Document N2w5DOvLY20O06pywwYNdNR0g

376 CHAPTER 20 1946 Guide take into account factors which are difficult to evaluate such as opening of windows, abnormal heating of the building, poor heating systems, winter heat gains, such as sun effect, and many others. In order to apply this method the hourly heat loss from the building under maximum load, or design condition, is computed following the principles discussed in Chapters 6 and 8 and the method described and illustrated in Chapter 14. In some cases, however, depending on the presence of interior par titions, the computed heat loss is modified when used for estimating the heat or fuel consumption. If the building has no interior walls or par titions then, by the method of Chapters 8 and 14, the infiltration losses are calculated by using only half the total window crack. In such a building the calculated loss need not be modified in order to prepare heat or fuel estimates by this method. Where the building does contain interior walls or partitions instead of using as the calculated heat loss (H) which is equal to the sum of the transmission losses (Ht) and the infiltration losses (Hi), it is more desirable to let H = Ht + t~' In predicting fuel consumption for heating a building by the Calculated Heat Loss Method, the general'equation is: where H{t - t*) N E (td -- to) C (1) F = quantity of fuel or energy required (in the units in which C is expressed). H = calculated heat loss, Btu per hour, during the design hour, based on to and /d ^generally H -- Ht + Hi but may on occasion equal Ht + t -- average inside temperature maintained during heating period, Fahrenheit degrees, /a ~ average outside temperature through estimate period, Fahrenheit degrees (for cities with an Oct. 1-May 1 heating season, see Table 1, Chapter 14). <d ~ inside design temperature, Fahrenheit degrees (usually 70 F). to = outside design temperature, Fahrenheit degrees (see Table 1 in Chapter 14). > N = number of heating hours in estimate period (for an 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 sbuth 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 in Chicago is to be heated to 70 F from 6 a.m. to 10 p.m. and 55 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 at --10 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 pound, and since it is purchased steam, the efficiency can be assumed as 100 per cent. From Table 1, Chapter 14, /a = 36.4 F. The average inside temperature is: (16 X 70) + (8 X 55) _ 65 F Substituting in Equation 1: 120,000 (65 - 36.4) 5088 1.00 [70 - (-10)] 1000 218,275 lb. Estimating Fuel Consumption for Space Heating 377- Example 2. How much would the fuel cost to heat the building in Example 1 during an average heating season with coal at $8 per ton and with a calorific value of 11,000 Btu per pound, assuming that the seasonal efficiency of the plant was 55 per cent? ,, 120,000 (65 - 36.4) 5088 ,,,, ,,,,,, ,L Solution. Substituting in Equation 1: F= 0 55 [70' -- (--10)1 11 000 = 36,079 lb = 18 tons, which, at $8 per ton, costs $144. Example S. What will be the estimated fuel cost per year of heating a building with gas,.assuming that the calculated hourly heat loss is 92,000 Btu based on 0 F, which includes 26,000 Btu for infiltration? The design temperatures are 0 F and 72 F. The normal heating season is 210 days, and the average outside temperature during the heating season is 36.4 F. The seasonal efficiency will be 75 per cent. The heating plant will be thermostatically controlled, and a temperature of 55 F will be maintained from 11 p.m. to 7 a.m. Assume that the price of gas is 7 cents per 100,000 Btu of fuel con sumption, and disregard the' loss of heat through open windows and doors. Solution. The average hourly temperature is: = (72 X 16) + (55 X 8) = g6 g p The maximum hourly heat loss will be: or non H = 92,000 - TSiEES = 79,000 Btu. M = 79,000 (66.3 - 36.4) X 24 X 210 100,000 X 0.75 X (72 - 0) = 2204.6 hundred thousand Btu. 2204.6 X $0.07 = $154.32 = estimated fuel cost per year of heating building. Several time-saving procedures have been devised for quickly esti mating the heat consumption of one and two-story residences in order that fuel estimates can be predicted more quickly from Equation 1. . A graphical method of calculating heat losses has beep developed1 which makes possible a quick solution if the gross wall, ceiling, or floor areas,and respective transmission coefficients are known. The Federal Housing Administration has originated a short-cut formula for residential heat loss determinations which makes use of the floor area and three selected transmission coefficients. Equation 2 is for a one-story residence and Equation 3 is intended for two-story structures. H, = A (0.45 + U,,+Vc + Ut) (ld- to) (2) .H,"= A (0.45 + 1.2 l/,, + 0.5 Uc + 0.5 Ut) (td - to) (3) where Hi = heat loss from one-story residence, Btu per hour. Hi = heat loss from two-story residence, Btu per hour. A -- floor area, square feet. f/w = coefficient-of transmission for outside wall. Uc = coefficient transmission for ceiling, from air in rooms to air in attic space (for 1 ventilated attics). Ut -- coefficient of transmission for floor from air in rooms to air in basement. /d = inside design temperature, Fahrenheit degrees. <0 = outside design temperature, Fahfenheit degrees. ' Both the graphical method and short-cut formula have been found to give accurate and consistent results for the average residence, but if precise estimates are required, the procedure outlined in Chapter 14 should be used. In the case of gravity warm air heating installations, the load was formerly expressed in square inches of leader pipe which can be converted