Air Pressure Switches for the most common boilers. An air pressure switch is commonly used in older boilers to detect the correct running of the fan. When the fan runs it creates a pressure difference that make the airpressure switch activate and close an electrical contact. This is monitored by the pcb which then allows the boiler to fire up. It is a vital safety part of the boiler to ensure the correct venting of toxic gases prodoced by the boiler.