Whole house fan

A whole house fan can be a smart cooling option for the home. Before we had mechanical air conditioning systems, the homes were cooled by opening the windows and allowing breezes into the home. This was especially effective at night and early in the morning.
An attic fan speeds up that natural process. A fan pulls the hot air from inside the home and exhausts it into the attic or to the outside. This allows more air to enter through the open windows and even can create an artificial breeze when there is little or no natural air flow.
The cooling effect can be controlled on a room by room basis by the amount of window opening.
In rooms that are not being used the windows can be left closed and more cooling will take place in the rooms with open windows.
In some parts of the world, this is the only type of cooling system required. In other places these systems are used during more moderate seasons such as Spring and Fall while a mechanical air conditioner is used during the Summer.
The typical way these units are used is that the unit is off and the windows are closed during the daytime. In the evening the unit is turned on and the windows are opened and the home is cooled throughout the night. This can normally save 75% or more of the energy required to run the central air conditioner.
Types of systems

There are several types of whole house fans. The simplest and least expensive is a central fan mounted in the ceiling. This is typically mounted in the hallway of a home. It can be controlled by a pull chain, electrical switch on the wall, a timer switch on the wall, or a remote control. These units normally have a single speed motor that is connected to a fan by a belt. These units therefore require regular maintenance such as replacing the belt and in some cases greasing/oiling bearings.

The next type of fan is mounted higher in the attic instead of on the ceiling which makes them a lot quieter during operation. These systems normally have flexible ducts connected to them and ran to different rooms. This allows air to be drawn more evenly throughout the home. These units have direct drive motors and typically do not require any maintenance. Some of the more advanced models even have a variable speed motor for maximum energy efficiency and comfort. In some cases, these units can have a duct connected to the outlet of the unit and exhaust the hot air directly to the outside.
What size do I need?

A rule of thumb for determining the proper sized whole house fan for the home is to take the square footage of the home and multiply that number by the ceiling height. This will give you the volume of the home in cubic feet. Then you should divide that number by two to find the desired cfm rating of the fan.
Precautions for proper installation of systems

There are some precautions in choosing these units. For units where the air is exhausted into the attic, proper attic ventilation is required to allow the hot air out of the attic. If the attic ventilation is not sufficient, the hot air can be pushed back into the home from the attic which would eliminate the cooling effect.

It is also important that fossil fuel burning appliances have sufficient combustion air. If the whole house fan is operated without a sufficient amount of windows open, it can cause air to be drawn in from the appliance vents. This is called back drafting and can lead to combustion products such as carbon monoxide in the home.

Most manufacturers have a recommended attic ventilation for their unit. It is best to check your application prior to purchasing.

These problems can be avoided with a unit that directly exhausts the air to the outside.