100. The loss of heat per hour by conduction through windows, walls, etc. being given, and also the temperature of the hot-air supply and the desired temperature of the room, the required volume per hour may be computed by the following rule:

## Rule 8

Multiply the amount of heat lost by conduction per hour, in heat units, by 58; divide the result by the difference, in degrees, between the given temperatures of the room and the hot-air current. The quotient will be the required volume of hot air, in cubic feet per hour.

## Example

The loss of heat by conduction from a certain building is 200,000 heat units per hour; the temperature of the fresh hot air may be taken as 110°, and the rooms are to be maintained at 70°. What should be the volume of the hot-air supply?

 - 200,000 X 58 = 290,000 cu. ft. per hr. Ans. 110-70

101. The volume of hot-air supply is generally determined by the requirements of ventilation, and its temperature is made just sufficient to afford the amount of heat required.

The desired volume of the fresh-air supply in cubic feet per hour, the amount of heat lost by conduction, in heat units per hour, and the desired temperature of the rooms being given, the following rule may be used to compute the temperature which the hot air should have on entering the rooms:

## Rule 9

Multiply the amount of heat lost by conduction, in heat units per hour, by 58, and divide the result by the given volume of the air-current. Add the quotient to the desired temperature of the room; the sum will be the required temperature of the hot-air supply.

## Example

A certain building requires 480,000 cubic feet of air per hour for heating and ventilation. The amount of heat lost from the rooms by conduction is 200,000 heat units per hour, and the rooms must be maintained at a temperature of 70°; what must be the temperature of the fresh-air supply at entering the room?

200,000X58/480,000 + 70= 94.17º.Ans Solution. --.