An oversized air conditioner is susceptible to short cycling, inadequate dehumidification and large temperature variations in the house. Oversized air conditioners are usually the result of a design or installation problem.
Oversized units will have a shortened life expectancy and will provide a less comfortable environment. The largest comfort issue is the lack of dehumidification. Because the temperature drops rapidly with an oversized unit, there is not an adequate volume of air movement across the coil to extract the water from the house air. This results in a house that is cold, but with a humid, swamp-like environment. Since compressors experience most damage on startup, short cycles also mean more startups and a shorter life.
Other than the rough guideline test, it is difficult to know whether and how much the unit is oversized. Some public utilities indicate that a unit may be as much as 25 percent oversized without adverse effect. The temptation to oversize may become apparent when we talk about heat pumps. Since heat pumps have to deal with a much large temperature differential from outside to inside, the tendency is to make the heat pump large enough to meet the heating demand. This makes it too large for the cooling load. There are some strategies to address this problem, but within this context, we are watching for oversized cooling units. One way to identify an oversized air conditioner is by sensing the cold damp environment when walking into a house. Also, an air conditioner that short cycles (turns on and off every five minutes) is a suggestion that the unit may be oversized. Two surveys have shown that one third to one half of all residential air conditioning systems are oversized.