Central Air Conditioning Sizing: Choosing the Right AC for Your Home

By Lyle Weischwill | Jul. 10, 2016 8:43 pm PST

Find the right sized air conditioner for your home

Have rising temperatures got you sweating over the cost of your energy bills?

It might be time to consider investing in a new air conditioning unit. Improved year-round energy efficiency is something you can see and feel, with lower energy bills and improved comfort throughout the day. But when it comes to AC units, not all are created equal.

Why Air Conditioner Size Matters

Air conditioning is not only about cooling the ambient temperature. Dehumidifying the inside air is another critical function of AC units. Removing moisture is necessary to keep your house free of mold.

Keep these two functions in mind when sizing a new AC unit. If the air conditioner is too small it will have to work harder and run longer, costing you comfort and money. But bigger is not necessarily better. If your AC unit is oversized for the size of your home, it will turn on and off more frequently. This “short cycling” can shorten the compressor’s lifespan and prevents your unit from operating long enough to properly remove moisture.

Sizing Your New AC Unit

Cooling power is measured in BTUs (British thermal units), and air conditioner sizing is determined by the square footage of the area to be cooled. Whether you need a window AC unit or a central system, the question is the same: How much power is required to cool and dehumidify a given room or living area? You can make a rough estimate of the unit you’ll need by calculating the square footage of your home.

Most living spaces can be divided into three basic shapes: square, rectangle, or triangles. To calculate the total square area, add together the area measurements for all the rooms and spaces on each floor, use the following formulas:

  • Square/Rectangles: length x width
  • Triangles: (length x width) / 2

Once you have the total square area, consult this chart provided by the ENERGY STAR program, to determine how much power your AC unit will need to produce. The load size of air conditioning systems is measured in tons, where 12,000 BTUs = 1 ton. For example, if you calculate a new system would need to produce 36,000 BTUs, then you would be shopping for a 3-ton AC unit (36,000/12,000).

Of course, this calculation is only a starting point. There are other variables to take into consideration. Chief among these factors is the R-rating of the insulation in your home. A poorly insulated 800 square-foot space will require more cooling power than a tightly sealed space of the same size. Other things to think about include: south- or north-facing parts of your home, the local climate, or unfinished spaces that you plan on building out.

Installing a new HVAC unit is great way to make your home more energy efficient. However, an improperly sized unit will cost you more in the long run. Be sure to consult local and state laws that might require you to install units with a minimum Seasonal Energy Efficiency Ratio (SEER) rating. Moreover, some local or state governments may offer tax incentives for more efficient models. A certified Sears technician will be happy to help you find the best and most efficient air conditioner size for your home, make an appointment today!

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