Choosing the correctly sized air conditioner unit or HVAC -- heating, ventilation, and air conditioning -- system for a house or individual rooms not only makes the home feel more comfortable, but saves you money as well. Since air conditioners remove both heat and moisture from the air, a unit or system that is too large will cool the space too quickly, making it feel damp and humid. It will also use too many BTUs per hour, resulting in a higher electric bill. An air conditioner unit or HVAC system that is too small for a home won't cool it properly and can overtax the appliance.
You can get a general feel for sizing on your own, with a formula commonly used to get a ballpark estimate: [(House square footage x 25)/12,000] – 0.5 = required tons. This is good way to approximate, but your installer may suggest adjustments to your estimate based on factors specific to your house, such as ceiling height, type of windows, climate and insulation. You can also use a chart organized by square feet and climate zone to get an estimate.
Calculating Air Conditioner Size
Determine the total square footage of the room or rooms you want to cool. For square- and rectangular-shaped spaces, multiply the length of the area by its width. For a triangular-shaped room, multiply the length of the area by the width and divide this figure by two.
Consult a chart that compares room square footage with the air conditioner unit capacity needed in BTUs, such as the one provided by the U.S. Environmental Protection Agency. For instance, if you have a 450- to 550-square-foot-room, you'll need an air conditioner unit with a capacity of 12,000 BTUs.
Once you know how many BTUs per hour you need, make any necessary adjustments for special circumstances. As one example, you can reduce the unit capacity by 10 percent if the room is heavily shaded. If the space is very sunny, increase the capacity by 10 percent. If two or more people are typically in the space, add 600 BTUs for each extra person. Also, kitchen air conditioner units should be increased by 4,000 BTUs to make up for heat generated while cooking.
If you have a large home, such as one with 2,000 to 2,500 square feet, which requires 34,000 BTUs per hour, you may need to install more than one HVAC unit to thoroughly cool each room. Do a room by room load calculation by utilizing the Air Conditioning Contractors of America's Manual J Residential Load Calculation Procedure software, which is the industry standard for sizing residential HVAC systems.