Are multiple room air conditioners causing your electric bills to skyrocket? Or, perhaps worse, is your house virtually unliveable because of the heat and/or humidity? Maybe it's time to consider central air conditioning.

Central air conditioning, also called "central air" in the United States or "air-con" in the UK, is a system that cools more than one room in a house or building (in most cases, the entire building). A conventional air conditioning system blows cooled and/or dehumidified air through ductwork to deliver it throughout the interior room spaces. In most cases, it employs the air handler (blower) and ductwork of a forced air furnace for this cool-air delivery. This, of course, is different than a room or window air conditioner that pokes through the wall or window and is meant to cool only a single room.

Air conditioners use refrigeration principles to cool the air. As discussed under How an Air Conditioner Works, a condenser, compressor, and evaporator work together to do this job. A typical split system air conditioner has a compressor and condenser located outdoors and an evaporator unit mounted on the air handler or furnace. A package A/C system combines all of these components in one outdoor unit.

A central air unit draws room air through return-air ductwork, cools, dehumidifies, and filters it, and then blows it back to the rooms through air supply ductwork. The A/C unit is controlled by a thermostat, which turns the unit off and on as needed. In most cases, the same thermostat doubles for both the air conditioning and heating system (you need a thermostat that's meant to do this).

Be aware that retrofitting an existing home with central air conditioning may not be an easy job. Ductwork must run from the air handler to the rooms and, to do this, must usually be routed through the attic or under the floor. If your home is suited for this type of installation, room air conditioners may be your best answer.

For information on room or portable air conditioners, see Air Conditioners: Room.