HVAC 403A Practice Exam

Interlocks are safety devices that actively prevent HVAC equipment from operating when conditions aren’t safe. A door interlock stops power to the unit or its moving parts if a service panel is open, protecting anyone who might reach into the equipment. Flame safety switches verify that a burner flame is actually present and will shut off fuel flow if the flame goes out, preventing fuel buildup. High and low pressure switches monitor refrigerant or combustion pressures and will shut the system down if pressures drift into dangerous ranges, protecting both the equipment and occupants. Fan interlocks ensure the blower or induced-draft fan is running when needed, guaranteeing proper airflow and preventing overheating or unsafe operation. These interlocks work together to enforce safe startup and shutdown, so the system cannot operate under unsafe conditions. Alarms alone don’t stop the machine; interlocks provide the automatic protective action. That comprehensive set of interlocks—door, flame safety, pressure, and fan—best safeguards people and equipment.