Central air conditioning runs the air throughout the entire house with the actual unit located outside. A thermostat controls the temperatures of the house. Older thermostats need to be changed manually to the desired temperature. Newer thermostats are programmable. Temperature settings can be programmed to change at certain times of the day. This can help lower electric bills by allowing the house to be warmer when the house is empty. Running central air conditioning uses a lot of electricity but it is generally more energy efficient than window air conditioning. Many older homes use either window units or have no air conditioning at all.