Fans may be centrally located in an attic or other suitable space (such as a hallway), and arranged to move air proportionately from several rooms. A local unit may be installed in a window to provide comfort cooling for one room only when desired. Attic fans are usually propeller types and should be selected for low velocities to prevent excessive noise. The fans should have sufficient capacity to provide at least 30 air changes per hour.
To decrease the noise associated with air-exchange equipment, the following rules should be observed:
? The equipment should be properly located to prevent noise from affecting the living area.
? The fans should be of the proper size and capacity to obtain reasonable operating speed.
? Equipment should be mounted on rubber or other resilient material to assist in preventing transmission of noise to the building.
If it is unavoidable to locate the attic air-exchange equipment above the bedrooms, it is essential that every precaution be taken to reduce the equipment noise to the lowest possible level. Since high-speed AC motors are usually quieter than low-speed ones, it is often preferable to use a high-speed motor connected to the fan by means of an endless V-belt, if the floor space available permits such an arrangement.