Document pDKON3Vk94mDqw8eg4qD09oj

424 CHAPTER 17 1952 Guide opening of windows, abnormal heating of the building, poor design of heat ing system, sun effect, and other variations. The Degree-Day Method is the more practical since it is based, in part, on actual consumption data. The presentation of the two illustrative estimating methods will .be pre ceded by a discussion of seasonal efficiency. SEASONAL EFFICIENCY The seasonal efficiency differs from the measured efficiency of the fuelfired heating unit because it is affected by the many , minor sources of extraneous heat gain and heat loss. Throughout a season, useful heat is supplied to a building not only by the heating unit, but also by the external surfaces of the heating unit, the flue pipe, and the chimney. In addition, heat is gained from fights, occupants, cooking and other appliances, and from the sun. Indeterminable amounts of heat are lost through radiation directly to basement or utility-room walls and floors, from the heating unit, the distribution system, and the flue pipe, from stand-by operation of the unit, from opening of doors and windows, and from faulty adjust ment and operation of the combustion unit. Fortunately, data which are available permit the making of reasonable estimates of seasonal efficiency for residences. The average, fuel consumption of various types of approved gas-fired equipment has been obtained from a large number of heating systems.1 Corresponding seasonal efficiencies can be calculated from these data. They show a variation from approximately 72 to 88 percent, depending upon the type and size of system. Laboratory tests on a gas conversion burner in a heating boiler operated with on and off cycles gave about 72 percent efficiency.2 Other tests on coal-fired room heaters indicate sea sonal efficiencies of 65 to 75 percent, when the heat from the flue pipe was included.3 A survey of 30 residences in one locality showed a variation of 45 to 75 percent in utilization efficiency, depending upon the condition of the equipment and the fuel used.4 A summary5-6 of many tests in two research residences at the University of Illinois, using many fuels and systems, gave values of 67 to 90 percent for overall house efficiency (the ratio of the heat loss from the structure to the heat input to the unit for an average test day). These data were used by the Small Homes Council1 of the University of Illinois in a tabulation of the comparative costs of heating the same build ing with various fuels. The approximate seasonal efficiencies shown in Table 1 are suggested as a guide. Table 1. Approximate Seasonal Efficiency Type of Fuel-Burning Unit Approximate Sea sonal Efficiency, Percent Type op Fuel-Burning Unit Approximate Sea sonal Efficiency, Percent Bituminous coal, hand fired Bituminous coal, hand fired Bituminous coal, stoker-fired... 75-80 65-80 60-80 50-65 40-60 50-70 Anthracite, hand-fired with Anthracite, hand-fired with- 60-80 50-65 60-80 60-80 50-65 100 Estimating Fuel Consumption for Space Heating 425 CALCULATED HEAT LOSS METHOD In the Calculated HeatrLoss Method a constant average outdoor temper ature is assumed throughout the heating season.- This average tempera ture is considerably above the design temperature given in Chapter 11. The method becomes unreliable whenever data on the seasonal average temperature are not available for the particular locality:. If the length qf time is shown oyer which the degree-day data: apply; in the particular locality, it is possible to calculate the average temperature from the defini tion of the degree-day (see Chapter 1). When this is done, the two meth ods described in this chapter become identical.- The average temperature for the period October to April inclusive, is fisted in Table 1, Chapter 11, for TJ. S. and Canadian cities. Computation and Application - In order to apply the Calculated Heat-Doss Method, the hourly heat loss from the building under maximum' load, or design condition, is com puted following the principles discussed in' Chapters 9, 10, and 11. The fuel: requirement is then computed by the equation ,, Hit - ON " Wi- ufc (1) where F = quantity of fuel or energy required (in the units,in which C is expressed). H = calculated heat loss including infiltration loss, Btu per hour, during the design hour, based on t,, and id. ' ' t = average inside temperature maintained during heating period, Fahrenheit degrees. t. = average outside temperature through estimate period, Fahrenheit degrees (for cities with an.Oct. 1-May 1 heating season--see Table 1, Chapter 11). ' U = inside design temperature, Fahrenheit degrees (usually 70 F). . t0 = outside design temperature, Fahrenheit degrees (see Table 1 in Chapter 11). 17 .= .number of heating hours in estimate period (for ah Oct. 1-May 1 heating . season, 212 days X 24 hr = 5088). ... E = efficiency of utilization of the fuel over the period, expressed as a decimal; not the efficiency at-peak or rated load condition. C =. heating value of one unit of fuel or energy. Although the assumption of an Oct. 1-May 1 heating, season is reason ably accurate in the well-populated New York-Chicago zone, it is not valid as far north as Minneapolis nor farther south than Washington, D. C. and St. Louis. Consequently, it is suggested that allowance be made for this variation, especially in the far north or southern cities. Example 1: A residence building is to be heated to 70 F from 6 a.m. to lO p ih. and 65 F from 10 p.m. to 6 a.m. The calculated hourly heat loss is 120,000 Btu per hour based on 70 F inside at10 F outside. If the building is to be heated by metered steam, how many pounds would be required during an average heating season? Solution: The heating value of steam may be taken as 1000 Btu per lb, and since it is purchased steam, the efficiency can be assumed as 100 percent. Assume average outside temperature as 36.4 F. The average inside temperature is: Oft X 70) + (8 X 65) 24 coow