Neat Loss from Buildings

NEAT LOSS FROM BUILDINGS A British Thermal Unit, or B. T. U., has been defined as the amount of heat required to raise the temperature of one pound of water one degree F. This measure of heat enters into many of the calculations involved in the solving of problems in heating and ventila tion, and one should familiarize himself with the exact meaning of the term.

Causes of Heat Loss. The heat loss from a building is due to the following causes: (1) radiation and conduction of heat through walls and windows; (2) leakage of warm ai; around doors and win dows and through the walls themselves; and (3) heat required to warm the air for ventilation.

Loss through Walls and Windows. The

loss of heat through the walls of a building depends upon the material used in construction For solid stone walls, multiply the figures for brick of the same thickness by 1.7. Where rooms have a cold attic above or cellar beneath, multiply the heat loss through walls and windows by 1 .1.

Correction for Leakage. The figures given in the above table apply only to the most thorough construction. For the average well-built house, the results should be increased about 10 per cent; for fairly good construction, 20 per cent; and for poor construction, 30 per ceni.

Table V applies only to a southern exposure; for other exposures multi ply the heat loss given in Table V by the factors given in Table VI.

of the wall, the thickness, the number of layers, and the difference between the inside and outside temperatures. The exact amount of heat lost in this way is very difficult to determine theoretically, hence we depend principally on the results of experiments.

Loss by Air-Leakage.

The leakage of air from a room varies from one to two or more changes of the entire contents per hour, depending upon the construction, opening of doors, etc. It is com mon practice to allow for one change per hour in well-constructed buildings where two walls of the room have an outside exposure. As the amount of leakage depends upon the extent of exposed wall and window surface, the simplest way of providing for this is to increase the total loss through walls and windows by a factor depending upon the tightness of the building construction. Authorities differ con siderably in the factors given for heat losses, and there are various methods for computing the same. The figures given in Table V have

been used extensively in actual practice, and have been found to give good results when used with judgment. The table gives the heat losses through different thicknesses of walls, doors, windows, etc., in B. T. U., per square foot of surface per hour, for varying differences in inside and outside temperatures.

In computing the heat loss through walls, only those exposed to the outside air are considered.

In order to make the use of the table clear, we shall give a num ber of examples illustrating its use: Example 1. Assuming an inside temperature of 70°, what will be the heat loss from a room having an exposed wall surface of 200 square feet and a glass surface of 50 square feet, when the outside temperature is zero? The wall is of brick, 16 inches in thickness, and has a southern exposure; the win dows are single; and the construction is of the best, so that no account need be taken of leakage We find from Table V, that the factor for a 16-inch brick wall with a difference in temperature of 70° is 19, and that for glass (single window) under the same condition is 85; therefore, Loss through walls = 200 X 19 = 3,800 Loss through windows = 50 X 85 = 4,250 Total loss per hour = 8,050 B. T. U.

Example 2. A room 15 ft.. square and 10 ft. high has two exposed walls, one toward the north, and the other toward the west. There are 4 windows, each 3 feet by 6 feet in size. The two in the north wall are double, while the other two are single. The walls are of brick, 20 inches in thickness. With an inside temperature of 70°, what will be the heat loss per hour when it is 10° below zero? Total exposed surface = 15 X 10 X 2 = 300 Glass surface = 3 X 6 X 4= 72 Net wall surface = 228 Difference between inside and outside temperature 80°.

Factor for 20-inch brick wall is 18. Factor for single window is 93.

Factor for double window is 62.

The heat losses are as follows: Wall, 228 X 18 = 4,104 Single windows, 36 X 93 = 3,348 Double windows, 36 X 62 = 2,232 9,684 B. T. U.

As one side is toward the north, and the other toward the west, the actual exposure is N. W. Looking in Table VI, we find the correction factor for this exposure to be 1.26; therefore the total heat loss is