What switch must operate properly before the ignition sequence in a gas furnace with an induced draft fan?

In a gas furnace with an induced draft fan, the switch that must operate properly before initiating the ignition sequence is the differential air pressure switch. This switch is vital for ensuring that adequate airflow is present and that the furnace is safely venting combustion gases.

The differential air pressure switch monitors the pressure difference between the intake air and the flue gases, ensuring that the induced draft fan is functioning correctly and that there is no blockage in the venting system. If the pressure switch does not close (indicating that the airflow is insufficient), the ignition sequence will not commence, thereby preventing potential hazardous conditions, such as backdrafting of combustion gases into the living space.

By requiring the differential air pressure switch to operate correctly, the system enhances safety and ensures that combustion processes occur only when appropriate airflow is established. This is an essential component of the safety interlock system in gas furnaces.