solar radiation enters with little loss and is converted to heat when it is absorbed by any of the surfaces upon which it falls. The long-wave radiation from the interior surfaces, on the other hand, can escape only by conduction through the glass after being converted into sensible heat by absorption in the glass at the inner surface. The magnitude of heat gain through the window becomes an important component of the cooling load. For certain types of buildings, it may be as high as 75 per cent of the total heat gain. A greenhouse is an excellent practical example of the utilization of this physical phenome- non. Internal Heat Sources. — That portion of the cooling load which is due to internal sources is relatively independent of climate. The energy dissi- pation of each occupant depends, as has been mentioned, upon his weight and activity. The energy consumed by lamps, motors, and other appli- Cm. 351] Air Conditioning 15 ances is eventually, if not immediately, converted into heat. Gas-burning appliances, such as stoves and water heaters, likewise produce heat. These three sources contribute directly to the cooling load. AIR MOVEMENT Air Circulation. — The circulation of air within a dwelling reduces the temperature variation within the conditioned rooms, preventing stratifi- cation of warm and cold air. Circulation removes the air from one part of the room, generally at floor level, and thus achieves a thorough mixing. Since a current of air must also be maintained across the cooling surfaces in order to effect the transfer of sensible and latent heat, the circulation of air accomplishes both purposes. Air may be circulated by either the thermosiphon or the forced-draft method. The thermosiphon system is used for hot-air furnaces in which the heated air is lighter than the cold incoming air and a circulation is naturally set up because of the density difference. It would be difficult, however, to arrange a satisfactory thermosiphon system for cooling a house : any such system would have to be located in the attic because, in cooling, the temperature of the conditioned air is lowered and its density thereby increased. Also, where the rate of air movement is low, the tem- perature differences attainable are small, and the cooling-surface areas and duct sizes needed are very large. The same equipment, furthermore, could not be used for both heating and cooling. As a result, all cooling systems use some form of forced-draft circula- tion. These are of either the central or the unit type. The former, as its name implies, has the equipment centrally located in the residence, dis- tributing the conditioned air from this central unit to the different parts of the residence through a duct system. The unit system, on the other hand, is individual to a single room or suite, with a purely local effect. Whereas in the central systems a fresh-air supply usually provides the necessary ventilation with circulation, the unit systems are generally for circulation within one room. Ventilation. — The proper rate for circulating air and supplying fresh air will depend upon several factors. In a cooling system, a velocity of more than 40 feet per minute with a difference of 2 degrees between the surrounding air and the stream of air will cause an unpleasant drafti- ness. Obviously, no such sensations are experienced with a heating system where the air current is warmer than the surroundings. In normal resi- dence installations, draftiness may be avoided and a uniform tempera- ture distribution maintained by having proper outlet placement and about eight air changes per hour, the air change being the movement of a 16 University of California — Experiment Station volume of air equal to the volume of the air-conditioned space through the cooling equipment. For the fresh-air requirement the minimum amount is about 10 cubic feet per minute per occupant ; this, in a house of normal size and occupancy, means one or two air changes per hour. Evidently, then, 12 to 25 per cent of the air discharged into the conditioned space by the equipment should be fresh air; the remainder recirculated air. In conventional residence construction the leakage around the windows and doors and some infiltration through the walls will normally satisfy the fresh-air requirements. This incoming air must be cooled to the in- door temperature. With central air-conditioning systems, the fresh-air ventilation is accomplished by appropriate ducts and dampers. Since these systems operate under a slight pressure from the blower, the air leakage is generally from the air-conditioned spaces to the out-of-doors. Fresh air is required to replace the oxygen consumed in respiration and to dilute the odors from tobacco, cooking, and body sources. PURITY Pollution of air is usually due to three common types of impurities : dusts, which are small solid particles, often created by cultivation or industrial operations ; fumes, due to acrid vapors or products of combus- tion; and smokes, generally particles of carbon from incomplete com- bustion. Often the air also contains pollen. Frequently