Indoor design temperatures are calculated from test data compiled for men and women with various amounts of clothing and for various degrees of physical exertion. For lightly clothed people doing light, active work in relatively still room air, the design dry-bulb temperature can be determined from t  180
1.4t (13.22) r where t  dry-bulb temperature,
F DB tr  mean radiant temperature of the space or room (between 70 and 80
F) When temperatures of walls, materials, equipment, furniture, etc., in a room are all equal, t  tr  75
F. With low outside temperature, the building exterior becomes cold, in which case the room temperature should be maintained above 75
F to provide the necessary heat that is being lost to the cold exterior. In accordance with Eq. (13.22), the design dry-bulb temperature should be increased 1.4
F for each 1
F of mean radiant temperature below 75
F in the room. In very warm weather, the design temperature should be decreased correspondingly. Humidity is often controlled for human comfort. Except in rare cases, relative humidity (RH) usually should not exceed 60%, because the moisture in the air may destroy wood finishes and support mildew. Below 20% RH, the air is so dry that human nostrils become dry and wood furniture often cracks from drying out. In summer, a relative humidity of 45 to 55% is generally acceptable. In winter, a range of 30 to 35% RH is more desirable, to prevent condensation on windows and in walls and roofs. When design temperatures in the range of 75
F are maintained in a space, the comfort of occupants who are inactive is not noticeably affected by the relative humidity. Variations from the design criteria are generally permitted for operational facilities. These variations are usually established as a number of degrees above or below the design point, such as 75
F DB  2
F. For relative humidity, the permitted variation is usually given as a percent, for example, 55% RH  5%. Design conditions vary widely for many commercial and industrial uses. Indoor design criteria for various requirements are given in the Applications volume of the ASHRAE Handbook. 13.3.11 Outdoor Design Conditions The outdoor design conditions at a proposed building site are very important in design of heating and cooling systems. Of major importance are the dry-bulb temperature, humidity conditions, and prevailing winds. Outside conditions assumed for design purposes affect the heating and cooling plants physical size, capacity, electrical requirements, and of considerable importance, the estimated cost of the HVAC installation. The reason for this is that in many cases, the differences between indoor and outdoor conditions have a great influence on calculated heating and cooling loads, which determine the required heating and cooling equipment capacities. Since in most cases the design outdoor air temperatures are assumed, the size of equipment will be greatly affected by

Each of these systems usually consists of one or more facing subsystems and a structural subsystem that supports them. The facing subsystems may be the surfaces of the structural subsystem or separate entities that enclose that subsystem. They serve esthetic purposes, provide privacy, and bar, or at least restrict, passage of people or other moving objects, water, air, sound, heat and also often light. Wood structural subsystems are discussed in Sec. 10, and concrete is discussed in Sec. 9. Basic principles of waterproofing building exteriors are presented in Art. 3.4.2. This section describes techniques applicable to unit masonry and curtain walls. Floors provide not only a horizontal separation of interior building spaces but also a surface on which human activities can take place and on which materials and equipment can be stored. The structural subsystem usually consists of a slab or deck and also often of beams that support it. These are described in Secs. 7 through 10. This section discusses constructions used for the upper facing, or floor coverings, which serve esthetic purposes and act as a wearing surface. The bottom facing, or ceiling, may be the bottom surface of the slab or deck or a separate entity, such as a gypsum-plaster membrane, which is also discussed in this section, or acoustical tile.

Power roof ventilators are usually roof mounted and utilize either centrifugal or axial blade fans. Both types are generally used in low-pressure exhaust systems for factories, warehouses, etc. They are available in capacities up to about 30,000 ft3 /min. They are, however, limited to operation at a maximum pressure of about 1/2 in water gage. Powered roof ventilators are also low in first cost and low in operating costs. They can provide positive exhaust ventilation in a space, which is a definite advantage over gravity-type exhaust units. The centrifugal unit is somewhat quieter than the axial-flow type. Fans vary widely in shapes and sizes, motor arrangements and space requirements. Fan performance characteristics (variation of static pressure and brake horsepower) with changes in the airflow rate (ft3 /min) are available from fan manufacturers and are presented in tabular form or as fan curves. Dampers. Dampers are mechanical devices that are installed in a moving airstream in a duct to reduce the flow of the stream. They, in effect, purposely produce a pressure drop (when installed) in a duct by substantially reducing the free area