City of Chicago Boiler Practice Exam 2026 – Complete Study Resource

An interlock is a safety/control connection in boiler systems that prevents the burner from operating unless certain prerequisites are met. It enforces the correct sequence and state of the system so that a unsafe condition cannot occur—for example, ensuring there is adequate water, that the ignition system is ready, and that fuel and air valves are in safe positions before firing.

This is why it’s the best answer: the whole purpose of an interlock is to automatically check key safety conditions and block operation if any are missing or out of sequence. That prevents dangerous situations like firing a boiler with too little water (which can cause overheating or damage) or starting fuel flow when the ignition isn’t ready, thereby reducing the risk of explosions or equipment damage. It also helps ensure other safety steps are completed in order, protecting both the equipment and the operator.

For context, interlocks differ from a manual start switch, which simply initiates firing without confirming safety prerequisites. They also aren’t the steam isolation valves, which control flow for other reasons, nor devices that measure oxygen content, which monitor combustion but don’t themselves enforce safe sequencing.