droplets of moisture from coughing and sneezing will evaporate before reaching the floor and leave particles of infective material floating freely in the air. Removal of undesirable impurities is generally accompanied by filtration or air washing. Air-conditioning systems with such filtering equipment simplify housekeeping by reducing the amount of dust and smoke enter- ing as a result of ventilation. Cooking odors and products of combustion from stoves and heaters can be promptly drawn by overhead flues or exhaust fans to the out-of-doors. HUMIDITY The choice of a desired humidity is more difficult than the specification of temperature, movement, and purity of the air. The difficulty arises largely from differences in personal preference. For high-temperature conditions, the evaporative cooling of the body will be facilitated by low relative humidities, whereas lower air temperatures with higher humidi- ties will produce similar sensations of comfort. Sometimes the humidity is not independently controlled, but depends upon the type of equip- ment or upon the operation. Relative humidity varies with temperature for a constant moisture CiR. 351] Air Conditioning 17 content. When, therefore, an air-vapor mixture is sufficiently cooled, the space becomes saturated with vapor, and fog forms — for example, in the low-lying radiation fogs of the central California valleys and of south- ern California. Similarly, a cold surface will condense moisture (dew) out of the atmosphere if its temperature is below the dew point. As the air grows warmer, its capacity for uncondensed water vapor is increased. For instance, air with a dew point of 55° F — in other words, air saturated with water vapor at 55° — has a relative humidity of about 20 per cent at 105°. Usually, for comfort, the relative humidity should be 30 to 70 per cent. For a desired temperature of 80°, air may be cooled from 105° at 25 per cent relative humidity without increasing the relative humid- ity of the cooled air to above 50 per cent. When, however, the outdoor humidity at 105° is substantially above 25 per cent, moisture should be removed if high humidity is to be avoided in the cooled air coming from the equipment. Moisture may be removed from the air by condensation or by chemical drying. In refrigeration-type air-conditioning equipment, the cooling surfaces may be maintained below the dew point of the entering air, and some moisture will condense on these cooled surfaces and will thereby be removed. If the air is dried chemically, its temperature is raised by release of the latent heat of condensation. Some cooling equip- ment that depends upon the evaporation of water actually adds moisture to the air ; consequently, where the initial humidity is moderate or high, the cooled air will be exceedingly humid. Since warming the air decreases the relative humidity, all air-heating systems for winter should include devices for bringing the relative humidity up to normal. Warm summer weather in California is generally accompanied by low relative humidities. The cooling capacity of refrigeration-type air- conditioning systems may be increased if the cooled surfaces are kept above or only slightly below the dew point of the entering air; there will then be a reduction in the amount of latent heat that the equipment is called upon to remove. Similarly, these low relative humidities will per- mit various types of evaporative air coolers to be used without increasing the relative humidity of the air to undesirable limits. FULFILLMENT OF AIR-CONDITIONING REQUIREMENTS For desirable living conditions, a building may be so designed that the cooling load is negligible. The transfer of heat through the walls, ceilings, and floors may be kept too small to raise the temperature ap- preciably. Instead, however, equipment adequate to dispose of all heat 18 University of California — Experiment Station that gains entry may be installed. Practically, the most economical solu- tion is to combine the two courses, first of all reducing heat load by ap- propriate structural methods. Let us consider the requirements for air ^-m-^ Fig. 5. — Floor plan of the model cottage. conditioning — first, as they apply to the reduction of cooling loads; second, as they affect equipment for accommodating whatever cooling load may remain after it has been reduced as far as economically prac- ticable. THE MODEL COTTAGE As an aid toward the application of these principles and requirements to the subsequent discussion of comfortable living conditions, and as a basis for estimating costs, the floor plan (fig. 5) and the specifications of a model cottage will be considered. Although the original plan does not specify a basement, it will be assumed in discussing the air-condition- Cm. 351] Air Conditioning 19 ing equipment, that basement space is available. The simplified diagrams illustrating the various types of installations all show a basement. Many- actual installations use, instead of a basement, an equipment room at floor or ground level. Brief specifications of the model cottage are as