Following are the important variables to keep in mind when estimating the Btu requirements for a room cooling installation:
- Room size in square feet of floor area
- Wall construction (whether light or heavy)
- Heat gain through ceiling
- The proportion of outside wall area that is glass
- The exposure of the room’s walls to the sun
Additional factors to be taken into account are room ceiling height, number of persons using the room, and miscellaneous heat loads (such as wattage of lamps, radio and television sets in use in the room, and so on).
Table 5-1 should be used to obtain the approximate cooling capacity in Btu. The following steps outline the procedure for using the table (which shows the Btu requirements for various sizes of rooms). In each case, it is assumed that the rooms have an 8-foot ceiling height. The procedure is as follows:
1. Measure the room to be cooled for square footage (length in feet times width in feet).
2. Determine the direction the room faces to determine which exposure to use.
3. Determine the condition of the space above the room to be cooled (such as occupied, attic, or insulated flat roof).
For example, to determine the required amount of cooling for a room 23 feet long and 13 feet wide, multiplication of the two figures will give 299 square feet. It is further assumed that the space above the room to be cooled is occupied and that the room has an exposed wall with windows facing south. Table 5-1 indicates that the room described will require a unit having a capacity of approximately 8000 Btu. A room of the same size and facing south with an uninsulated attic above it will require a cooling capacity of approximately 10,500 Btu.
Table 5-1 indicates the approximate Btu capacity required to cool different rooms with the approximate area and other conditions as described. The table will also permit checking an installed unit where unsatisfactory performance might be suspected resulting from improper sizing or inadequate unit capacity.