Gas Heat ESCO Practice Exam

A gas furnace has an induced draft fan. Which switch must operate properly before the ignition sequence starts?

1. Differential air pressure switch

2. End switch

3. High limit switch

4. Roll-out switch

The correct answer is: Differential air pressure switch

The correct answer is the differential air pressure switch. This switch plays a crucial role in ensuring that there is adequate airflow through the furnace before the ignition process begins. It monitors the pressure difference created by the induced draft fan, confirming that the fan is operating correctly and that there is sufficient venting of combustion gases. If the differential air pressure switch does not function properly, the system will not allow the ignition sequence to start, thus helping to prevent potentially dangerous conditions such as backdrafts or improper combustion. In contrast, other switches like the end switch, high limit switch, and roll-out switch have different specific functions within the furnace's safety and operational mechanisms. The end switch is typically associated with the operation of a fan or burner, while the high limit switch protects against overheating by shutting off the burner if the temperature exceeds a predetermined limit. The roll-out switch detects whether flames are escaping from the combustion chamber, adding another layer of safety. Each of these switches is important for overall safe operation, but they do not initiate the ignition sequence directly like the differential air pressure switch.