It is generally accepted that "the right way" to specify an air conditioning system is to calculate the loads and select a piece of equipment that will provide comfort to the customer in a wide variety of conditions. Unfortunately, this is rarely practiced.

Let's look at an example. Our friend (we'll call him Bob) needed a new air conditioning system in his house. Bob called us for some advice and we told him €Be sure that the contractor calculates the cooling load and that the air conditioner is sized to that load.€ Well, Bob contacted four different air conditioning contractors for pricing and specifications for the proposed new system. Out of the four, only one would provide load calculations. Two just asked for the square footage of the home and the last one simply wanted to know the size of the existing system. Not knowing any better, Bob went ahead and hired the contractor who was willing to provide the load calculations. But poor Bob did NOT live happily ever after. It turns out that the contractor had fudged his calculations by increasing the outside air temperature and decreasing the desired indoor air temperature and then added another 20% on top of THAT to his calculations €just to be safe€. The end of the story is that Bob overpaid for a system that was too large for his home and ended up living with high energy bills and a cold and clammy house. Why?

An over-sized air conditioning system may seem like a good thing at first thought but, in reality, oversized AC units can lead to a series of issues and problems. Specifically, many would think that a short cycle time (the amount of time the unit is running) would save on your electric bill but the truth of the matter is that air conditioners are very inefficient when they first start operation. It is far better for the air conditioner to run longer cycles than shorter ones. The efficiency of the typical air conditioner increases the longer it runs.

For most of the cooling season, the cooling loads are well below the capacity of properly sized air conditioners, and for oversized units the short cycling is a substantial problem. Because of the short cycles, an oversized system is less efficient. Furthermore, the ability of the air conditioner to remove moisture (latent capacity) is lowest at the beginning of the air conditioner cycle. The moisture removed from the indoor air is dependent upon the indoor coil temperature being below the dew-point temperature of the air. The moisture then wets the indoor coil and, should the unit run long enough, will begin to flow off the coil and be removed out of the condensate drain.

For short cycles, the coil does not have time to operate at the low temperature and when the unit stops, the moisture on the coil evaporates back into the indoor air. Thus, in humid climates such as Florida, a properly sized air conditioner will do a far better job of removing moisture from the air than oversized units. If an oversized air conditioner cannot remove enough moisture from the air, the home will be cold and clammy.

And, finally, an oversized air conditioner can mask problems from duct leaks, improper flow across the coils, and improper coolant charge.