air conditioner cooling capacity

Do You Know Why Air Conditioner Capacity Is Measured in Tons?

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Tonnage in the HVAC profession refers to a specific amount of heat energy the air conditioner must extract from your home every hour to keep it cool and comfortable. It’s measured in BTUs (British Thermal Units) and a “ton” of cooling equals 12,000 BTUs extracted per hour. Your contractor will use both his own expertise as well as sophisticated, industry-standard software to determine your home’s BTU requirements, known as “sizing the house,” then recommend an air conditioner that offers that cooling capacity.

You shouldn’t accept anything less. 

In the past, calculating the required BTU tonnage per hour with precision was often not a high priority, particularly in new construction. In an era when energy costs were much lower than today, builders frequently installed oversized A/C units as a matter of course to standardize the make and model and cover for all variables in a home. This standard procedure was so widespread that contractors who perform air conditioning upgrades of older units nowadays report that more than 50 percent of the existing installed air conditioners are larger than they need to be. Not only do these oversized units consume unnecessary amounts of energy and produce high utility costs, ironically, they also cool less effectively and often have higher rates of component failure and shorter service life, too. Undersized units aren’t a bargain, either. They won’t extract enough BTU tonnage to adequately cool the home. In addition, a unit that’s too small for the thermal characteristics of the house will run extended “On” cycles, using excessive amounts of electricity.

Getting a Handle on the Cooling Load

In the summer, your home carries a heavy burden. It’s called the cooling load and it’s the sum total of a range of factors that influence how much cooling power in BTU tonnage is required to keep your home at a comfortable indoor temperature. This comfort goal is called the “design temperature” and it’s usually around 78 degrees. To maintain that temperature, the contractor has to know the number of BTUs of heat energy that need to be taken out of the house every hour by the air conditioner.

The cooling load is influenced by factors including the:

 
  • Local climate

  • Square footage of the house

  • Orientation of the home and the amount of solar exposure it receives

  • Total area of windows and the type of glass

  • Amount and type of insulation

  • Condition of the ductwork

  • Number of major appliances

  • Amount of lighting and the type of bulbs

  • Number and age of occupants

What Is Cooling Capacity?

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Cooling capacity refers to the rate heat is removed from a space, under standard conditions. These conditions take into account such factors as the volume of the space being cooled and the air temperatures both inside and outside the space. Typically, the cooling capacity of a modern air conditioner is printed on the outside of the unit.

Generally, cooling capacities are rated in British Thermal Units (BTUs) per hour, or in tons. A ton of cooling capacity is the same as 12,000 BTUs per hour, and the term has its origin in the cooling effect of a ton of ice. Often, capacity measurements are given in tonnage for cooling units and BTUs for heating units. Broken down, a BTU is based on the amount of energy it takes to raise the temperature of a pound of water by just one degree Fahrenheit.

Cooling capacity is directly related to the size of the area being cooled. As an illustration, a typical 2,000-square-foot home, depending on the climate it is in, may require an air conditioning system with a cooling capacity of 48,000 BTUs per hour. In contrast, an average 150-square-foot bedroom may be able to get by with a unit rated at 5,000 BTUs per hour.

Air conditioners cool a heated room by transferring the heat to the fluid-filled evaporator coils of the unit. The units condense some of the moisture in the air, too. That makes the atmosphere in the room not only cooler, but less humid and more comfortable as well.

 

Cooling capacity ratings are usually printed directly on the sticker of newer air conditioning units. On older models, the capacity is sometimes coded within the serial numbers. These ratings apply not only to residential and commercial air conditioning systems – including heat pumps that operate in the air conditioning mode – but also to industrial chillers, cooling towers, and other cooling equipment.

The Air-Conditioning, Heating and Refrigeration Institute (AHRI) independently certifies heating, ventilating, air conditioning, and refrigeration (HVACR) equipment and components to ensure these systems meet industry efficiency standards. Generally, the Institute – known as the Air-Conditioning and Refrigeration Institute (ARI) prior to 2009 – works with manufacturers to develop standards and guidelines that result in uniform ratings, including cooling capacity ratings, to give consumers a basis of comparison. An ARI number on a heating or air conditioning unit is a guarantee that its cooling or heating capacity has been properly tested.