Whole house fans
Whole house fans are specifically designed to cool your home at night using the outside air. They are best suited for dry climates, where the temperature drops down at night. When should you turn on a whole-house fan? The answer depends on your climate and your comfort range. The outdoor temperature should certainly be below 80°F — or, better yet, below 70°F. The fan is mounted in the floor of the attic, sitting above a central hallway in the home. To operate, windows in the house are opened and the fan is turned on. Outside air is then drawn in through the windows and up through the attic where the hot inside air is vented out of the home.
Whole-house fans are intended to be used in homes that are not air-conditioned. It makes no sense to introduce lots of (potentially humid) exterior air into a house at night if you intend to turn on an air conditioner the next day. They are very energy efficient; drawing only 10-15% of the energy drawn by a central air conditioning system. They are also efficient from a time standpoint. Generally speaking, they can cool down a house in less than an hour. Once the house has cooled, the fan can be turned off and the windows closed. The windows remain closed until the next evening to keep the cool air from escaping.
Whole house fans take all the hot air from the house and move it up into the attic and out through the attic vents. Because of this, they require significant ventilation in the attic. Often their ventilation requirements exceed the minimum required by building codes. Although people had complained that whole-house fans are noisy, much quieter models are available today. In the winter months, the fan opening needs to be sealed. Whole house fans are now also available with built-in motorized doors as a part of the unit. These doors open and close without having to crawl into the attic to seal the unit.
Ventilation fans, or powered attic ventilators, are designed to cool the attic, pushing out the hot attic air and rushing in a flow of cool outside air. Powered attic ventilators are housed in the slope of the roof or in the gable wall of the attic. The intention of the ventilation fan is to save energy and the roof. Energy would be saved by reducing the burden of the air conditioner in the home. The roof would be saved by decreasing the amount of heat moving from the attic through the roof. The lifetime of a roof with asphalt shingles is significantly reduced by excess heat moving from the attic below through the shingled roof above.
Even though the hope with these fans is that cool outside air is pulled into the attic through the attic vents, it is common that the fan instead pulls interior, air-conditioned air up from the home into the attic through small cracks in the ceiling. This negative pressure in your house, in turn, pulls in hot exterior air to replace the exhausted cooled air. Your air conditioner is then running longer as it is working to cool more warm air. Attic fans are intended to cool hot attics by drawing in cooler outside air from attic vents (soffit and gable) and pushing hot air to the outside. However, if your attic has blocked soffit vents and is not well-sealed from the rest of the house, attic fans will suck cool conditioned air up out of the house and into the attic. This will use more energy and make your air conditioner work harder, which will increase your summer utility bill. You don’t want your unfinished attic cooled by your air conditioner.