Prior to the design of the heating system, an estimate must be made of the maximum probable heat loss of each room or space to be heated. There are two kinds of heat losses: (1) the heat transmitted through the walls, ceiling, floor, glass, or other sur- faces; and (2) the heat required to warm outdoor air entering the space. The sum of the heat losses is referred to as the heating load.
The actual heat loss problem is transient because the outdoor temperature, wind velocity, and sunlight are constantly changing. The heat balance method discussed in Chapter 8 in connection with the cooling load may be used under winter conditions to allow for changing solar radiation, outdoor temperature, and the energy storage capacity of the structure. During the coldest months, however, sustained periods of very cold, cloudy, and stormy weather with relatively small variation in outdoor tem- perature may occur. In this situation heat loss from the space will be relatively con- stant, and in the absence of internal heat gains will peak during the early morning hours. Therefore, for design purposes the heat loss is often estimated for the early morning hours assuming steady-state heat transfer. Transient analyses are often used to study the actual energy requirements of a structure in simulation studies. In such cases solar effects and internal heat gains are taken into account.
The procedures for calculation of the heating load of a structure are outlined in the following sections. The ASHRAE Cooling and Heating Load Calculation Manual (1) may be consulted for further details related to the heating load.
6-1
OUTDOOR DESIGN CONDITIONS
The ideal heating system would provide just enough heat to match the heat loss from the structure. However, weather conditions vary considerably from year to year, and heating systems designed for the worst weather conditions on record would have a great excess of capacity most of the time. The failure of a system to maintain design conditions during brief periods of severe weather is usually not critical. However, close regulation of indoor temperature may be critical for some industrial processes.
The tables in Appendix B contain outdoor temperatures that have been recorded for selected locations in the United States, Canada, and the world. The data for selected locations (2) are based on official weather station records for which hourly observations were available for the past 12 years. The tables contain the basic design conditions for both heating and cooling load calculations. Only those data for the heat- ing load will be discussed here.
Columns 2 through 4 in the Appendix B tables, for heating design conditions, give latitude, longitude, and elevation for each location. Columns 5 and 6 give 99.6 and 99 percent annual cumulative frequency of occurrence of the given dry bulb temperature. That is, the given dry bulb temperature will be equaled or exceeded 99.6 or 99 percent of the 8760 hours in an average year. Conversely, in an average year, the dry bulb tem-
perature will fall below the 99.6 percent temperature for about 35 hours. Columns 7 and 8 give the mean wind speed (MWS) and prevailing wind direction in degrees measured clockwise from north coincident with the 99.6 percent dry bulb temperature. The humid- ity ratio outdoors for heating load calculations can be assumed equal to the value for sat- urated air at the dry bulb temperature. A thorough discussion of ASHRAE weather data is given in the ASHRAE Handbook, Fundamentals Volume (2) and Harriman III et al. (3). The outdoor design temperature should generally be the 99 percent value as spec- ified by ASHRAE Energy Standards. If, however, the structure is of lightweight con- struction (low heat capacity), is poorly insulated, or has considerable glass, or if space temperature control is critical, then the 99.6 percent values should be considered. The designer must remember that should the outdoor temperature fall below the design value for some extended period, the indoor temperature may do likewise. The per- formance expected by the owner is a very important factor, and the designer should make clear to the owner the various factors considered in the design.
Abnormal local conditions should be considered. It is good practice to seek local knowledge relative to design conditions.
6-2
INDOOR DESIGN CONDITIONS
One purpose of Chapter 4 was to define indoor conditions that make most of the occu- pants comfortable. Therefore, the theories and data presented there should serve as a guide to the selection of the indoor temperature and humidity for heat loss calcula- tion. It should be kept in mind, however, that the purpose of heat loss calculations is to obtain data on which the heating system components are sized. Indeed, the system may never operate at the design conditions. Therefore, the use and occupancy of the space is a general consideration from the design temperature point of view. Later, when the energy requirements of the building are computed, the actual conditions in the space and outdoor environment, including internal heat gains, must be considered. The indoor design temperature should be low enough that the heating equipment will not be oversized. ASHRAE Standard 90.1 does not specify specific design tem- perature and humidity conditions for load calculations, but does specify that the condi- tions shall be in accordance with the comfort criteria established in ASHRAE Standard 55 (see Chapter 4). A design temperature of 70 F or 22 C is commonly used with rela- tive humidity less than or equal to 30 percent. Although this is in the lower part of the comfort zone, maintaining a higher humidity must be given careful consideration because severe condensation may occur on windows and other surfaces, depending on window and wall insulation and construction. Even properly sized equipment operates under partial load, at reduced efficiency, most of the time; therefore, any oversizing aggravates this condition and lowers the overall system efficiency. The indoor design relative humidity should be compatible with a healthful environment and the thermal and moisture integrity of the building envelope. Frequently, unheated rooms or spaces exist in a structure. These spaces will be at temperatures between the indoor and out- door design temperatures discussed earlier. The temperature in an unheated space is needed to compute the heat loss and may be estimated, as described in Chapter 5, by assuming steady-state heat transfer and making an energy balance on the space.
The temperature of unheated basements is generally between the ground temper- ature (about 50 F, 10 C) and the inside design temperature unless there are many win- dows. Therefore, a reasonable estimate of the basement temperature is not difficult. However, for a more precise value, the energy balance procedure may be used with data from Chapter 5.