Proper Sizing of Central Air Conditioning

Whether you are looking to install a new central air conditioning unit or replacing a current unit, it is important to use the correct size unit. A that is too small will run continuously without adequately cooling the house, while a unit that is too big is overly costly and, due to frequent on/off cycles, can cause humidity problems.

Proper System Sizing 

In order to scientifically calculate the right central air conditioner size, you can perform a Residential Load Calculation. The sizing chart above is useful to help you determine your approximate central air conditioner size requirements.

Another way to size replacement air conditioners is to determine the system size that is currently in place. The manufacturers do not list the air conditioner size on the unit. The system capacity will be coded into the model number of the outdoor unit. For example, model CKL24-1 is a 2 ton unit.
Why? There are 12,000 Btu's per ton. The number 24 in the model number indicates the unit is 24,000 Btu's, divided by 12,000 Btu's per ton, equals 2 tons. 24,000 Btu's/12,000 Btu's per ton = 2 tons Use the following conversion information to determine your existing system size (use the system model number NOT the serial number) 18 = 1.5 tons 24 = 2 tons 30 = 2.5 tons 36 = 3 tons 42 = 3.5 tons 48 = 4 tons 60 = 5 tons.

When sizing air conditioners, be careful not to make the mistake of getting one that is too large. If the system you install is too large for the space, it will short cycle. In other words, the compressor will not run long enough to dehumidify the space and will limit your comfort. In addition, it will cycle on and off more frequently, increasing operating costs and reducing the system life.

Insulation Variables Variables such as your insulation, type and number of windows, number of stories, construction type, etc. will greatly affect the required Btu's per square for both heating and cooling. A general rule of thumb is that if your home is well insulated with newer style windows, you can select the smaller size system that falls within your total square footage.

If your home is two story it will place less of a load on the system in the downstairs area as the second floor acts as additional insulation. If your home is not well insulated, has older style windows, and/or a larger than average number of windows, you will want to select the larger system which falls within your square footage range. The less insulated and more windows within the environment, the more likely you will experience greater air and heat loss.

Proper air conditioner sizing is critical to optimize performance.