Heating and air conditioning systems are a sizable investment. Knowing these terms will help you to make educated decisions.

AFUE (Annual Fuel Utilization Efficiency) - the standard measurement of efficiency for gas and oil-fired furnaces. Given in percentages, this number tells you how much of your fuel is used to heat your home and how much fuel is simply wasted. The higher the AFUE rating, the greater the efficiency.

Air flow volume - the amount of air the system circulates through your home, expressed in cubic feet per minute (cfm). Proper air flow depends on the outdoor unit, the indoor unit, the ductwork and even whether the filters are clean.

Bels - the rating system by which sound is measured. The bels scale is a logarithmic scale like the Richter scale...even a small difference in rating can mean a significant difference in actual sound. The higher the number, the greater the noise level. A diesel truck 10 feet away is rated at 9.2 bels. The sound of a refrigerator running is rated at 7.0 bels.

BTU (British Thermal Unit) - the amount of energy needed to change the temperature of one pound of water by one degree Fahrenheit. The heat extracted from your home by an air conditioner is measured in BTUs.

Coil - a network of tubes filled with refrigerant. A coil located inside the home takes heat and moisture out of the air as liquid refrigerant evaporates. Sometimes called the cooling or evaporator coil.

Compressor - the pump that circulates refrigerant from the indoor coil to the outdoor condensing unit and back again. It's called the heart of the system, an apt analogy.

Condenser - a network of tubes located in your unit outside the home, that removes the heat from the hot, gaseous refrigerant to make it liquid again. A fan discharges the heat to the outdoors. Sometimes called the condenser coil.

HSPF (Heating Seasonal Performance Factor) - the efficiency measurement used to gauge the efficiency of the heating mode of heat pumps. Again, the higher the number, the greater the efficiency. Today's models are required to have a 6.8 HSPF.

Refrigerant - the real name for what you might call "freon," it's fluid that evaporates at relatively low temperature, sucking heat and humidity out of the air as it does. Learn more about environmentally friendly refrigerants.

SEER - Seasonal Energy Efficiency Rating. The higher a unit's SEER, the more efficient it uses electricity. All new central systems sold in the United States must be at least 10 SEER.

Split system - a central air conditioning or heat pump system in which some components are located inside the home and others outside. This is the most common type of home system.

Ton - the unit of measure for an air conditioning system's capacity. One ton of air conditioning removes 12,000 BTUs of heat energy per hour from a home. Residential air conditioners usually range from one to five tons.