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Office of Energy Efficiency Links


Personal: Residential


Cooling and Ventilation Equipment –
Central Air Conditioners


Central air conditioners are designed to cool the entire house. The large compressor and outdoor coil are located outdoors and are connected by refrigerant lines to an indoor coil mounted in the furnace. The same duct system is used for both heating and cooling air distribution.

Installed Central Air Conditioner

Installed central air conditioner

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How Does a Central Air Conditioner Work?

A central air conditioner uses energy to take heat away. The most common type uses a compressor cycle (like a refrigerator) to transfer heat from the house to the outdoors. Using a special fluid called a refrigerant, heat is absorbed and released when the refrigerant changes back and forth between a liquid and gas state. As it changes from liquid to gas, it absorbs heat; in changing back to a liquid from a gas, it releases heat.

The compressor cycle passes liquid refrigerant through an expansion device, changing the liquid to a low-pressure liquid/gas mixture. In the indoor coil or evaporator, the remaining liquid absorbs heat from household air and becomes a low-temperature gas.

The Operation of a Central Air Conditioner

How does a central air conditioner work

The low-temperature gas is compressed by a compressor that reduces its volume and increases its temperature, causing it to become a high-pressure, high-temperature vapour. This vapour is sent to the outdoor coil or condenser where its heat is transferred to the outdoor air, causing the refrigerant to condense into a liquid. The liquid returns to the expansion device and the cycle is repeated.

Household air is cooled and dehumidified as it passes over the indoor coil. The moisture removed from the air, when it contacts the indoor coil, is collected in a pan at the bottom of the coil and sent to a house drain.

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Operating Cost

The operating cost of a central air conditioner is influenced by a number of factors:

  • how much you use the air conditioner
  • its efficiency
  • the amount of insulation and glazing in your home
  • the extent to which the coil and filters allow air to flow freely
  • the frequency and duration of door and window openings when the system is operating
  • proper refrigerant charge
  • activities in your home
  • use of other equipment and appliances that increase the load on the air conditioner
  • local climate
  • electricity costs
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Life expectancy and warranties

The life expectancy of a central air conditioner is 15 years or longer. When the air conditioner starts giving more problems than seem cost-effective to fix – particularly when major components, such as a compressor, require replacement – it may be time to replace it. New units offer greater efficiency and lower operating costs; it may be more cost-effective in the long run to replace rather than repair.

The warranty on your equipment will vary according to the manufacturer. Air conditioner warranties range from one year for complete parts and labour to five years for the compressor. Some manufacturers are now offering 10-year warranties on compressors. Ask the contractor or manufacturer for an explanation if you do not fully understand the terms of a warranty.

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