What Happens when a Run Capacitor Goes Bad?

While start capacitors are an essential part of HVAC systems, run capacitors are generally the first to fail. These unique electrical components are used in everything from refrigerators and microwaves to furnaces, and a part that technicians are frequently called out to replace. We’re going to take a closer look at run capacitors in this guide, and discuss how you can tell if one is going bad.

The Importance of a Run Capacitor

In our guide to AC capacitors, we discussed the differences between start and run capacitors. Both are used on HVAC motors, but run capacitors “run” continuously as long as the fan or compressor motor are running. 

By comparison, a start capacitor simply gives a motor a boost to get it started before disengaging. Run capacitors are designed to hold a charge which keeps motors running smoothly during operation. 

They also store energy for use while the motor is running. This allows the cap to prepare for the next heating or cooling cycle, and why it’s important to discharge capacitors before attempting to replace one.

What Happens When a Run Capacitor goes Bad?

Given the fact that run capacitors are designed to operate on a continuous basis when one fails, it can have a significant impact on your system. It’s also harder to diagnose as an air conditioner or furnace will continue to function which can hide a faulty run capacitor.

If a run capacitor fails, the motor will still run but can experience spikes in voltage. It can overheat, which will cause parts or the motor itself to break down prematurely. Damage to run capacitors can occur from a variety of things as well.

Voltage spikes are a concern, but as they age, capacitance drops over which weakens the capacitor itself. Needless to say, purchasing a new condensing fan motor and having it installed in an HVAC system is a lot more expensive than replacing a failing capacitor.

Testing Capacitors vs. Calling a Pro

While we feel that replacing a capacitor is something most handy homeowners can tackle, it is important to remember that they can hold a charge for some time. You need to be comfortable working on your HVAC unit before attempting to repair a capacitor and also discharge the run or dual capacitor before attempting to remove it from your system.

Once you locate and remove the capacitor, you may need to test it unless it shows visible signs of damage. Doing that requires a multimeter, but you’ll need to be able to know how to identify the replacement part and install it as well. In other words, think about these factors before proceeding to the next section.

How to Test a Run Capacitor

Testing a start capacitor is fairly straightforward and something most homeowners can diagnose in a matter of minutes. There is only one quick way to check a run capacitor, however, and that’s through a visual inspection.

If you notice that a run capacitor seems “swollen” that’s a bad sign, and some can actually split open in extreme cases. When the capacitor appears to be intact and in good form, the next step is to remove the capacitor from the system for testing, a process you can follow in this guide. If you are unfamiliar with your system, it’s a good idea to take photos of the wiring before removing the start cap.

The best way to test a run capacitor is with a multimeter, which is essentially a variant of an ohmmeter but with more testing capabilities. Select Ohms on the meter while ensuring to set the dial in the high range of at least 1000 Ohms. Ensure the meter is reading “0” and if it isn’t, you can calibrate it by touching the probes together.

With a single run capacitor, take each probe and connect them to a terminal to get a reading. For dual run capacitors, you’ll need to connect one probe to the common terminal, and then put the second terminal on HERM or COMP terminal to test the other.

  • Good Capacitors – If the capacitor is good and has no problems, it will show a low resistance near 0 before bouncing back to infinite resistance.
  • Bad Capacitors – When the probes are properly connected and the meter stays near 0 or doesn’t move at all, it’s time to pick up a replacement run capacitor for your system.

Conclusion

While it can be difficult to diagnose symptoms on a run capacitor before it goes bad, performing routine maintenance is one way to stop problems before they start. That includes the little things that are easy to forget like routinely changing air filters along with seasonal maintenance from a certified HVAC technician.

Run Capacitor FAQ

Q: Do all HVAC systems need a capacitor to work?

A: While all single-phase motors use a start capacitor, some electric motors are designed to work without a run capacitor.

Q: Can you use a start capacitor in place of a run capacitor on an air conditioning unit?

A: No. A start capacitor cannot deal with continuous current. The only alternative is a dual run capacitor which has both a start and run capacitor in one housing.

Q: How long should I expect an original run capacitor to last?

A: Many factors can affect the lifespan of a run capacitor including its age, the quality of the part, and damage to your system. In most cases, you can expect around 20 years of operation before a factory or OEM run capacitor will need to be replaced.

Q: Why do start capacitors have higher ratings than run capacitors if a run capacitor is used continuously?

A:  A start capacitor requires a high amount of energy to produce enough torque to kick start motors in an AC system.

Q: What happens if the wrong size run capacitor is used?

A:  It will cause a variety of problems and cause undue stress on the motor including overheating or a reduction in speed.

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