Furnace blower capacitor definition and function

You may have heard the term “capacitor” in regards to your furnace, which is called furnace blower capacitor. What exactly does the capacitor in your furnace do, and what are the symptoms of capacitor problems?


The most important job of a furnace is to move heated air from the furnace out into the home. Every furnace has a blower motor connected to a large “squirrel cage” fan that performs this job. Every blower motor needs an extra “jolt” of power to lose its inertia and get it turning when it first starts up.


The start capacitor’s job is to provide that extra burst of energy to spin the blower motor and get the fan moving in your furnace. It does that by storing power until it’s needed.

What is a furnace blower capacitor?

A furnace blower capacitor helps regulate power input to the motors located in your furnace. These motors can include blower motors and, in some cases, inducer motors.

Visually, a furnace blower capacitor is a small component that you may see mounted on the blower fan compartment in some furnaces.

A furnace typically has multiple capacitors that regulate power for different components. We can break capacitors down into two types: start capacitors and run capacitors.

Furnace capacitors fall into the category of run capacitors. Run capacitors manage power to a component while it’s actively running.


What does a furnace blower capacitor do?

Capacitors are a part of the fan motor in your furnace. They are generally divided into two types, both of which provide electricity to the furnace apparatus at key times. The first kind of capacitor is a start capacitor, which helps the motor start up when you first turn on your heater.

There is also a run capacitor: used in furnaces, HVAC systems and AC units of all varieties. They provide a steady supply of electricity to the fan motors (used to help the blowers move hot air through your system).

Obviously, both types of capacitor need to be running smoothly in order for your furnace to function. Otherwise, your furnace won’t be able to heat your home. Electrical problems can usually be hunted down and corrected by a trained professional armed with a multimeter or similar piece of equipment.

We’ve already discussed a furnace capacitor’s basic function: to regulate the flow of electricity to components like the blower and inducer motors. But why is regulating power input necessary?

The furnace houses the system’s blower motor, which powers the fan that helps move conditioned air throughout the home year-round. Every time the system heats or cools the home, the furnace’s blower motor kicks on.

You may even choose to run the fan even when the HVAC system isn’t actively heating or cooling your home. This can help improve circulation and air quality.

Regardless of its fuel source, HVAC equipment requires high-voltage electricity to heat or cool house. This electricity powers components like the blower motor.

Furnace capacitors regulate power for the blower motor during startup and as it continues to run.

Without a capacitor, power input can fluctuate. With a capacitor, the input is steady. This helps maintain consistent performance and helps keep the motor from straining itself.


Common signs a furnace blower capacitor isnt functioning properly

When something is amiss with the capacitor, your furnace will often speak in its own way. The following are signs it might give you to make you alert.

1. No power. A faulty capacitor will prevent the fan from turning and make the system halt. If your system is unresponsive, first check your circuit panel to see if the breaker might have tripped. If it did, switch it back on, increase the thermostat and see if the furnace turns on. If it doesnt, there might be a capacitor problem.

2. Inconsistent blower motor. A failing capacitor can cause the blower to overheat or operate slowly or inconsistently. This means the capacitor isnt holding the proper charge to start the blower. Be alert to any short or odd heating cycles.

3. Clicking sound. If the furnace makes a clicking sound when trying to start but the blower doesnt move air or turn, it could be a failed capacitor.

4. the capacitor are frayed or corroded.

5. Humming sound. With a properly functioning furnace, the blower motor might hum a bit as it works up to full speed. If you hear a hum but the blower isnt starting, something is holding back the fan or the blower. It could be an electrical problem or an obstruction caused by debris or a rodent.

6. System shutdown. Todays furnaces include a thermal overload switch for a furnace that gets too hot. A failing capacitor can cause the whole unit to overheat, which may trip the safety switch and shut down the system.

Why do furnace blower capacitors fail?

A furnace capacitor typically only fails for a few reasons:

1.The capacitor reaches the end of its lifespan

2.The blower motor is under strain

As with any mechanical component, furnace capacitors have an average lifespan. Capacitors typically last 10-15 years. In some cases, a capacitor just reaches the end of its lifespan.

But sometimes, the capacitor is just a symptom of a bigger issue. For example, if the blower motor is under strain, it’ll make the capacitor work harder as well. 

So what happens when a furnace capacitor fails? In many cases, you can get the furnace back up and running by just replacing the capacitor. Other times, you may need to replace your blower motor to address larger issues with your system.

HVAC service technician should conduct a full inspection of the furnace. This will help determine whether the issue goes beyond the capacitor.

Professional Furnace Capacitor Replacement

HVAC service technician should be able to replace the faulty furnace capacitor quickly and safely.

Once the new capacitor is installed, the technician should check that the capacitor is functioning correctly. This involves taking a reading and observing the blower motor as it starts up and runs.