2. Air Conditioning Terminology

2. Air Conditioning Terminology

Here are some common terms you will come across when comparing and determining the best choice for air conditioners:


The refrigerant is a substance that circulates through the air conditioner, alternately absorbing, transporting and releasing heat.

A coil is a system of tubing loops through which refrigerant flows and where heat transfer takes place. The tubing may have fins to increase the surface area available for heat exchange.

The evaporator is a coil that allows the refrigerant to absorb heat from its surroundings, causing the refrigerant to boil and become a low-temperature vapour.

The compressor squeezes the molecules of the refrigerant gas together, increasing the pressure and temperature of the refrigerant.

The condenser is a coil that allows the refrigerant gas to give off heat to its surroundings and become a liquid.

The expansion device releases the pressure created by the compressor. This causes the temperature to drop and the refrigerant to become a low-temperature vapour/liquid mixture.

The plenum is an air compartment that forms part of the system for distributing warmed or cooled air through the house. It is generally a large compartment immediately above the heat exchanger.


A Btu/h or British thermal unit per hour, is a measure of the heat output of a heating system. One Btu is the amount of heat energy given off by a typical birthday candle. If this heat energy were released over the course of one hour, it would be the equivalent of 1 Btu/h.

A kW or kilowatt, is equal to 1000 watts. This is the amount of power required by ten 100-watt light bulbs. A ton is a measure of cooling capacity. It is equivalent to 3.5 kW or 12 000 Btu/h.

The capacity of an air conditioner is a measure of the maximum rate at which it can remove heat from the conditioned space. Capacity is expressed in British thermal units per hour or tons and is determined under a specific set of best conditions.

The cooling load also stated in British thermal units per hour, is the maximum amount of heat that builds up in a space without a cooling system operating. It is calculated to determine the capacity of air conditioner required.

Heat gain is a term applied to various components of the heat load, such as appliance heat gain and solar heat gain. All of the heat gain components are summed to calculate the cooling load.

Over sizing is the practice of selecting an air conditioner with a cooling capacity greater than the cooling load.

Under sizing is the practice of selecting an air conditioner with a cooling capacity smaller than the cooling load.

The energy efficiency ratio (EER) is a measure of how much cooling effect is provided by the air conditioner for each unit of electrical energy that it consumes under steady-state operation. It is determined by dividing the cooling output of the unit, in British thermal units per hour, by the electrical power input, in watts, at a specific temperature. The higher the EER, the more efficient the unit.

The seasonal energy efficiency ratio (SEER) is a measurement of the cooling efficiency of the air conditioner over the entire cooling season. It is determined by dividing the total cooling provided over the cooling season, in British thermal units per hour, by the total energy used by the air conditioner during that time, in watt/hours. The SEER is based on a climate with an average summer temperature of 28°C.

The bel (B) is a unit of sound measurement equivalent to 10 dB (decibels). One bel is the threshold of human audibility. The sound level in a busy typing and accounting office would be approximately 6.5 B.

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Source: Natural Resources Canada (NRCan) - Office of Energy Efficiency