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Furnace capacitor in Newark Ohio

What is a Furnace Capacitor and How to Tell If It’s Bad?

Expertise from a Professional HVAC Contractor in Newark Ohio

One of the most important parts of a gas forced air furnace is the blower motor. This motor is responsible for moving fresh air through the furnace, over burners to heat it (or over the evaporator coil to cool it), then distribute it out to the home via a series of ducts. The blower motor is connected to a fan and fan cage (also called a “squirrel cage”) either directly or via a belt, and requires a “jolt” of extra power to get it turning to start blowing.

The job of the start capacitor is to store that extra power. When the thermostat and control board sends a request to start the motor turning, the capacitor sends that burst of power to get it spinning. A capacitor is a sealed unit, typically shaped like a small can, and can only be replaced, not repaired.

Signs of a Bad Furnace Capacitor

There are some telltale signs that the capacitor is at fault when your furnace or AC won’t start. Look for these signs and symptoms.

Symptom: Clicking Noise

Fortunately, a capacitor can show symptoms of failure before, or during a failure event. The first symptom is a clicking noise as the furnace is trying to start up, as the relays and switches attempt to send power to the blower motor. If you hear this noise, but the blower doesn’t start, switch the thermostat to off and call a professional HVAC contractor to check the furnace.

Symptom: Acrid Burning Smell Coming from the Furnace

Often, the furnace will give off a burning smell in the vents, indicating electrical issues in the furnace. If you get any sense of something burning in the area of the furnace, switch the furnace off at the thermostat. You may also want to switch off the breaker at the home’s electrical breaker box to ensure further damage doesn’t occur to the heating system.

Note: A musty, dusty smell could be from an accumulation of dust on the burner and in the ductwork, but that type of smell usually dissipates within a short time of the furnace running again after a long summer off.

Symptom: Humming Sound from the Blower

When the start capacitor can’t send that burst of power, or can’t send enough power to the blower motor to start turning, often you’ll hear a “humming” sound coming from the blower motor. Sometimes, the motor will try to turn but seems like it’s binding or stuck. You can check this by setting the temperature on your thermostat a bit higher, then stand by the furnace to listen as it tries to start. You may hear a click as a relay sends power to the capacitor or motor, then the humming sound. A bad capacitor can also cause the circuit breaker or other safety devices within the system to trip, to prevent damage to the blower motor or other sensitive electronic parts.

Symptom: No Power At All

When the capacitor goes bad in a furnace, it may also cause circuit breakers to trip, or may not provide power at all to the blower motor to begin its operation because of an electrical fault in the capacitor. Either way, the blower motor isn’t getting power. Double-check the circuit breaker in your home’s electrical panel, and be sure the heating system switch is turned on (there may be a light-switch style switch near your furnace that controls power at the furnace). Turn up the thermostat, and wait for it to start. If nothing happens, it’s time to call for a professional HVAC contractor to check out your heating system.

Symptom: Erratic Motor Operation

The capacitor acts like a battery, holding a charge to get the blower motor spinning. All electricity going to the motor passes through the capacitor, even after it’s running. A bad capacitor might cause your blower motor to run slower than normal, run hot, and short cycle. If you notice your furnace stopping and starting erratically, it could very well be the capacitor.

Older furnace models may use separate start and run capacitors. The start capacitor is responsible for the extra burst of electricity to get the motor running, when the run capacitor takes over to regulate the power running to the motor. Newer furnace models have split capacitor motors that handle both functions, making it easier for our service technicians to diagnose and repair.

Don’t let a bad capacitor in your furnace ruin your day! If it appears you’re having one of the above issues with your furnace, call Dor-Mar Heating & Air Conditioning. We are available 24 hours a day, 7 days a week, every day of the year. We can arrive at your doorstep same day, if necessary.

Dor-Mar to the Rescue!

Schedule an appointment for HVAC system installation, troubleshooting or maintenance with us today by using our online contact form, or call one of our seven neighborhood offices listed below. We pride ourselves on our customer-focused service, and our reviews show it.

NEWARK  740.345.6639 • COLUMBUS 614.238.6689 • DUBLIN 614.545.8939 • REYNOLDSBURG 614.365.1579 • WESTERVILLE 614.381.1540 • GROVE CITY 614.595.3098 • ZANESVILLE 740.454.2420

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