How to Test Start Capacitors on Furnace Blowers

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Many electrical devices use some type of capacitor, including a furnace blower. A capacitor is a device that stores an electrical charge. A start capacitor is typically found on an air conditioning unit or heat pump where extra torque is needed to start the compressor. A furnace usually uses a run capacitor to maintain a relatively constant voltage supply to the blower motor. If you're concerned that your furnace capacitor isn't operating properly, you can test it by following a few simple steps.

Things You'll Need

  • Protective eyewear
  • Screwdrivers
  • Voltmeter

Inspect the capacitor visually for signs of damage. Bulging, or fluid leaking out of the unit, are common signs that the capacitor should be replaced.

Turn off the power to the furnace and put on protective eyewear. Completely disconnect the furnace from the power source.

Remove the wires that are connected to the terminals. These are the only wires connected to the capacitor, and the terminals are the only terminals. They're located on the top of the unit.

Discharge the capacitor. Discharging a capacitor simply means releasing any remaining current held in the device. You can use your voltmeter or a watt resistor to connect to the terminals. This allows any stored current a path out of the capacitor.

Check the capacitor rating on the side of the unit. It will have two numbers. The first is the amount of charge the unit can store, expressed in microfarads -- or mfd -- and the second is the supply voltage. In addition to the rating, the capacitor should have a plus or minus sign and a percentage which indicates the range within which the unit can fluctuate and still be considered to be operating properly.

Set your voltmeter to the capacitor setting and connect the meter to the terminals on the capacitor. Check the reading you get against the rating on the capacitor. If your rating is within the allowable range of the capacitor's rating, then your capacitor is still good. A reading outside the allowable range means you need to replace your capacitor.

Tips & Warnings

  • Many people use a screwdriver to discharge a capacitor. This can be dangerous and may also damage the screwdriver.

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