The preceding articles present the basic knowledge necessary for accurate determination of the heat losses of a building. Such procedures make possible sizing and selection of heating equipment that will provide reliable and satisfactory service. The basic procedure used in sizing a heating plant is as follows: We isolate the part of the structure to be heated. To estimate the amount of heat to be supplied to that part, we must first decide on the design indoor and outdoor temperatures. For if we maintain a temperature of, say, 70
F inside the structure and the outside temperature is, say, 0
F, then heat will be conducted and radiated to the outside at a rate that can be computed from this 70
temperature difference. If we are to maintain the design inside temperature, we must add heat to the interior by some means at the same rate that it is lost to the exterior. Recommended design inside temperatures are given in Table 13.10. Recommended design outdoor temperatures for a few cities are given in Table 13.5, p. 13.27. (More extensive data are given in the ASHRAE HandbookFundamentals, American Society of Heating, Refrigerating and Air-Conditioning Engineers.) Note that the recommended design outdoor winter temperatures are not the lowest temperatures ever attained in each region. For example, the lowest temperature on record in New York City is
14
F, whereas the design temperature is 0
F. If the design indoor temperature is 70
F, we would be designing for (70
0) /(70  14), or 83.3%, of the capacity we would need for the short period that a record cold of
14
F would last. Once we have established for design purposes a temperature gradient (indoor design temperature minus outdoor design temperature) across the building exterior, we obtain the heat-transmission coefficients of the various building materials in the exterior construction for computation of the heat flow per square foot. These coefficients may be obtained from the manufacturers of the materials or from tables, such as those in the ASHRAE HandbookFundamentals. Next, we have to take off from the plans the areas of exposed walls, windows, roof, etc., to determine the total heat flow, which is obtained by adding the sum of the products of the area, temperature gradient, and heat-transmission coefficient for each item (see Art. 13.3). TABLE 13.10 Recommended Design Indoor Winter Temperatures Type of building Temp,
F

13.4.5 Mechanical Ventilation Mechanical ventilation is almost always preferred over natural ventilation because of reliability and the ability to maintain specific design requirements, such as air changes per hour and face velocities for exhaust hoods. Natural ventilation permits wide variations in ventilation-air quantities and uncertain durations of ventilation. (In critical areas, such as in carcinogenic research laboratories, natural ventilation is never relied upon.) For this reason, mechanical ventilation systems are almost always used where ventilation requirements are critical and must be highly reliable. Mechanical ventilation is often required by various building codes for various applications as follows: 1. Control of contaminants in the work area for health protection and compliance with OSHA standards for achieving the legal limits set on employee exposure to specific toxic and hazardous substances 2. Fire and explosion prevention for flammable vapors 3. Environmental protection 4. Reuse of valuable industrial materials 5. Human comfortremoval of heat, odors, and tobacco smoke 6. Humidity control 7. Corrosive fumes and noxious gases Mechanical ventilation may be a single system without heating, cooling, filtration, humidification, dehumidification, etc., or it may include various combinations of these functions. In other words, the systems can be heating-ventilating units or heatingventilatingair-conditioning (HVAC) units. In many complex and specialized buildings, certain functional areas will be required to have various degrees of positive pressurization, negative pressurization, or balanced atmospheric conditions. The ventilation air as a part of the system supply air is used to provide the positive and balanced pressures. An exhaust system is utilized to maintain the negative-pressure areas. In many designs, air lost by pressurization is exfiltrated from the system and does not become part of the returnair

Because of the variations in material constituents, temperature, and workability required at jobsites, theoretical approaches for determining ideal mix proportions usually do not give satisfactory results on the jobsite. Most concrete therefore is proportioned empirically, in accordance with results from trial batches made with the materials to be used on the jobsite. Small adjustments in the initial basic mix may be made as a project progresses; the frequency of such adjustments usually depends on the degree of quality control. When new materials or exceptional quality control will be employed, the trialbatch method is the most reliable and efficient procedure for establishing proportions. In determination of a concrete mix, past field experience or a series of trial batches is used to establish a curve relating the water-cementitious materials ratio to the strength and ingredient proportions of concrete, including admixtures if specified, for the range of desired strengths and workability (slump). Each point on the curve should represent the average test results on at least three specimens, and the curve should be determined by at least three points. Depending on anticipated quality control, a demonstrated or expected coefficient of variation or standard deviation is assumed for determination of minimum average strength of test specimens (Art. 9.10). Mix proportions are selected from the curve to produce this