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Why Sizing Your Air Conditioner Properly Is So Essential

Bigger isn’t always better—especially when it comes to your central air conditioner. When your HVAC contractor says he needs to properly size your new A/C, he’s not talking about something that's done with a measuring tape. “Sizing” in the air conditioning sense refers to matching the BTU capacity of the unit with the cooling requirements of the home, a figure known as the “cooling load.” It’s a precision procedure and when it’s done right, you’ll get the efficiency and comfort performance promised by the manufacturer’s specifications.

Unfortunately, accurately sizing a new air conditioner hasn’t always been the rule since energy efficiency used to be a low priority in home construction. Any HVAC contractor can tell stories of service calls to homes with grossly oversized A/C units that were installed when the home was built and have been an ongoing source of poor performance and energy inefficiency for the residents. Today, with high utility costs being a major concern, an improperly sized unit is simply unacceptable.

Why Sizing Your Air Conditioner Properly Is Essential

  • An undersized air conditioner can’t remove BTUs of heat from the home fast enough to keep up with outdoor temperatures. These units run day and night without providing satisfactory cooling. This results in high operating costs and low comfort levels..
  • An A/C with oversized BTU capacity cools living spaces too fast. While a quick cooldown might sound good on a hot day, it’s an efficiency drag and a comfort destroyer, too. Because they cool so rapidly, oversized units end up “short cycling,” turning on and off in cooling cycles that last less than 5 minutes. An air conditioner operates most efficiently only when its “On” cycles exceed 10 minutes. Short on/off cycles consume more energy and also cause excessive wear and tear. The end result is premature failure of expensive components, such as the compressor.
  • Humidity extraction is a critical part of air conditioning. Humid air holds heat energy and makes it more difficult to cool the house. A humid indoor environment also feels less comfortable to occupants. Effective humidity reduction at the A/C evaporator coil only begins when the coil temperature is lowest, usually after the unit has been running for at least 10 minutes. If the A/C always turns off prematurely due to being oversized, the humidity extraction is effectively zero and indoor comfort is reduced.
  • Proper indoor air filtration depends on air circulation through the system filter. All filtration ceases when the unit cycles off. An oversized, short-cycling system spends a greater percentage of the day in an “off” cycle with no circulation, allowing particulates and contaminants to concentrate in living spaces.   
  • Temperature swings above and below the thermostat setting are more noticeable with an oversized system, particularly on hot days. When the system turns off after a brief “on” cycle, room temperatures rise quickly above the target temperature; when the oversized equipment cycles back on, the blast of overly cold air flooding the rooms causes temperatures to plunge below the thermostat setting and rooms become uncomfortably cold. Then the cycle repeats. 
  • Oversized air conditioners make more noise. The constantly alternating on/off sound of the system can be very noticeable compared to a properly-sized A/C that runs more consistently. Also, oversized blower output is often inappropriate for the diameter of the home’s ductwork. This generates higher decibels of air noise inside the ducts and at supply registers where air enters the room.  

Cooling Load Calculation With Manual J

You can avoid having an undersized or oversized air conditioner installed. Your HVAC professional will calculate the cooling load by using Manual J software, which is the standard for accurately determining the BTU requirements of the home. He will perform a room-by-room survey and input information relating to the thermal characteristics of the house. These factors include:

  • Local climate
  • Number and type of windows
  • Amount and quality of insulation
  • Condition of ductwork
  • Air infiltration due to leakage
  • Building materials and roof composition
  • Ceiling heights
  • Major appliances and type of lighting
  • Number of occupants in the home

Manual J software calculates the amount of BTUs per hour that must be removed from the home to maintain a comfortable temperature—usually 75 degrees—on the hottest expected day of the year. With that information, the HVAC contractor can select an air conditioner with the capacity specs that match the cooling load for optimum efficiency and performance. Not too large, not too little.

For more information on properly sizing an air conditioner the Chicago area, contact us at Comfort24-7.com .

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