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Annual Fuel Utilisation Efficiency. Indicated as a percentage, your furnace's AFUE tells you how much energy is being converted to heat. For example, an AFUE of 90 means that 90% of the fuel is being used to warm your home, while the other 10% escapes as exhaust with the combustion gases.

Air Conditioning, Heating and Refrigeration Institute. An industry association that establishes the norms for measuring and certifying product performance.

British Thermal Unit. Used for both heating and cooling, BTU is a measure of the heat given off when fuel is combusted. For cooling, it's a measure of heat extracted from your home. (One BTU is approximately equal to the heat given off by a wooden kitchen match.)

British Thermal Units per Hour. A British Thermal Unit (BTU) is the unit of heat required to raise 1 pound of water by 1 degree Fahrenheit.

Cubic Feet per Minute. A measurement of airflow that indicates how many cubic feet of air pass by a stationary point in one minute. The higher the number, the more air is being forced through the system.

The ability of a heating or cooling system to heat or cool a given amount of space. For heating, this is usually expressed in BTUs. For cooling, it is usually given in tons.

A colourless, odourless, highly poisonous gas produced when carbon-based fuels, such as natural gas, burn without sufficient air nearby.

A system by which air is treated in one location and then distributed between rooms using fans and ductwork.

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