How To Know if an HVAC Capacitor Is Bad

Capacitors are an essential part of many HVAC systems. All central air conditioners and heat pumps have capacitors that help power the compressor motor and the unit’s fan motor. The blower inside your home that circulates air through the HVAC system may also have its own capacitor or rely on the capacitor in the outdoor unit. However, some blower motors are small enough that they don’t require a capacitor. If your home has an oil furnace, electric furnace, or boiler, this unit most likely also has a capacitor.

Most of these HVAC units have both a start and run capacitor. If the start capacitor malfunctions or wears out, it usually prevents the HVAC unit or the blower from turning on. A bad run capacitor won’t prevent the unit from starting, but it can lead to the unit shutting down prematurely, overheating, or making more noise than usual. Determining if the problem is related to a bad start capacitor is usually easy, but a bad run capacitor is often more difficult to spot. Here is an overview of capacitors’ role in an HVAC system and how to know if it’s bad.

What Do HVAC Start and Run Capacitors Do?

The compressor and internal fan motors in an air conditioner or heat pump require a ton of energy to power on. This is also true for some blower motors and most electric furnaces, oil furnaces, and blowers. In most cases, the energy required for these motors to start up is more than the electrical circuit can safely provide without getting overloaded and tripping the circuit breaker.

The easiest way to think about a capacitor is like a battery or a jump starter. Instead of trying to draw the extra power needed for the motors to start from your electrical system, the start capacitor provides this extra surge of electricity.

The start capacitor essentially stores a strong electrical charge, which it then releases to assist the motor in starting up. Once the unit is running, the run capacitor activates and provides the additional voltage needed for the motor to operate smoothly without overheating or overloading the electrical circuit.

How to Spot the Signs of a Bad Capacitor

Over time, a start or run capacitor can wear out to where it can no longer store enough of a charge for the motors to power up. If you ever find that your outdoor HVAC unit or your blower fan won’t run, you can check to see if the problem is related to a bad start capacitor fairly easily. However, the job does require two people as one person will need to turn the system on at the thermostat while the other person listens to the blower or HVAC unit when it tries to start up.

The first thing to do is shut your HVAC system off at the thermostat. If your home has both central heating and air conditioning, you may then want to switch the thermostat over to the opposite mode from what you’re currently using, i.e., from AC to heat or vice versa. This is because the reason that the blower or the outdoor unit isn’t starting could also be related to the thermostat itself. If the system works in one mode but not the other, you can usually at least rule out the thermostat as the cause of the problem.

After having your helper shut the system off, you will want to stand next to the blower or the HVAC unit to hear if the unit makes any noises when it tries to start. If you don’t hear anything and the unit doesn’t try to start, the problem is due to something other than the capacitor.

You will hear the capacitor clicking every few seconds if it is bad. This noise is the capacitor trying to release the energy needed for the unit’s motors to start. If the start capacitor fails, you will typically hear the motor make a loud humming noise. This sound occurs when the motor tries to start on its own without the extra energy supplied by the capacitor. If you do hear these sounds, your system won’t be able to run until you hire an HVAC technician to replace the capacitor.

In most cases, you won’t be able to easily tell if your unit’s run capacitor is bad. Many of the issues a bad run capacitor can create could also be related to various other problems with the unit. That being said, if the unit vibrates excessively, continually shuts off without completing a full cycle, or has issues with overheating, you will want to have the run capacitor and the rest of the unit inspected.

How Does a Technician Replace an HVAC Capacitor?

The good news is that replacing a failed start capacitor is one of the easier HVAC repair tasks. A qualified technician can usually complete the entire job in just a few minutes.

On most HVAC units, the capacitor is a large metal cylinder that resembles an oversized battery. Sometimes a capacitor fails because it can no longer store a charge. But other times, the issue is that its electrical connections are damaged or corroded to where it can’t release the charge. This is why the capacitor always needs to be discharged before touching it.

After ensuring that the unit’s electricity is shut off at the main electrical panel, the technician will need to discharge the capacitor. This is done by laying a piece of metal across the capacitor to connect both terminals. This will cause it to instantly release any energy it has stored. Once the capacitor is discharged, the technician can test it to see if it needs replacing.

If the capacitor is bad, the technician will disconnect the wire from each of the two posts. The bad capacitor can then be removed and the new one mounted. The technician will then reconnect the wires to the posts before finally turning the power back on and checking to make sure the unit will now start properly. If the unit still doesn’t turn on, the technician will need to shut the system back off and continue troubleshooting to determine what other issues the unit has.

Capacitors can vary in terms of voltage load, capacitance, and tolerance. It is essential that the right capacitor is used to ensure the unit works correctly and prevent possible damage. Not installing it correctly may also cause serious damage to the unit, so it is always best to leave this job to a trained professional.

Expert HVAC Services

If you’re dealing with a bad capacitor or any other heating or cooling issue, you can trust the IT Landes Home Service Team for help. We repair most types and brands of heating and cooling equipment, including furnaces, air conditioners, heat pumps, ductless mini-splits, boilers, and smart thermostats. We also offer duct cleaning and plumbing services. We can install carbon monoxide detectors, whole-home surge protectors, air purification systems, and whole-home humidifiers. Call us today if you need an HVAC capacitor replaced or to schedule any other home service in Harleysville, PA or the surrounding areas.