Wall 5/6 X 19 = 15.8

Glass 1/6 X 85 = 14.1/ 29.9

Increasing this by 16% for total exposure and 10% for loss through ceilings we have 29.9 X 1.16 X 1.10 = 38.1. 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 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, corrections must be made as follows: Multiply by 50 for 20° below zero, by 44 for 10° below, by 33 for 10° above.

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 to a cold attic or other unmanned 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 temperature 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 may be found by the following formula: cu. ft. of air per hour X number of degrees rise = B. T. U. required.

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, then compute the amount required for ventilation as above and take the sum of the two. An inside temperature of 70° is always assumed unless otherwise stated.

Example - 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?

100,000 X 80/ 55 = 145,454 B. T. U. Ans.

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 II. we find that the total air required per hour is 1200 X 500 = 600,000 cu. ft.; therefore 600,000 X 70/ 55 = 763,636 B. T. U.

Example. - A corner room in a grammar school 28' X 32' and 12' high is to accommodate 50 pupils. The walls are of brick 16" in thickness and there are 6 single windows in the room, each 3' X 6'; 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? The total window surface is

6X3X6= 108 square feet. The exposed surface of the room is

32 X 12/ 28 X 12 = 720 square feet. The exposed wall surface is

720 - 108 = 612 square feet. Heat loss through windows = 108 X 85 = 9180 Heat loss through walls = 612 X 19 = 11628

Total heat loss = 20808

Total corrected for S. E. exposure

= 20808 X 1.06 = 22,056. Ans. Air supply required per hour

= 2400 X 50 = 120,000 cubic feet. B. T. U. required for ventilation

= 120,000 X 70 = 152,727. Ans. 55