Air conditioner capacity is measured in tons, but that term doesn’t refer to how much heavy lifting the crew installing your unit will have to do. Tons in the HVAC profession relates to the amount of heat energy in British thermal units (BTUs) an air conditioner’s capable of extracting from the air. One ton equals 12,000 BTUs per hour.

“Cooling load,” another industry term, refers to the number of BTUs an A/C must remove from the air inside your home to achieve a comfortable interior temperature target — usually around 75 degrees. The cooling load of a house also incorporates a figure that estimates the amount of humidity inside the home that needs to be extracted for maximum comfort. Humidity reduction is a critical part of the cooling process. 

While the BTU tonnage of an air conditioner is a fixed specification determined by testing under laboratory conditions, the differing thermal characteristics of every house mean that the cooling load figure varies widely among typical homes. Accurately matching the air conditioner capacity of a new A/C to the cooling demands of the home — a process known as “sizing” — is vital to ensure optimum efficiency and performance.

The bad old days

In past eras, correct A/C sizing was often given low priority, largely because energy was cheap and installing an A/C unit that was larger than necessary was believed to add a margin of error that would cover all circumstances. Studies show that a large percentage of existing A/C units are larger than they need to be and use more energy than necessary. Today, with high utility bills and more accurate methods to match the cooling demands of a home to the air conditioner capacity, a sizing procedure performed by an HVAC professional is a standard part of any competent air conditioner installation.

Not too much, not too little
There are a number of reasons to get the match between air conditioner capacity and cooling load right. An oversized unit that has more capacity than the home’s cooling load will cool rooms with too-rapid on/off cycles of the compressor. Known as short-cycling, this phenomenon wastes energy, puts excess wear and tear on components and interferes with the process that extracts humidity from the air, making rooms feel clammy. Undersized air conditioners, on the other hand, underperform and don’t cool rooms adequately. These units run excessively long “on” cycles, struggling to meet thermostat settings, inflate utility costs and may have shortened service life.

Sizing by the book
An experienced professional contractor utilizes the industry standard software called Manual J to size your home and determine the BTU capacity required to cool it. Conducting a room-by-room survey, the tech will input specific data about your home, such as square footage, orientation to the sun, the number of windows, type and amount of insulation, major appliances and the number of occupants. With this data, Manual J generates an exact figure, expressed in BTU tons, called the sensible load. This represents the A/C capacity required to comfortably cool the home in the most energy-efficient manner.

A secondary figure is also derived from the data. Known as the latent load and also expressed in BTUs, this figure refers to the amount of humidity extraction required to keep the home comfortable. 

Match making
Since manufacturer’s specifications of BTU tonnage capacity are calculated under controlled conditions that may not accurately reflect your home’s characteristics nor your local climate, your contractor will utilize another program called Manual S to further refine the selection of the right air conditioner.

This software comprises a database of makes and models of air conditioners along with their individual performance specs for cooling capacity and air handler volume. Utilizing the sensible and latent cooling load data from Manual J and adding additional data about the local climate and the overall airtightness of the home, Manual S will crunch the numbers and search the database for the models that best match the revised cooling load after the more individualized information has been factored into the equation.

As the largest full-service HVAC contractor in Montgomery County, Conroe Air is positioned to meet your cooling and heating needs with expert sales and service. Call us when it’s time for an A/C upgrade and make sure you get a unit with the precise air conditioning capacity to meet your needs.