follows, all lumber being Douglas fir ("Oregon pine") except as noted : Foundation Concrete Mudsills 2" X 6" heart common redwood, bolted to concrete Underpinning 2" x 6" Posts 4" X 4" on 8" x 8" redwood post caps Floor joists 2" x 10" and 2" x 12", 16" on centers Floor plates 2 — 2" x 6" Ceiling joists 2" x 4", 16" on centers Studding 2" x 4", 16" on centers Sills 2" X 4" Solid blocking Same as joists, over all bearings Roof rafters 2" x 4", 16" on centers, alternate rafters trussed to ceiling joists with 1" X 6" Sheathing 1" x 6" or 1" x 8", surfaced one side Eough flooring 1" x 6" or 1" x 8", surfaced one side Finish flooring Kitchen, 1" x 4", no. 1 vertical grain, tongue and groove ; baths, tile ; screen porch, composition; remainder, %" x 1%" plain clear oak Siding 1" X 8" standard redwood rustic, pattern as selected Note: Place 2-ply building paper between sheathing and siding Roofing No. lA split-cedar shingles, laid 4%" to the weather Porch ceiling 1" x 4" heart common redwood Doors Front, 3'0" x 6'9" x 1%" thick Interior, 2'8" and 2'4" X 6'8" X 1%" single-panel Rear, 3'0" X 6'8" X 1%" glazed single-pane, 24-oz. glass Sash Stock casement windows, single-pane, 24-oz. glass ; bath and kitchen stock double hung. Sizes as shovm, 2'8" up Story height: first-floor joists above ground, 2'0" clear; ceiling height, 8'6" clear Interior walls and ceilings are %" hard wall plaster on wood lath, except kitchen and bath, which are cement. A basement, adequate in size to contain the necessary equipment, is assumed under the house. INSULATION The first structural step toward comfort is to provide insulation. In new construction, insulation may well be included in the original con- tract. In older houses, insulation may be readily installed in the attic. Figure 6 shows on a percentage basis the computed values of heat gain during the summer {A-D) and the heat loss during the winter (E-H) for the model cottage, four cases being considered for each. The first ex- ample (A, ^) is a model cottage with no insulation and with single- glass, unshaded windows ; and the study concerns the heat gain or loss through walls, roof, and windows, the transmission of solar radiation 20 University of California — Experiment Station through the windows, and the air changes necessary for the occupants. The second example {B,F) is for the same conditions except that ceiling insulation has been used and the windows have been shaded. The third Si/mmer coo/wg^ /oad iVmier fieai/fjg /oacf Walls Roof Glass ^ "^ ^ A/r f 5amj5 approx. S/% fioof /67o. f Sai/l/igs approx. 66% f Ifi/alls Walls /^aof fioof Glass Glass /llr cl?a/?^e Air sl!an(^e Occapa/!li Oca/pa/ils approx. 70% Walls Roof Air Walls Poof Glass F/aor Air i i Samp approx. 40% //// Samp approx. 56% i Walls Walls Roof Roof Glass Glass Floor Floor Air c/ian^e Air cl!a/!fe SaWap Walls Roof Pi ^i/ble^/c Floor Air A 3 C D E F 6 H Fig. 6. — Effect of insulation upon the percentage cooling and heating loads of the model cottage, calculated for 3% inches of shredded redwood-bark insulation and 17- and 30-degree Fahrenheit temperature difference for summer and winter conditions, respectively : A and B, no insulation, unshaded windows; J5 and ¥, ceiling insulation, shaded windows ; C and G, ceiling and wall insulation, shaded win- dows; J) and B., ceiling and wall insulation, shaded and double-glazed windows. ((7, G) is for fully insulated ceiling and walls and shaded windows. The fourth (D, R) is the same as the third except that the windows have double glass. Note that the total amounts of heat gained or lost in the two seasons are not equal. Ceiling Insulation. — According to the National Bureau of Standards," " Phillips, Thomas D. Effect of ceiling insulation upon summer comfort, [TJ. S.] National Bureau of Standards Building Materials and Structures Eeport BMS52:3-8. July 1, 1940. CiR. 351] Air Conditioning 21 and also private companies in California, attic air temperatures gen- erally rise above 130° F on hot days. As already mentioned, the heat transferred through a surface is directly proportional to the temperature difference. Evidently, the heat entering a room through the ceiling is disproportionately large in comparison with that coming through the walls. Computations made for the model cottage show that more than 40 per cent of the heat gain for the uninsulated cottage is through the ceiling. Exclusive of attic and basement, this cottage has an exterior wall surface of 1,080 square feet, and a total ceiling surface of 1,120 square feet. Obviously, therefore, excellent results can be obtained by insula- tion of the ceiling. In many houses the relation of ceiling to wall area will be less favorable than for the model cottage, and the heat gained through the ceiling will be proportionately less. Since ceiling insulation is relatively easy to install, this particular improvement can be made in older houses almost as cheaply as in new construction. The recommended thickness of insulation will cost from 5 cents per square foot for shredded redwood bark to about 8 cents per square foot for mineral wool. Ceilings