A whole-house fan can do an effective job of circulating house air in areas that have cool, dry nights. They are not quite as effective as air conditioners, but they do a good job. Whole-house fans are cheaper than air conditioners, and use much less electricity. The fan, usually located in the attic over a central hallway, is designed to draw hot house air into the attic, where it escapes through attic vents. At least one door or window has to be left open when the fan is operating to pull the air through the house. Many fans come equipped with timers, speed controls, and thermostats. During the winter, the fan opening must be covered with an insulated panel and sealed airtight to prevent moist, warm air from escaping into the attic. For this reason, whole-house fans are rarely used outside of the southern United States.