Your HVAC system relies on various components to operate effectively, and one crucial element in this intricate system is the HVAC capacitor. The capacitor plays a pivotal role in facilitating the smooth startup and continuous running of your HVAC equipment. Regularly checking the condition of your capacitor during your routine maintenance is vital to ensure optimal performance and longevity of your HVAC system. Read on to learn more about the importance of an HVAC capacitor and how a professional tests it.

The Role of AC Capacitors

In an AC system, capacitors are responsible for storing and releasing electrical energy, which is essential for starting and running motors, fans, and compressors within the system. There are three types of AC capacitors.

Start Capacitor

Start capacitors have a higher capacitance value than run capacitors and are designed to provide a temporary boost of power during the motor’s startup phase. The start capacitor stores electrical energy and delivers it to the motor, enabling it to overcome the initial inertia and reach its operating speed. Once the motor is running, the start capacitor is disconnected from the circuit.

Run Capacitor

On the other hand, run capacitors are continuously connected to the motor circuit throughout its operation. These capacitors maintain the motor’s efficiency and power factor by providing a steady electrical energy supply. Run capacitors assist in enhancing the motor’s torque, improving its performance, and reducing power consumption. By correcting the power factor, run capacitors ensure that the motor operates at maximum efficiency while minimizing reactive power losses.

Dual Capacitors

Some HVAC systems use dual capacitors that combine the functions of both start and run capacitors into a single component. These capacitors simplify the wiring and operation of the system by eliminating the need for separate start-and-run capacitors.

Reasons Why a Capacitor Might Be Faulty

Age-Related Wear and Tear

Like many other mechanical components, HVAC capacitors are not immune to the effects of age. Over time, the dielectric materials within the capacitor can degrade. This degradation often results from continuous charge-discharge cycles during the HVAC operation, potentially leading to a reduced capacitance value below the manufacturer’s specifications. When the capacitor’s performance dips below a certain threshold, the HVAC system may struggle to start up or operate efficiently, indicating that the capacitor is no longer functioning as intended.

Overheating Due to Excessive Load

Capacitors are designed to handle a specific power load. When an AC system is pushed beyond its capacity or runs too hard for too long — often during extremely hot weather — the capacitor may overheat. This can cause the dielectric materials within the capacitor to break down and even the outer casing to deform or burst.


Electrical overvoltage usually happens due to a power surge or a malfunction in the electrical supply system. When the voltage exceeds the capacitor’s rated capacity, it can cause the capacitor to blow out. Voltage irregularities can result from grid issues but can also be caused by problems within your home’s electrical wiring system.

Improper Installation

Even a perfectly good capacitor can become faulty if it is not installed correctly. An improperly secured capacitor may experience mechanical stress due to vibrations, leading to early failure. In addition, a capacitor installed in an inappropriate environment — for example, one with excessive moisture, high temperature, or corrosive gases — may also deteriorate prematurely. Another common issue is incorrect wiring during the installation process. If the capacitor is connected to incorrect terminals, it can damage the capacitor and cause failure.

Poor Quality or Defective Capacitor

Sometimes, the fault lies in the capacitor itself. Lower-quality capacitors may be less robust and reliable than those from reputable manufacturers and may fail earlier. Inferior materials or substandard manufacturing processes can compromise the capacitor’s performance and lifespan. Furthermore, even high-quality capacitors can be defective from the start due to production errors. When installed in an HVAC system, such capacitors may function initially but fail prematurely.

Effects of a Faulty AC Capacitor

As mentioned earlier, a capacitor helps start and run the AC compressor and fan motors. However, over time, capacitors can degrade or fail due to various factors, resulting in several noticeable effects.

1. Reduced Starting Ability

When a capacitor goes bad, it may lose its ability to deliver the required starting voltage. As a result, the compressor may struggle to start. This can lead to long startup times, frequent system cycling, or even complete system failure.

2. Motor Overheating

Capacitors also aid in maintaining the efficiency of AC fan motors. A faulty or weakened capacitor may not provide the necessary electrical boost to keep the fan motor running at its optimal speed. As a result, the motor may strain and overheat. Continuous overheating can cause irreversible damage to the motor windings and eventually lead to motor failure.

3. Increased Energy Consumption

When the capacitor is not functioning properly, the compressor will work overtime and may draw more electrical current to compensate for the reduced voltage. This increased energy consumption results in higher electricity bills without delivering the desired cooling performance.

4. System Cycling Issues

Capacitor problems can disrupt the proper cycling of the AC system. For instance, a bad capacitor may cause the compressor to short-cycle, repeatedly turning on and off rapidly. This not only puts unnecessary strain on the system but also leads to inefficient cooling and increased wear and tear on various components.

5. Component Damage

A faulty capacitor can also affect other AC system components. For example, when the fan motor does not receive the necessary electrical boost, it may experience excessive stress and premature wear, potentially leading to motor failure. Moreover, the increased electrical stress on the compressor due to a weak capacitor can cause compressor overheating, resulting in compressor damage or failure.

How To Check Your HVAC Capacitor

It’s crucial to recognize the risks involved when handling HVAC components, including capacitors. Capacitors store an electrical charge, which can be hazardous if not discharged properly. Therefore, it’s highly advised that only trained HVAC technicians with the appropriate knowledge and experience should work on capacitors. Replacing a capacitor should be done during an AC repair service, without professional expertise it can result in electrical shocks, injury, or even damage to the HVAC system.

Checking your HVAC capacitor involves several steps. The technician will start the testing process by inspecting the capacitor visually for damage, followed by discharging stored electrical energy for safety. The capacitor’s wires are then disconnected. Next, the technician will use a multimeter to measure the capacitor’s ability to store an electric charge, comparing the reading to the capacitor’s label. If the measurement falls outside the label’s range, it indicates a faulty capacitor that needs replacement. In case of a defective capacitor, the technician will replace it with one that matches the original’s specifications.

Contact HVAC Professionals Near You Today

By addressing capacitor problems promptly, you can prevent further damage, improve energy efficiency, and maintain a comfortable indoor environment. If you live in Boise and require professional assistance with your HVAC system or suspect capacitor issues, contact Express Plumbing Heating & Air today. You can also reach out to us for plumbing, sewer, and air quality services.

Brad Jordan

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