The air pressure switch is designed to detect the operation of the fan and to confirm it is spinning. It is connected to the fan via heat resistant pressure tubes directly on the fan venturies. This enables the printed circuit board to detect if the fan is turning because air flow from the fan closes the air pressure switch which then sends a signal to the pcb confirming the fan is running. At this time gas is sent to the burner and the ignition procedure can start.
If the fan was stop running or jam there would be too little air flow throughout this air pressure switch which would create the air pressure switch to open. A signal is then routed into the printed circuit board instructing that the Ideal boiler to stop sending gas to the burner and to turn the boiler off. This Ideal air pressure change is a fundamental safety device ensuring that the secure functioning of the boiler.