If an oil furnace will not ignite, which issue might be the cause?

The reason why a defective thermostat might be the cause of an oil furnace not igniting lies in the function that the thermostat serves in the heating system. The thermostat is responsible for detecting the room temperature and signaling the oil furnace to turn on when heating is required. If the thermostat is malfunctioning, it can fail to send the necessary signal to the furnace, thereby preventing it from igniting and starting the heating process.

Understanding the role of the thermostat is crucial for troubleshooting issues with an oil furnace. If the thermostat settings are correct but it is not functioning properly, the system will not operate as expected, leading to the scenario where the furnace does not ignite due to a lack of activation by the thermostat.

While other factors such as power supply, air in the oil line, or issues within the inside plenum can certainly affect the operation of an oil furnace, they do not directly relate to the ignition process as fundamentally as the thermostat's role does. A functional thermostat is essential for the ignition sequence to begin, making it a primary suspect in diagnosing ignition failures.