• How can I determine the proper size air conditioner / heater for my home?

• Bigger Is Not Better
  
  Why an Oversized Air Conditioner Is Bad News

• Oversized air conditioners run up your utility bill.  An oversized unit short-cycles (runs for shorter periods of time than it should), and uses more electricity than a properly sized unit. Air conditioners operate more efficiently the longer they run continuously, and should be sized so they don’t turn on and off frequently on hot days.
• Oversized air conditioners cost more to buy and don’t last as long.  The larger system will cost you hundreds of dollars more and give you fewer years of service— and bad service besides.
• An oversized air conditioner makes your house clammy.  A unit that’s too big can’t dehumidify the air effectively because it runs for only short periods at a time. The indoor coil never gets cold enough to do its job of removing moisture from the air. An oversized unit will leave your home cold and clammy, especially in humid climates.
• Oversized air conditioners are noisy.  The bigger the air conditioner the faster the air rushes through the ducts and grilles. Often the ducts and grilles are not designed for the larger unit, so the noise will get even louder as air tries to force its way through the small openings.

Studies show that 1/3 to 1/2 of all home air conditioners don’t work the way they should because they are oversized!  There is no happy medium with an oversized unit. It delivers noisy blasts of frigid air, then shuts off while moisture builds. On hot days, it will do this repeatedly, making the home more and more uncomfortable as the day passes.

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You'll pay hundreds of dollars more for an oversized air conditioner. Then you'll pay higher electricity bills to run it because an oversized unit is so inefficient.

Investing a little more in a good contractor who will take the time to properly size and install your system may be a very wise investment – both for your comfort and for your pocketbook.
 

• Don’t use a contractor who wants to size your unit solely on the square footage of your house. Contractors should calculate how much cooling a home needs according to guidelines such as the ACCA’s Manual J. To gather the information necessary, a contractor should spend about an hour poking around your house. He or she needs to measure floors, ceilings and walls—including all the windows—and check insulation in the attic and walls. Some other factors that go into the cooling load formula include indoor and outdoor temperatures, number of occupants, shading, siding, and roof color.
• Don’t be tempted by the lowest bid. Be willing to pay for the time the contractor must spend to do the job right.
• Buy a high efficiency unit. Central air conditioners are rated by SEER, or Seasonal Energy Efficiency Ratio. New air conditioners are required by federal law to have a SEER of 13 as a minimum. Even though it will cost more, it is usually worth it to buy at least a SEER 14 unit because over time that initial investment will be paid back with energy savings. If you are replacing an existing air conditioner, you must replace the whole unit, including the inside coil and often the blower fan, to achieve the rated efficiency.