Just What Is An HVAC System, Anyway?

Heating, ventilating, and air conditioning (HVAC) equipment can be a bit of a mystery to homeowners. You may be aware that your home has the ability to remain climate controlled at the touch of a button, but you find yourself wanting to understand more about the machinery that works so hard for you. Read on for a comprehensive definition of the components of your indoor climate control machinery.


In hot summer months, the air conditioner is the star of the climate control show. Your AC unit draws air from outside and cools and dehumidifies it as the air passes over a cold metal coil. This coil is itself an air-to-liquid heat exchanger, meaning the warm air absorbs cold from the coil, and vice-versa. Rows of tubes pass cold liquid through the coil, and flat surfaces connected to the tubes create a large cooling surface area, increasing the amount of air that can be cooled at one time. More cold surface area equals more cool air. The type of cold liquid used depends on the system you have, but can be a liquid refrigerant or chilled water with added freeze protection. Regardless of the liquid medium used, the liquid is delivered to the cooling coil at a cold temperature.

As this liquid passes through the cooling coil two things happen to the air that passes over the coil’s surface: the air’s temperature is lowered (cooled) and moisture in the air is removed (dehumidified). The cooling ability of your air conditioner depends on its efficiency at cooling and dehumidifying air. It’s important to note that factors such as outdoor temperature can affect the cooling capacity of some machinery. When cooling is inversely proportional to outdoor temperature, a machine’s ability to cool your indoor air is reduced when outdoor temperature increase. Similarly, indoor temperatures and humidity affect your machinery in much the same way. Most manufacturers of AC equipment provide a user guide that shows the efficiency change of the machinery when indoor and outdoor temperatures and humidity changes.


Central air conditioning units are usually paired with a gas or oil burning furnace to provide heat to your home through the same set of air ducts. Alternatively, there are central HVAC units called heat pumps, which combine both the heating and cooling functions. If you heat your home with electricity, a heat pump system can be more efficient in moderate climates, providing up to three times the heating per electrical energy it consumes.

Air conditioners and heat pumps use different components to functionally cool or heat your home’s air, but the basic principles of operation are the same. The heat pump transfers heat from the evaporator coil to air and circulates the now-heated air through your home. A heat pump’s heat coil actually extracts heat from outdoor air (yes, even cold winter air carries heat that can be extracted by your system). Heat pumps are often installed with a back-up furnace to help heat your home during those times when only a small amount of heat can be extracted from outdoor air.

Types of Systems:

Typical homes employ a “split system” HVAC, with the air condensing unit placed outside the home, and the coil inside. The other available type is called a “packaged” air conditioner, and it combines the condensing unit and the coil into one outdoor unit. The best unit for you depends entirely on your home’s location and construction.