may be insulated with reflective or insulating materials. In the reflective type, a sheet of metal-foiled paper is stretched across or between the joists. A reflective sheet between joists, with sealed air space above and below, is substantially better than that on joists; and two sheets between joists with three air spaces are better than one sheet. Paper foiled on one surface costs about 2.8 cents per square foot and double-foiled paper about 4 cents. Installation costs are in addition to this figure. The values given in figure 7 for the reflective type of insula- tion are for winter conditions with heat flow across the air spaces by circulation because the ceiling is warmer than the attic. In summer the heat flow is in the reverse direction, the air stratifies, and the values are about 60 per cent less. The insulating materials may be rigid, semirigid, or fill. Although rigid panels are often used as a plaster base, their in- sulating value is only slightly superior to that of normal construction. Semirigid insulation is laid between the joists, whereas loose-fill insula- tion is spread between. A 2-inch thickness of insulation is about the minimum recommended; 3 to 4 inches is most common. Thicknesses exceeding 4 inches are usually not justified economically. Figure 7 also shows the heat conduction through this type of ceiling construction. Loose-fill insulation such as rice-hull ash must be prevented from sift- ing through small openings into the rooms below. Rice-hull ash must also be covered with heavy processed paper or plywood to keep it from blow- ing away. If loose-fill redwood-bark fiber is used, one may well spray the exposed /.z 0.9 I 0.5 OJ III 1 1 nr /^ d C D fVa//s F G H I J Ce/7/ngs K L M N O iV/'/?£/OtVS Fig. 7. — Conduction of heat through construction repre- sentative of common building practice, for conditions of 15-mile-per-hour wind. A-E, Exterior walls, with 1-inch wood siding on 1-inch sheathing on the outside and plaster on metal lath on stud- ding on the inside: A, no insulation; B, %-inch flexible in- sulation between studding in contact with sheathing; C, same insulation, but with two air spaces ; B, 1-inch flexible insulation with two air spaces; E, 3% -inch rock -wool fill. F-J, Ceiling, with no attic flooring, and ceiling of plaster on metal lath on joists: F, no insulation; G, reflective sheet on joists, bright on one side; H, reflective sheet on joists, bright on both sides, or 1-inch flexible insulation ; I, 2-ineh flexible insulation; J, 3%-irich rock -wool fill. K-0, Windows and glass-block panels, conduction heat transfer only for air-temperature difference: K^ single thickness of window glass ; L, double thickness of window glass with not less than %-inch space; M, triple thickness of window glass with not less than two %-inch spaces; N, smooth-surface glass blocks 7%x7%x3% inches thick; 0, ribbed-surface glass blocks 7%x7%x3% inches thick. CiR-351] Air Conditioning 23 surface with whitewash to prevent a possible flash-over in case of fire; having been fireproofed, this fiber is not hazardous except that fine surface fibers may momentarily ignite. Electric wiring must be isolated from any form of corrosive or combustible insulation. Eoof Insulation. — If insulation is applied to the under surface of the roof between the rafters rather than to the ceiling, a much lower attic temperature will result.'' When insulation is used under composition shingle or felt and asphalt roofs, the temperature of the roof surface will rise high enough to soften the asphalt unduly unless a reflective white or aluminum paint has been applied. Wall Insulation. — As was pointed out, the wall area of the model cot- tage is less than the ceiling area even without deductions for door and window openings. For reducing the cooling load, insulation will be less effective when applied to the available areas of the wall than when ap- plied to the ceiling, both because of the smaller area and because of the smaller temperature difference. For the model cottage the insulation of the w^alls would reduce the total cooling load about 15 per cent. Heat flow does not occur until the temperature difference is established. For the uninsulated construction employed in the model cottage, therefore, solar radiation on the outer surface will not raise the inside wall tempera- ture until about 2 hours after the sun has begun to shine on the outside wall. The corresponding time lag for the insulated construction, about 5 hours, shows that insulation will postpone the peak of inside wall tem- perature — in some cases until the need for cooling has passed. The heat stored in the walls, however, will result in a higher inside surface tem- perature during the evening than would otherwise occur. Similarly, a concrete floor or the ground will begin to lose the heat stored in it during the day as soon as the temperature of the surroundings falls appreciably. Figure 7 gives the relative heat conductions of different representative building constructions without insulation and with different thicknesses of insulating materials commonly used. SUPPLEMENTARY STRUCTURAL AND EXTERNAL TREATMENT Window Treatment. — The contribution to the cooling load due to the heat flow through the windows has already been mentioned in the section on air-conditioning loads and requirements (p. 11). As figures 6 and 7 indicate, double or triple glazing is desirable : it effectively reduces the heat flow through the windows due to the temperature difference between the air indoors and outdoors. (It has, however, only a relatively slight ^* Molenaar, Aldert, and E. L. Perry. Flow of heat through roofs. Agricultural Engineering 20(6) : 222-24. 