If an oil burner motor starts only when the shaft is turned by hand, what is likely the issue?

When an oil burner motor starts only when the shaft is turned by hand, it suggests that there is a problem with the motor's ability to initiate from a standstill. In this scenario, the most likely issue relates to the start switch and/or capacitor.

The start switch is crucial for enabling the initial current necessary to power up the motor. If the switch is defective or malfunctioning, it may not close properly, preventing the motor from receiving the power it needs when prompted. Additionally, the capacitor plays a vital role in providing extra torque at startup. If the capacitor is faulty or has degraded over time, it might not provide enough energy to start the motor, leading to the need for manual assistance to get it running.

In contrast, a fuel blockage would likely prevent the burner from functioning altogether, not just from starting the motor. A thermostat malfunction would result in operational issues related to temperature regulation rather than affecting the motor's ability to start. Finally, oil viscosity typically pertains to temperature and fuel delivery, and while it can affect burner performance, it wouldn't cause a motor to start only with manual intervention of the shaft. Therefore, the start switch and/or capacitor are the most relevant elements in this particular situation.