AC Capacitors Explained: Types, Functions, and When to Replace Them

When an HVAC system refuses to start, repeatedly trips breakers, or runs without cooling, one of the first components a skilled technician checks is the AC capacitor. That’s because capacitors fail more often than compressors, fans, or thermostats, especially during peak summer when high heat and constant cycling put them under maximum stress. A faulty capacitor doesn’t just stop your air conditioner; it can shorten motor life and drive up energy costs.

For professionals and serious DIYers, knowing how to identify, test, and replace an AC capacitor means faster service calls, fewer return visits, and systems restored to peak efficiency in less time. And with high-quality replacements from trusted sources, you can extend system lifespan and avoid costly callbacks.

What Does an AC Capacitor Do?

An AC capacitor stores electrical energy and delivers it in short bursts to start and run motors in your system, including the compressor, blower motor, and fan motor. Without it, motors would struggle to start or fail. In HVAC systems, capacitors either work as start capacitors (providing a strong jolt to get the motor running) or run capacitors (supplying a steady voltage to keep the motor running efficiently).

In some units, a dual capacitor combines both functions in one housing, making replacement simpler.

Types of AC Capacitors

1. Start Capacitors – Deliver high voltage for a quick burst of energy to start the motor.

2. Run Capacitors – Maintain a consistent flow of energy while the motor is running.

3. Dual Capacitors – Combine start and run functions in one unit, saving space and simplifying wiring.

Example:

• ICM Controls 35-460D125 – A round motor starting capacitor with 460–552 MFD and 110/125 VAC. Ideal for garage door openers and gate opener motors, it ensures efficient and reliable motor startups.

Bad AC Capacitor Symptoms

Recognizing early signs of a failing capacitor can prevent major system breakdowns:

• AC unit not turning on at all

• System is running but not cooling

• Frequent short cycling (turning on and off quickly)

• Humming noise from the outdoor unit

• Swollen or leaking capacitor housing

• Burning smell near the condenser unit

If you notice these symptoms, power down your system and inspect visually: bulging, cracks, or leaking oil are clear signs it’s time for replacement.

How to Test an AC Capacitor

Testing involves a multimeter set to capacitance mode.

1. Turn off the power to the unit.

2. Discharge the capacitor safely using an insulated screwdriver.

3. Disconnect the capacitor terminals.

4. Attach the multimeter probes and read the measurement.

If the value is more than 10% lower than the rated microfarads (MFD), replacement is recommended. For most homeowners, this should be handled by an HVAC technician due to the risk of electric shock.

How to Replace an AC Capacitor

1. Disconnect all power to the unit.

2. Locate the capacitor (usually in the outdoor condenser unit).

3. Discharge it safely.

4. Take a photo of the wiring before removal.

5. Remove the old capacitor and install the new one, matching microfarad and voltage ratings exactly.

6. Reconnect wires according to your photo or diagram.

Use quality replacements to ensure long service life, like the Carrier P291-7014R dual capacitor, which powers both the compressor and fan motor at 440 VAC, ensuring system stability.

AC Capacitor Wiring Diagram

Wiring varies depending on whether you have a single, dual, start, or run capacitor.

• The C (Common) terminal connects to the power source.

• The FAN terminal connects to the fan motor.

• The HERM terminal connects to the compressor.

Always verify with your system’s wiring diagram to ensure correct placement. Incorrect wiring can damage motors or the new capacitor.

How Much Does It Cost to Replace an AC Capacitor

Professional replacement typically costs $100-$400, depending on capacitor type, brand, and labor rates in your area. DIY replacement can be far cheaper if you source the correct part and install it safely.

Can I Replace the AC Capacitor Myself?

If you have electrical knowledge, the correct tools, and follow strict safety steps, you can replace a capacitor yourself. However, for most people, the safer choice is hiring a professional. Capacitors store high voltage even after power is cut, so mishandling them can result in serious injury.

Where to Buy an AC Capacitor

For reliable, in-stock replacements, choose a supplier with a wide range of HVAC components and fast delivery. Voomi Supply offers everything from start capacitors to dual run capacitors for residential, commercial, and specialty applications. Examples include:

• Copeland 914-0053-04 – 189-227 MFD, 165V, designed for stable HVAC efficiency and optimal power factor correction.

• Copeland 914-0006-00 – 108-130 MFD, 330V start kit, ideal for HVACR service parts needing strong startup torque.

• Hayward HPX51100010001 – 80 + 7.5 uF, 440VAC dual run kit for in-ground pool heaters, boosting performance and longevity.

• Carrier P291-7014R – 440VAC dual capacitor for compressor and fan operation, ensuring balanced HVAC performance.

With Voomi Supply, you get expert-selected parts and the benefit of one of the largest online inventories for HVAC, plumbing, and electrical components.

Final Word on AC Capacitors and Reliable Replacements

In HVAC service, a faulty AC capacitor is one of the fastest and most cost-effective repairs you can make to restore system function. We’ve covered what capacitors do, the main types, common failure symptoms, testing methods, replacement steps, and wiring essentials.

We stand out with fast shipping, reliable sourcing, and the expertise to ensure you get exactly what your system needs. Explore our selection of Capacitors today and keep your systems running at peak efficiency all year round.