A furnace pressure switch is designed to control the power that comes from the source to the furnace. When the pressure switch is closed, electrical currents come flowing into the furnace. When switch is open, nothing can get through. A furnace pressure switch is controlled automatically by pressure from the inducer. The Inducer fan connects to your pressure switch, usually through a visible rubber tube. The inducer fan, or the draft inducer, is turned on and creates suction. That suction is what activates the furnace pressure switch. Once the switch is activated, it allows the current to flow to the igniter, which heats your furnace.