Air Switch
An air switch is a device designed to control a two-speed fan in response to air temperature changes in the furnace plenum (see Figure 6-3). When the temperature in the plenum rises to the set- point, the air switch will change fan operation from low to high. In other words, the air will be removed from the plenum at a higher rate of speed in order to lower the temperature of the air in the fur- nace. Because this device is frequently used in conjunction with a fan controller or a combination fan and limit controller, it is some- times referred to as an upper fan control.
Air switches are generally available with fixed temperature set- tings (e.g., 125F° , 135F° , 165F° , or 200F) or with an adjustable temperature range (125F°
An air switch can also be used to shut off the burner and turn on the fan when the temperature between the filter and heat exchanger rises to the setpoint. In this manner, it functions as a limit control (see Secondary High-Limit Switch in this chapter).