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What’s a Furnace Capacitor and Why Do They Fail?

Furnace capacitors are small components that have the power to affect your comfort. Let’s break down what furnace capacitors are, how they work, why they fail, and how to maintain them.

What’s a Furnace Capacitor and Why Do They Fail?

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Jerad Kingery


May 31st, 2022

Furnace capacitors are small components that rarely come up unless there’s an issue.

If you’re searching for information on a furnace capacitor, it’s likely your HVAC service partner recently diagnosed your furnace with a bad or faulty capacitor. 

Like other industries, HVAC comes with its own jargon. If you’re not familiar with the technical terms, it can be challenging to understand your service technician’s professional diagnosis.

At Fire & Ice, we have almost two decades of experience with HVAC. Over those decades, we’ve recognized that providing high-quality HVAC service and repair goes beyond even the technical skills of our technicians. 

During service calls, we take the time to help our customers understand any issues with their HVAC system. We’ve created this article to answer questions about furnace capacitors. 

In this article, we’ll cover what a furnace capacitor is and how it works, signs and reasons why furnace capacitors fail, and how to get the most out of your furnace capacitor. We’ll also weigh DIY and professional capacitor replacement.

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What Is a Furnace Capacitor?

A furnace 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 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. 

Since your HVAC service technician may also discuss capacitors in terms of “start” and “run,” let’s take a moment to break these down as well.

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

Start capacitors are responsible for regulating power to a component that requires substantial electricity to turn on. Start capacitors are most commonly found in air conditioners or heat pumps. In furnaces, run capacitors also double as start capacitors.

The number and type of capacitors in a furnace can vary depending on the manufacturer and model.

What Does a Furnace Capacitor Do?

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?

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

You may even choose to run the fan even when your 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 your home. This electricity powers components like your blower motor.

Furnace capacitors regulate power for your 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. 

2 Common Signs a Furnace Capacitor Isn’t Functioning Properly

When it comes to furnace capacitors, you may not always notice that there’s an issue right away. 

A furnace capacitor itself typically doesn’t show signs of failure. Instead, you may notice a few indirect signs that a capacitor isn’t functioning properly.

Two of the most common signs that your furnace capacitor isn’t functioning properly or has failed:

  • Your blower motor is noisier than usual – If the blower motor is noisier than usual when the fan kicks on, the capacitor could be weakening. The most common noise is a whirring or humming noise that sounds like the fan is ramping up.

  • Your blower motor that doesn’t turn on at all – If you notice that your fan isn’t running at all, this can be a sign of a bad capacitor.

These signs are most common in permanent-split capacitor blower motors, a type of blower motor that was phased out across the industry in 2019. 

However, if your system has a variable-speed blower motor, the chances of it failing are relatively low.

Why Do Furnace 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 your 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 your 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.

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

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Can You DIY Furnace Capacitor Replacement?

The short answer: For safety reasons, we would never recommend that a homeowner replace any capacitor themselves.

As we’ve discussed, capacitors regulate high-voltage electricity. Working with high-voltage components without the proper experience or tools can have painful and even fatal consequences.

DIY furnace capacitor replacement also risks damaging your furnace if the capacitor is installed incorrectly. In some cases, DIY work can void your furnace’s warranty, which means that you’d be stuck with any repair or replacement costs if something goes wrong.

Although there are countless online guides on DIY repairs for furnace capacitors, we highly recommend leaving this to an HVAC professional.

Professional Furnace Capacitor Replacement

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

Once the new capacitor is installed, your 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.

Blower motor and fan replacement can be more costly and labor-intensive. Although it’s not always possible, preventing strain on the blower motor can help avoid some of these costs. 

Your service technician should take the time to walk you through your options. Regardless of the situation, your technician should never pressure you.

How to Maintain a Furnace Capacitor

The best way to keep your furnace capacitor and, by extension, blower motor running properly is to regularly maintain your furnace

Regular maintenance helps keep your furnace at the top of its game. And your HVAC service provider can help track any changes in your furnace’s performance over time.

We typically recommend two tune-ups each year for your HVAC system: one for your furnace and one for your air conditioner or heat pump.

During a furnace tune-up, your service technician should check the condition of your blower motor. This also includes taking a reading of the furnace capacitor.

However, a major part of capacitor and blower motor maintenance is regularly changing your furnace’s filter. 

As we mentioned, blower motors help circulate conditioned air throughout your home. But as the filter collects dust and debris, the blower motor has to work harder to circulate the air. 

Dirty filters can affect your comfort and your wallet. Lack of airflow caused by dirty filters also means your entire HVAC system has to work harder to heat or cool your home. And the harder your system works, the more you’ll likely spend on energy costs.

Your filter’s manufacturer should provide more information on how frequently to change your filter. But in some cases, you may need to change your filter more often.

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For more information, check out our article on how, when and why to change your furnace filter.

Get the Most Out of Your Furnace

A furnace capacitor may be one small part of your furnace. But when it comes to machines, even small components can affect the broader system’s performance.

It’s important to take the time to understand how components influence your system’s performance—and you’re doing just that! As you continue to maintain your HVAC system, it’s important to have an HVAC partner you can trust.

At Fire & Ice, we believe in helping our customers learn more about their HVAC system. Because when you understand your system, you gain better control over your own comfort.

If you live in the greater Columbus, Ohio, area, we’d love to help maintain your furnace! Use the zip code lookup below to check if you’re in our service area.

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