Perimeter induction units – usually located under windows – blend primary air from the air-processing unit with secondary air from within the room. The high-velocity processed air delivery is induced into the unit through restrictive nozzles. This creates a negative pressure in its wake, drawing in the room secondary air for mixing and discharge. A damper regulates the volume of room air passing through a thermostatically controlled heating coil.
These coils may be used with chilled water as cooling coils in the summer months. If heating only is used, the system is known as the ‘two-pipe induction system’ With the additional two pipes for cooling water, the system is known as the ‘four-pipe change-over induction system’. The latter system gives excellent control of the air temperature in various zones but is very capital intensive, and therefore expensive to install.