Whole house fans will keep you and your home cool at a fraction of the cost of air conditioning. Fans had been supplanted by air conditioners for the past several decades but are now re-surging in popularity. Why? First, they sip electricity while air conditioners guzzle power. Second, the latest generation of fans sport self-sealing insulated shutters that close when the fan’s not operating. (Unless you jury-rig your own cold-weather cap, older units behave just like an open window in the winter.) This auto-insulating feature prevents vast amounts of heat from escaping through the fan when it’s not in use.