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Save Hundreds of Dollars on your Next AC... and Be More Comfortable to Boot!

April 2, 2013

Sooner or later, you will need a new air conditioner. What’s the best size? Bigger is better, right? Turns out this is not true. Just like the start-stop city miles per gallon in your car is less than highway miles per gallon, the most efficient AC is one that runs all afternoon on the hottest days of the year. In addition, having the most efficient AC unit will make you feel more comfortable - a steady stream of cool air, rather than blasts of cold air followed by an increased temperature in the living space.

Rich Franz-Under, R.A., LEED AP and Green Building Program manager for Pima County offers a way to properly size your air conditioner. Simply follow these steps:

  1. Wait for a hot afternoon. It must be 105 degrees or above.
  2. Next, set a timer and monitor how long the AC runs, versus how long it remains off. Repeat this two or three times.
  3. Record the average of run-times and "off" times.
  4. Next, use the following formula:
    Minutes on
    Minutes on + Minutes Off = % of current system capacity used
  5. Multiply the % of current system capacity used by the size of your current AC. This will indicate how large your next AC needs to be to provide a comfortable environment.

For example: if your average cycle is 22 minutes running and 21 minutes off, then the load is 51% of system capacity:

22/(22+21) = .511 or about 51%

Then multiply the capacity of your existing AC by the percentage of capacity used: if your existing AC is 3.5 tons:

3.5 tons x .51 = 1.8 tons of load. Therefore, a new 2-ton AC should be fine for you. Less expense, greater comfort!