Unit ventilator is an operable air-inlet damper, which furnishes outdoor air to an interior space; may be provided with a filter and heating and/or cooling coils. It consists of a heating coil, fan assembly, dampers, filter and controls contained in a metal cabinet. Unit ventilators are usually located on the outside wall of classrooms, although they are sometimes suspended at or above the ceiling level. Outdoor air is brought directly into the cabinet via a grille located on the outside wall of the classroom. The unit ventilator is designed to mix room air with outside air, heat the air if necessary, and deliver it to the classroom through a grille located in the top of the unit ventilators. The proportion of outside air is controlled by the position of the fresh air damper. This damper can be adjusted to provide as much or as little fresh air as desired. A typical unit ventilator will circulate a total of 1000 or 1250 cfm of air, of which a minimum of 400 cfm is outdoor air. With a unit ventilator, 100% of the airflow can be outside air when it is needed for free cooling.