57,344 X 1.16 X 1.10 X 1.10 X 1.20 = 96,338 B. T. U.
If one pound of coal furnishes 8,000 B. T. U., then 96,338 _ 8,000 = 12 pounds of coal per hour required to warm the building to 70° under the conditions stated.
Approximate Method. For dwelling-houses of the average con struction, the following simple method for calculating the heat loss may be used. Multiply the total exposed surface by 45, which will give the heat loss in B. T. U. per hour for an inside temperature of 70° in zero weather.
This factor is obtained in the following manner: Assume the glass surface to be one-sixth the total exposure, which is an average propor tion. Then each square foot of exposed surface consists one-sixth of glass and five-sixths of wall, and the heat loss for 70° difference in temperature would be as follows: 29.9 Increasing this 20 per cent for leakage, 16 per cent for exposure, and 10 per cent for cold ceilings, we have: 29.9 X 1.20 X 1.16 X 1.10 = 45.
The loss through floors is considered as being offset by including the kitchen walls of a dwelling-house, which are warmed by the range, and which would not otherwise be included if computing the size of a furnace or boiler for heating.
If the heat loss is required for outside temperatures other than zero, multiply by 50 for 10 degrees below, and by 40 for 10 degrees above zero.
This method is convenient for approximations in the case of dwelling-houses; but the more exact method should be used for other types of buildings, and in all cases for computing the heating surface for separate rooms. When calculating the heat loss from isolated rooms, the cold inside walls as well as the outside must be considered.
The loss through a wall next toacold attic or other unwarmed space may in general be taken as about two-thirds that of an outside wall.
Heat Loss by Ventilation. One B. T. U. will raise the tempera ture of 1 cubic foot of air 55 degrees at average temperatures and pressures, or will raise 55 cubic feet 1 degree, so that the heat required for the ventilation of any room can be found by the following formula : Cu. ft. of air per hour x Number of degrees rise B. T. U. required.
55 To compute the heat loss for any given room which is to be ventilated, first find the loss through walls and windows, and correct for exposure and leakage; then compute the amount required for ventilation as above, and take the sum of the two. An inside tem
perature of 70° is always assumed unless otherwise stated.
Examples. What quantity of heat will be required to warm 100,000 cubic feet of air to 70° for ventilating purposes when the outside temperature is 10 below zero? How many B. T. U. will be required per hour for the ventilation of a church seating 500 people, in zero weather? Referring to Table III, we find that the total air required per hour is 1,200 X 500 = 600,000 cu. ft.; therefore 600,000 X 70 - 55 = 763,636 B. T. U.
The factor Rise in Temperature is approximately 1.1 for 60°, 1.3 for 70°, and 1.5 for 80°. Assuming a temperature of 70° for the entering air, we may multiply the air-volume supplied for ventilation by 1.1 for an outside temperature of 10° above 0, by 1.3 for zero, and by 1.5 for 10° below zero—which covers the conditions most commonly met with in practice.
1. A room in a grammar school 28 ft. by 32 ft. and 12 feet high is to accommodate 50 pupils. The walls are of brick 16 inches in thick ness; and there are 6 single windows in the room, each 3 ft. by 6 ft.; there are warm rooms above and below; the exposure is S. E. How many B. T. U. will be required per hour for warming the room, and how many for ventilation, in zero weather, assuming the building to be of average construction? Ans. 24,261 + for warming; 152,727 -I- for ventilation.
2. A stone church seating 400 people has walls 20 inches in thickness. It has a wall exposure of 5,000 square feet, a glass expos are (single windows) of 600 square feet, and a roof exposure of 7,000 feet; the roof is of 2-inch pine plank, and the factor for heat loss may be taken the same as for a 2-inch wooden door. The floor is of wood on brick arches, and has an area of 4,000 square feet. , The building is exposed on all sides, and is of first-class construction. What will be the heat required per hour for both warming and ventila tion when the outside temperature is 20° above zero? Ans. 296,380 for warming; 436,363 -}- for ventilation.
3. A dwelling-house of average wooden construction measures 200 feet around the outside, and has 3 stories, each 9 feet high. Compute the heat loss by the approximate method when the tem perature is 10° below zero.