2 figs. 2 tables. June, 1939. ^ University of California — Experiment Station effect on the heat transmitted by solar radiation.) Figure 7 shows the conduction of heat through window construction due to the difference between indoor and outdoor air temperatures. Figure 8 gives the relative amount of radiation transmitted through a single-glazed window and shows the effect of shading and other window treatment. The contribu- tion of windows to the total cooling load depends upon the window area because of the much higher conductivity of windows as compared with ■ # + 8 C F 6 Fig. 8. — Solar radiation transmitted through single-glazed windows: A^ solar radiation transmitted through unshaded window ; B, canvas awning, plain outside finish; C, canvas awning, aluminum out- side finish ; D, inside shade, fully drawn, aluminum outside finish ; E, inside shade, half drawn, buff outside finish; F, inside Venetian blind, covering window, slats at 45-degree angle, aluminum outside finish ; G, outside Venetian blind, covering win- dow, slats at 45-degree angle, aluminum outside finish. equal wall areas ; one should consider this factor when projecting a house design. Glass blocks are useful where light is required but where a view is unnecessary or undesirable. Although multiple glazing increases the cost of construction, it reduces the fuel bill about 5 per cent in winter and reduces the cooling load about 4 per cent in summer. An external awning or shade is the most effective type of shading shown in figure 8. Awnings, if used, should be ventilated so that warm air will not be imprisoned under them and increase the heat conducted through the windows because of the increased temperature of the air in contact with the glass. Some metal awnings are considered desirable CiR. 351] Air Conditioning 25 because they may be adjusted to admit light and yet to shade the win- doAvs from direct solar radiation. A window screen of narrow horizontal strips now available permits a direct view but eliminates direct solar radiation except in the early morning and late afternoon, besides func- tioning as a screen. The effects of the window appurtenances are approxi- mately the same for single or multiple glazing. An inside window shade will act as a radiation shield, but the heat absorbed will be liberated within the room. Projecting eaves or trees appropriately placed will shade the windows from direct solar radiation, at least during the hottest part of the day. Weather stripping of Windoivs and Doors. — The weatherstripping of windows and doors will prevent the infiltration of outside air. As already noted, however, the infiltration is approximately equal to the ventilation requirement; and when forced-draft air-conditioning systems are in operation, the slight pressure within the building will cause the air to flow outward through any cracks around doors or windows. Weather- stripping will effect substantial economies in winter heating, particu- larly during high winds. In regions where dust is blown into the house, weatherstripping promotes cleanliness. The filters used for air-condition- ing equipment are adequate to clean the air brought in for ventilation. Weatherstripping may be needed to reduce the winter heating load if the doors and windows are so poorly fitted that infiltration might be excessive, or where low temperature and high winds prevail. Treatment of Building Exteriors and Surroundings. — The air-condi- tioning load of a building is substantially increased if it is near a white- surfaced wall or a large paved area, because of the reflection of solar energy and direct radiation from the surface. This contribution may be reduced by lawns or shrubs. Leaf and grass surfaces are below the air temperature because of the evaporation of moisture that the plant is transpiring. As the temperature of these surfaces is thereby reduced, the radiation to the walls and windows of the building is lower than if no such treatment is given. The surface temperature of the exterior of a building may be reduced by w^hite or light-colored paints, since these will reflect much of the solar radiation that falls upon them. In addition, the emissivity of painted surfaces is high in the long-wave-length region, and the wall surfaces consequently radiate heat more readily. The rela- tive absorptivity and emissivity of different surfaces are given in table 1. As these data demonstrate, appropriately treated roof surfaces would result in lower attic temperatures and