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HEATING VENTILATING AIR CONDITIONING GUIDE 1944 Example 5. 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).^ . The maximum hourly heat loss will be: H = 92,000 - 26,^)Q0 = 79,000 Btu. w _ 79,000 (66.3 - 36.4) X 24 X 210 . ... M--------- 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. In the case of gravity warm air heating installations, the load is usually expressed in square inches of leader pipe. This can be converted into hourly heat loss by multiplying by the factors in Table 1. Table 1. Heat Carrying Capacity of Gravity Warm Air Furnace Round Leader Pipes 176 F Register Temperature First floor................ Second floor._____ Third floor. Leader Pipe Btu per Hour at Design Conditions fee Sq In. of Leader Pipe 111 167 200 Example A What would be the total gas consumption over a full heating season of a gas-fired gravity warm air furnace designed according to the Code1, and with four 12 in. and two 8 in. round leaders to the first floor and six 10 in. leaders to the second floor, if the gas has a heating value of 500 Btu per cubic foot, the plant operates at a 70 per cent seasonal efficiency and is designed to maintain an average inside temperature of 65 F when it is 10 F outside in a city where the average outside temperature is 45 F and the heating season is 5088 hr long? Solution. The area of the round leaders is: 12 in., 113 sq in.; 10 in., 79 sq in.; and 8 in., 50 sq in. From Table 1 the total Btu transmitted is: First Floor: [(4 X 113) + (2 X 50)1 X 111 = 61,272 Btu per hour. Second Floor: (6 X 79) X 167 = 79,158 Btu per hour. Total * 140,430 Btu per hour. Substituting this total heat loss value as H in Equation 1 gives: 140,430 (65 - 45) 5088 0.70 (70 - 10) 500 680,483 cu ft gas. ,, 'Standard Gravity Code for the Design and Installation of Gravity Warm Air Heating Systems (11th edition), and the Technical Code for the Design and Installation of Mechanical Warm Air Heating Systems, may be obtained from the Rational Warm Air Beating and Air Conditioning Association 145 Pubhc Square, Cleveland, Ohio. 226 CHAPTER 11. ESTIMATING FUEL CONSUMPTION DEGREE-DAY METHOD This method is based on consumption data which have been taken from buildings in operation, and the results computed on a degree-day basis. While this method may not be as theoretically correct as the Calculated Heat Loss Method, it is considered by many to be of more value for practical use. ' The amount of heat required by a building depends upon the outdoor temperature, if other variables are eliminated. Theoretically it is pro portional to the difference between the outdoor and indoor temperatures. Table 2. Base Temperature for the Degree-Day* Type or Building n*ihQ Warehouse and Loft No. or Buildings . Analtied 60 4 3 2 6 11 ` 12 ` 7 14 8 4 5 3 2 2 4 3 3 2 8 Temperature F Cor responds to Zero Steam Consumption 66.2 65.8 66.2 65.5 67.4 64.0 64.3 66.5 68.8 66.9 65.5 64.9 67.6 65.8 64.8 61.2 67.7 67.7 65.2 65.4 66.0 F Report of CnmmFrip'al Relations Committee, Proceedings, Notional District Seating Association, 1932. Some years ago the American Gas Association3 determined from experi ment in the heating of residences that the gas consumption varied directly as Idle difference between 65 F and the outside temperature. In other words, on a day when the temperature was 20 deg below 65 F, twice as much gas was consumed as on a day when the temperature was 10 deg below 65 F. For any one day there exists as many degree-days as there are degrees Fahrenheit difference in temperature between the mean temperature for the day and 65 F when the mean temperature is less than' 65 F. ' Studies made by the National District Heating Association of the metered steam consumption of 163 buildings located in 22 different cities and served with steam from a district heating company substantiates the fact that the 65 F base originally chosen by the gas industry is approxi mately correct as shown in Table 2. If the degree-days occurring each day are totaled for a reasonably long See-Industrial Gas Series, House Heating, {third edition) published by the American Gas Association. 227