The overall cooling capacity of an air-conditioning system is usually measured in Btus (British thermal units) or tonnage. One Btu is the heat required to raise the temperature of 1 pound of water 1° F. A rating in Btus per hour indicates the amount of heat that the unit will remove in that time. A ton of air-conditioning, historically, represented the cooling effect achieved by melting 1 ton of ice in twenty-four hours. An air-conditioning system that is rated at 1 ton will provide 12,000 Btus of cooling in an hour.
The cooling needed for a house depends on several variables, such as size of the rooms, whether there are cathedral ceilings, the number and type of windows, the amount of insulation in the walls and ceilings, and whether shading is provided by trees. The following rule of thumb can be used to determine the size of the air-conditioning system needed to cool the house that you are inspecting. One ton of cooling capacity (12,000 Btu) is needed for every 550 square feet in the structure. Therefore, if you are inspecting a house with 1,800 square feet, you will require an air-conditioning system rated at 3 tons. The exact requirement can be determined by a professional. However, the rule of thumb is quite effective in determining whether the unit servicing the house is grossly oversized or undersized. Calculate your cooling requirements and then ask the homeowner the capacity of the air-conditioning system.
You can check the capacity yourself by looking at the data plate on the compressor-condenser. Even though most manufacturers do not put the system’s capacity on the data plate, it can be approximated. The model number of the compressor on most air-conditioning systems contains digits that represent the approximate number of thousands of Btu of cooling capacity. See TABLE 17-1.
Another figure on the data plate that can be used for determining the cooling capacity is the full load amperage (FLA) or rated load amps (RLA). There are approximately 7 amps per ton of cooling. The FLA for the Tappan compressor in TABLE 17-1 is 27.1. When this figure is divided by 7, you get approximately 4 tons, which verifies the figure indicated by the model number.
An air-conditioning system should be properly sized or slightly undersized, but it should not be oversized. A unit that is too large operates intermittently. It quickly chills the air and then shuts down. When the system is shut off, moisture in the air does not condense, and the system does not dehumidify the air. For the system to remove enough moisture to make the air comfortably dry, the evaporator coil must be kept cold. This means that the compressor should run almost continuously for maximum comfort. Even though a system that is too small will run continuously, its cooling capacity is not adequate to remove enough heat to cool the room to a comfortable level.