What must operate properly for the ignition sequence to start in a gas furnace with an induced draft fan?

For the ignition sequence to start in a gas furnace equipped with an induced draft fan, the differential air pressure is crucial. This mechanism ensures that the induced draft fan is functioning properly by creating a negative pressure in the combustion chamber, which is necessary for the effective expulsion of exhaust gases. If the differential air pressure is not maintained, the furnace cannot operate safely, as this could lead to improper combustion and the potential buildup of harmful gases.

The induced draft fan plays a significant role in establishing this pressure difference, allowing for the proper air-to-gas mixture needed for ignition. If the system detects insufficient pressure, it will prevent the ignition sequence from starting, ensuring that safety measures are upheld. Thus, the correct operation of differential air pressure is essential for the ignition sequence to initiate successfully.