Most controllers are not built to directly switch large electrical loads such as fan or feed
auger motors. Instead, they typically have a control relay which is activated by the
controller. The control relay is connected to coil of a so-called power relay which
switches the larger load of the actual equipment on or off. At least one per ventilation
stage is needed. Several companies manufacture relay boxes consisting of several
power relays pre-wired with auto/on/off switches. The on position provides a manual
override of the controller, the off position is used to disconnect, and the auto position is
used during normal operation