Choosing the right size air conditioner

The chill cycle

Central air conditioning uses a compressor and condenser coil housed in an outside unit and an expansion valve and evaporator (also called an "A coil") mounted above the furnace heater inside your home's HVAC system. Air conditioning works by cycling refrigerant from gas to liquid and then to gas again through the use of pressure. The refrigerant (known as R-410A) begins the cycle as a low-pressure, low-temperature gas. It enters the outside unit where it's first compressed into a high-pressure, high-temperature gas.

Next it enters the condenser's coil where the fan cools the gas into a high-pressure liquid. The liquefied refrigerant flows through tubing inside your home to the HVAC unit. There it first enters the expansion valve which limits the flow of the liquid as it enters the evaporator. This valve lets the liquid expand inside the evaporator coil into a gas, chilling to about 7 degrees Celsius as air from the HVAC blower moves over the coil. The refrigerant then heads back to the outside unit as a low-temperature, low-pressure gas to begin the cycle again.

Air is cooled by being blown over the chilled coil, and it's then circulated throughout your home. The AC system continues running until the desired temperature is reached. Any humidity in the air condenses onto the coil as water and drips into a condensation pan. In this way, your home is both cooled and de-humidified by a central air conditioning system.

Why size matters?

Choosing the correct size of your air conditioner is a key factor for setting up a home cooling system. Sizing an air conditioner requires a consultation with a professional Energy Management Consultant who looks at the factors that affect your home's cooling:

• Total volume of the home's living space - not just the square footage.
• The area of sun-exposed exterior walls.
• The number, age, condition, orientation of windows - do your windows face south?
• The amount of sunshine on your home.
• The age of your home's construction.
• Degree of air sealing the amounts of insulation.
• Existing ventilation system and ductwork - are they right?
• Amount of shade from trees and shrubs.

Likewise, it's not a simple matter of dropping in any air conditioning system. The AC unit size is very important because the correctly sized air conditioner should run a proper full cycle to ensure your home stays cooler longer.

An oversized AC system will quickly overcool the air in your home and shut off too soon. However, the building structure itself (walls, wood, furniture, etc.) will not be cooled adequately and allow heat and humidity to creep back in, and the AC will kick-on again and again. As a consequence, oversized systems run for too short a time, far too often, which can cost you far too much.

An undersized system, however, will not effectively cool your entire home. It will run continuously, waste energy, and wind up costing you more.

Understanding size: What do TON and SEER mean?

Air conditioner unit sizes are rated in tons of cooling capacity. One ton of cooling is equal to 12,000 BTU's (British thermal units) per hour. For example, a typical residential unit is a 2-ton unit that cools 24000 BTU/hour and a larger 3-ton unit cools 36000 BTU/hour.

While two air conditioners can be the same tonnage, they could be completely different in terms of their energy efficiency or Seasonal Energy Efficiency Ratio (SEER). Once you have found the right tonnage-size to cool your home, you'll want to look over which ones have the best SEER rating as is practical within your budget. The higher the rating, the less energy it will use — roughly 5% less per SEER. So, an older 12 SEER upgraded to a higher efficiency SEER 16 will be approximately 20% more efficient and will cost less to run. Systems that are Energy Star qualified range between 12 and 23 SEER.

Because air conditioning is one of the most expensive conveniences you can have in your home, it's important to be able to rely on a trained professional that you can trust to listen to your needs and guide you through this complex process.

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