The Remeha air pressure switch, which is also referred to as the APS, is used as a safety device to ensure the fan is in full working order. The fan prevents a build-up of dangerous gases within the appliance by diverting them out of the property. The air pressure switch is monitored by the printed circuit board, ensuring that it is fully operational. If the air pressure switch is not working correctly, the printed circuit board will stop the appliance from firing up.