What Is the Function of the Capacitor in an Oil Furnace?

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The capacitor is an important component of a household oil-burning furnace, and is critical to ensure the furnace will start when the thermostat calls for heat. Most residential oil burners are the gun type where an electric motor drives a pump and blower that pressurizes and atomizes fuel oil for ignition by a high-voltage spark. The motor keeps the burner firing until the thermostat tells it to stop. The capacitor starts that motor.

How Capacitor Works

A capacitor stores electricity on a pair of closely-spaced plate conductors. When electricity is fed into the capacitor, each plate builds up an electric charge of equal intensity but opposite polarity. When a capacitor is discharged by connecting the plate conductors, all the stored electricity rushes out. In an oil burner's capacitor motor, the capacitor provides a momentary jolt of high voltage electricity to jump-start the motor, then cuts out of the motor circuit for recharging. In some oil-burner motors, the capacitor not only starts the motor but also serves to smooth out voltage sags and spikes while the motor is running, ensuring the motor receives unvarying voltage.

Capacitor Failure

Many times, problems with an oil burner that won’t run can be traced to a failed motor capacitor. In most oil burners, the capacitor is a cylindrical or oval component on or near the motor and hooked to the motor with wires. It will be marked with its storage capacity in microfarads (mfd) and its voltage output, usually 370 or 440 vac. Capacitors generally are accessible and removable for testing. A new capacitor is relatively inexpensive and relatively easy for the experienced handyman to replace. First, check the capacitor for obvious signs of trouble such as a bulging case or an oily substance leaking from it. Those signs indicate a dead capacitor that requires replacement.

Capacitor Testing

If the capacitor looks good, it still may have failed internally. Capacitor testing requires that you use a digital multimeter or analog volt/ohm meter. Remove the capacitor from the burner motor. Before testing, discharge the capacitor by simultaneously touching the wire leads with a metal screwdriver that has an insulated handle. Be careful when doing this. A fully-charged capacitor can give you a nasty, painful electric shock if you are careless. You also risk frying your testing meter if you forget to discharge the capacitor before testing.

Use Your Meter

With a digital multimeter, set it for capacitance testing. Attach the multimeter’s testing leads to the capacitor’s wires and take a reading. If the meter reading in microfarads is within 6 percent of the mfd value printed on the capacitor, the component is OK and your problem is somewhere else. If the meter reading is more than 6 percent below the stated value, the capacitor is bad and should be replaced. With an analog volt/ohm meter, you will test the capacitor’s resistance. Set the meter to its highest resistance test setting. Touch the meter leads to the wires. The needle should go from zero to the maximum. If the needle doesn’t move or only moves part way, the capacitor is bad.

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