In a gas furnace, what condition does the pressure switch verify before allowing operation?

The pressure switch in a gas furnace plays a crucial role in ensuring that the system operates safely and effectively. Its primary function is to verify that the blower is operating properly before allowing the furnace to ignite and produce heat.

When the system calls for heat, the pressure switch checks for the presence of adequate airflow, which is generated by the blower. If the blower is not functioning correctly or if there is insufficient airflow, the pressure switch will prevent the ignition sequence from starting. This is a vital safety feature because it helps to ensure that combustion gases are vented properly, reducing the risk of carbon monoxide buildup or other hazardous conditions within the furnace.

In this context, while other options are related to the overall performance of the furnace, they do not directly pertain to the specific function of the pressure switch. For example, temperature levels and gas pressure are monitored but are not the primary focus of the pressure switch's operational check. Ventilation quality is important for safe operation but is not the direct responsibility of the pressure switch. Therefore, confirming blower operation is the correct condition that the pressure switch verifies before allowing the furnace to operate.