A HVAC capacitor, also commonly known as an AC capacitor, plays a critical role in helping your system run efficiently. These components provide the necessary start-up power to motors and ensure smooth operation. Over time, these capacitors can degrade or fail, leading to poor performance or system shutdowns. Knowing how to properly test and replace AC capacitors is vital for technicians. This article will cover the uses of AC capacitors, how to test them, and tips for replacing them.
What is an air conditioner capacitor and what does it do?
Air conditioners have a capacitor that plays a critical role in their operations. This capacitor is commonly located in the outdoor unit of the system, near the compressor or fan motor. It assists in starting and running the motors of the system, particularly the compressor and fan motors. The HVAC capacitor provides an initial surge of power to help start the motors and maintain steady voltage during operation. This ensures efficient performance and reduces strain on the electrical components.
During startup, the capacitor provides an energy boost to help the motor overcome inertia and begin turning. This surge of power is crucial for getting the motor moving quickly and efficiently. Once the motor reaches operating speed, the capacitor helps maintain steady voltage, ensuring consistent performance and reducing electrical strain on the system.

Some systems use two separate capacitors: one for the start operation and another for the run operation. The start capacitor provides a quick energy boost, while the run capacitor helps maintain efficient motor operation. Most modern air conditioning systems utilize a dual capacitor, which combines both start and run functions into a single unit. This design simplifies the system, reduces space requirements, and enhances overall reliability. Figure 1 shows a circuit of an air conditioning system consisting of a start and run capacitor.
Causes of HVAC capacitor failure
Generally, air conditioner capacitors can last for over 20 years. However, certain factors can accelerate their failure. Excessive heat is one of the major contributors to early failure. This is usually caused by poor ventilation or a malfunctioning motor. Excessive heat leads to the breakdown of the dielectric material inside, resulting in leakage or loss of capacitance.
Electrical stress also plays a role in capacitor failure. Frequent voltage fluctuations or exposure to high voltage can wear out the capacitor quickly. Over time, the electrolytic fluid inside the capacitor may dry out, reducing its effectiveness. This leads to diminished performance and eventual failure.
Regular maintenance is key to preventing AC capacitor failure. Ensure proper ventilation to avoid overheating, as excessive heat can accelerate capacitor degradation. Clean or replace filters regularly to maintain optimal airflow. It’s also important to inspect the capacitor for signs of wear, such as bulging or leakage.
Using a surge protector can help stabilize the electrical system and prevent damage from power surges. Furthermore, avoid overloading the system by ensuring it operates within its designed capacity. This will help extend the life of the capacitor and other components.
Symptoms of a bad air conditioner capacitor
One common visual sign of a failing AC capacitor is bulging or swelling on the capacitor’s casing. This occurs when the internal pressure builds up due to overheating or internal damage. Additionally, if the capacitor is leaking any oily substance or residue around its edges, it’s a clear sign of failure. These physical changes indicate that the capacitor is no longer functioning properly and should be replaced.
Another symptom of a failing capacitor is reduced system performance. If the air conditioner struggles to start or makes a humming noise without the motor running, it could indicate a capacitor issue. The system may also run less efficiently, with the compressor or fan motor turning on intermittently or not at all. These symptoms suggest that the capacitor isn’t supplying the necessary voltage to the motors, leading to operational problems.
Testing a HVAC capacitor
Testing or replacing a capacitor should be done by a qualified technician to ensure proper handling and system safety. The first step in checking an AC capacitor is a physical inspection. Look for signs of damage such as bulging, leakage, or discoloration on the capacitor’s casing. If any of these physical signs are present, the capacitor should be replaced.
To test the capacitor electrically, technicians usually use a multimeter or capacitance tester. If the capacitor fails electrical tests or shows significantly reduced capacitance, it should be replaced. A failing capacitor can lead to poor system performance or motor issues, so addressing it promptly is essential.
Replacing a bad air conditioner capacitor
When replacing a bad AC capacitor, the technician should first ensure they choose a capacitor with the correct voltage and capacitance rating. Using a capacitor with incorrect specifications can cause system malfunctions or even further damage to the components. It’s also important to check the physical size of the replacement to ensure it fits properly in the system.
The technician should also consider the type of capacitor required, whether it’s a start capacitor, a run capacitor, or a dual capacitor. Additionally, the technician should inspect the overall condition of the system to ensure there are no other underlying issues, such as electrical problems or poor ventilation, that could have contributed to the capacitor failure. Proper installation and handling are essential to ensure the new capacitor functions effectively and safely.