Residential air conditioning is a very complex and complicated trade. Many homeowners do not realize the amount of technology and training it takes to properly calculate and install air conditioning equipment. Heating and air conditioning is by far the most technically advanced trade in residential construction. Technology advancements are common. Professional HVAC contractors must constantly attend seminars and classes to stay current.
However, even though HVAC technology produces high-tech equipment on a daily basis, some things do not change. One of the constants in residential cooling is the necessity of performing specific calculations to accurately determine the 'heat gain' of your home or business. It is impossible to properly size air conditioning equipment without performing these calculations. The recent advancements in personal computers have made this a much simpler task, however the calculations must be performed either by hand or by computer.
Another constant in residential air conditioning is humidity. Humidity is the moisture that occurs as water vapor in the air we breathe. The higher (more water vapor) the humidity in warmer air, the more uncomfortable we become. This principal is very easy to understand. The evaporation of water from a surface actually is a cooling process. It actually lowers the temperature of the object. Normal body perspiration is the way in which our bodies cool themselves. As the perspiration evaporates from our skin it cools it. However, the slower the rate of evaporation, the slower the cooling process.
When the humidity is high, the air has less ability to accept additional moisture. In other words, it is harder for water to evaporate. Air conditioning equipment has the ability to lower the humidity of air in your house. However, it can only do this while it is running. The moist air in your house passes through an evaporation coil inside of your furnace. This coil is cold, much like a cold can of soda or beer. Just as water droplets form on a cold can, the water from the air does the same thing on this coil. As long as you do not introduce any additional outside air or have numerous air leaks, the process happens within a few hours. The air in your house becomes cooler and drier. The perspiration on your skin can more readily evaporate and the net result is that you feel very comfortable.
However, if the air conditioner does not run long enough, it can not extract enough moisture from the air. The air will get cooler, but it will feel damp. You will feel cool but clammy.
The point of all of this is that the air conditioning unit outside of your house must operate long enough to properly 'cool' your house. It can only do this if it is sized properly. And, as I mentioned earlier, it can only be sized properly if you do the calculations. If a unit is sized too small, it simply won't have the ability to cool the house when it gets hot, even if it runs constantly. Whereas, a unit which is too large (too much capacity) will 'short' cycle. It will not run long enough to remove enough moisture from the air.