I want a 'big' air conditioner. Someone told me this is not good - what are the advantages & disadvantages?
An 'oversized' air conditioner will cool your house quicker, but it will use more electricity and will not remove humidity adequately. Contrary to popular belief - and intuition - long A/C system run cycles are far more desirable and energy efficient than short run cycles. You can think of this in terms of a car, where you'd prefer to buy a car that has mostly highway miles rather than city miles. The greatest amount of energy usage and wear and tear on your equipment occurs at system start up (it takes tremendous torque and, hence, amperage to get your compressor turning). Once started, it takes 10 to 15 minutes before your system is operating at peak efficiency and performance. Because an oversized system will often bring the temperature down to the thermostat setting in just a few minutes, your oversized system will never get to operate at peak performance and will end up 'short-cycling' -- starting and stopping over and over again - a fact which will cost you plenty in terms of comfort, utility bills and added repairs.
Another factor to consider with regard to system size is that comfort is a function of temperature and humidity. That is, you can have low humidity (relatively dry air) and be comfortable when the temperature is 77 degrees or be absolutely miserable when you have high humidity and a temperature of 72 degrees. One of the most important jobs your air conditioning systems performs is removing moisture from the air in your home or office. Removing moisture requires long run times so that enough moisture laden air has a chance to blow across the cold indoor coil and condense into water which then drains from your system. Today's high-efficiency central air conditioners may remove as much as two gallons of water from the air in your home every hour, which makes you far more comfortable. If your system is oversized, it will cool the temperature down to the thermostat setting too rapidly and the system will shut off before proper dehumidification of the air can occur. Taken to the extreme, a severely oversized system will create 'rain forest' like conditions in your home: you'll have water dripping from your cold supply grills and, very soon, mold and mildew covering your ducts and walls.
Duct system sizing must be considered too. If your duct system is not big enough to handle the increased airflow required by an 'oversized' air conditioner, the evaporator coil may freeze and the system won't work at all. Even if the coil doesn't freeze, reduced airflow will impact operating efficiency and effectiveness and cause premature failure of expense components such as the compressor.
Obviously the air conditioner must be sized big enough to cool your house, but too big is just as bad. We can measure the heating and cooling load on your particular home and then recommend a system that is properly sized to cool your home to 74 degrees when it is 100 degrees outside, then increase the size slightly, if needed, based on your temperature preferences.