thereby reduce the cooling load. For this purpose aluminum paint is sometimes used, although white paint or white-surfaced roofing materials would be more effective. TABLE 1 Solar Energy Absorptivity of Various Surfaces and Surface Emissivities at Ordinary Temperatures* Materials Short-wave absorption Long-wave emission Standards : "Hohlraum," theoretical perfectly black body Meteorological : Water Snow, fresh, bright, sparkling (maximum reflection) Snow, soiled Vegetation: Grass, 80-90 per cent new (in sunshine after rain) Grass, fresh, dry Building material, roofing, etc.: Paper, white Asbestos cement board, white Asbestos felt (white impregnated covering for corrugated iron roofing) Plaster, finished Glass Paints : Whitewash Enamel, ceramic white Porcelain enamel on steel plate, white Porcelain enamel on steel plate, green Bright aluminum, 2 coats Fine bronze Bronze with 2 coats varnish White Gloss-white Cream Light yellow Light blue Medium blue Light green Dark green '. Red Lampblack Metals: Aluminum foil Aluminum foil with coat of linseed oil Aluminum, polished Chromium Copper, polished Copper, rolled, tarnished Copper, black oxidized Galvanized iron, new Galvanized iron, oxidized Galvanized iron, very dirty Galvanized iron, whitewashed Iron, rusted Lead, old roofing 1.00 0.79 1.00 0.90t (vert.) 0.914,0.965 0.13 0.74 0.54 0.67 0.98 0.67-0.75 0.28 0.95 0.59 0.96 0.25 (approx.) 0.50 (approx.) 0.35 0.93 0.92t 0.90-0.95 0.22-0.25 0.90 0.34-0.40 0.90 0.76 0.35-0.54 0.28-0.45 0.51 0.88 0.11-0.18 0.95 (approx.) 0.35 0.95 (approx.) 0.23-0.26 0.35 0.39 0.92-0 96 0.64 0.52-0.53 0.88 0.87 0.96 0.97-0.98 0.96 0.08 0.56 0.26 0.04-0.05 0.49 0.08 0.18 0.04 0.64 0.64 0.78 0.65 0.23 0.28 0.91 0.22 0.63-0.5 ♦ From: Brooks, F, A. Solar energy and its use for heating water in California. California Agr. Exp, Sta Bui. 602: 18-19. table 3. 1936. t Includes transmissivity. . . j u Note: Table 1 shows what proportion of the short-wave radiation falling on the surfaces is absorbed by them. The short-wave energy absorbed is converted into heat and is lost by conduction, convection, and long-wave radiation. The data on long-wave emission show the relative abilities of the surfaces to lose heat by long-wave radiation. CiR- 351] Air Conditioning 27 EQUIPMENT FOR COMFORT COOLING AND AIR CONDITIONING COMMON ASPECTS OF COOLING, HEATING, AND AIR CONDITIONING Heating and cooling are, as already mentioned, merely two aspects of the problem of promoting health and comfort for the occupants of a house. If much of the equipment can be used for both these purposes, capital cost can be reduced. Furthermore, if a heating system in an existing house can serve in part for the cooling system, adding the latter becomes less difficult and expensive. Some contractors of air-conditioning equipment estimate the total cost to be the same whether all units are purchased initially or the heating system is installed first and the cooling unit added later, provided that such a plan is originally adopted.^^ Equip- ment installations are summarized in table 2. Classification of Equipment, — Cooling and air-conditioning equip- ment may be classified as to its type of service. Unit or room equipment, as its name implies, is designed to be installed and operated in the space where the cooling effects are desired. It has a relatively low first cost, and its design requires only the minimum expense for installation. It is frequently used in older buildings or where only a limited space need be cooled. Usually this type utilizes windows for the fresh air supply and in some cases for heat disposal. The most familiar example is the electric fan, but more recent developments include unit compressor systems and evaporative cooler units located at or near the window. The other classification is that of the central installation of cooling or air-conditioning equipment. Central units, being necessarily perma- nent, are best installed as original equipment when the building is con- structed. Because of the central location and the resultant need for ducts to distribute the cooled or conditioned air, mechanical equipment must be provided for forced circulation. Although conventional practice com- bines the heating and cooling facilities for central units, the required air movement and temperature difference for cooling differ from those for heating. If, therefore, the equipment will be used for both purposes, it must be adequately designed and must include the necessary change- over provisions. Some central units, notably those for night-air cooling, are relatively simple, having a centrally located grille for the air-circu- lating system; but they require, of course, the opening of doors and windows or the provision of grilles in the doors to obtain the desired distribution and flow pattern. ^^ From information furnished by companies submitting estimates that served as data for table 2. 3^S »c oo o a o ^5 o cr g 01 (B O «-. 13 Eh iO l^ -fJ lO o o o §000 :^ :s^::i^ §§ O W3 > c3 ■*3 "^a 13 ;=3 "5 a .5 c fl o ci3 ;»•"'—' o •^•a ^ "^ +i ^J r3 _d CI fl ^ Q) o j^i ■^ '5 -S odd .§ -^ -^ a> (u