Why AC Capacitors Fail & How to Prevent It

bad air conditioner capacitor

AC Capacitors are critical components in air conditioning systems, serving to start and run the compressor and fan motors efficiently. However, they can fail over time, leading to AC malfunction.

Causes of AC Capacitor Failure

  • Aging: Over time, capacitors naturally degrade and lose their ability to store and release electrical energy effectively.
  • Overuse: Excessive usage or running the AC unit for extended periods can strain capacitors, causing them to wear out prematurely.
  • Electrical Surges: Power surges or voltage fluctuations in the electrical supply can damage capacitors, leading to failure.
  • Poor Maintenance: Lack of regular AC maintenance, such as cleaning coils and filters, can contribute to overheating and stress on capacitors.

Preventive Measures

  • Regular Maintenance: Schedule annual AC maintenance by a professional technician. They can inspect and test capacitors for signs of wear and tear, replacing them if necessary.
  • Keep the Unit Clean: Ensure that the AC unit’s coils and filters are clean and free from debris to reduce strain on capacitors.
  • Invest in Surge Protection: Install surge protectors to shield your AC unit from electrical surges and voltage fluctuations.
  • Use a Programmable Thermostat: Opt for a programmable thermostat to control your AC system efficiently, reducing its workload and minimizing wear on capacitors.
  • Replace Capacitors Proactively: Consider replacing capacitors every five to seven years as a preventive measure, even if they show no signs of failure.
  • Monitor Performance: Pay attention to any unusual sounds, weak airflow, or reduced cooling capacity, as these can be signs of capacitor issues. Promptly address these symptoms to prevent further damage.
  • Professional Installation: Ensure that capacitors are installed correctly by hiring a licensed HVAC technician. Proper installation can extend their lifespan.

How to Tell if Your AC Capacitor is Blown

Some of the first signs that you have blown AC capacitor is when your air conditioner won’t start at all, or when it takes too long to kick in. Sometimes even when it starts, it will turn on and off randomly (short-cycling) because the capacitor just can’t supply the motor circuit with the needed current.

An increase in energy bills is another sign, since the air conditioner system will be working harder than usual. These signs could however also mean that something is wrong with another part of the AC system and not just the capacitor.

To be really sure, walk out to where your outside unit is located. If you hear a humming noise from the unit, it is a sign that the motor is struggling to come on, usually due to a bad capacitor.

Can you also notice a burning smell emanating from the unit? When the motor is overworked/strains, it is bound to overheat and hence the reason for the burning smell. That could be a good sign that your capacitor is out.

Peep inside the outside unit from the top grille. Is the fan spinning or not? If the fan is not spinning, you most likely have a bad capacitor.

You can also go ahead and visually inspect the capacitor. To do that, you will first need to turn off power to the AC at the breaker box and also turn off the thermostat.

Strat by removing the unit’s side panel which is secured using screws. Once the side panel is out, you should see the capacitor (usually cylindrical in shape and shiny exterior) with wires connected to its terminals at the top.

The first thing you will need to check (without touching the capacitor) if it is swollen. Can using a bulge on the capacitor, usually at the top or bottom?

swollen AC capacitor
swollen AC capacitor

That is a good sign that the capacitor is bad. Capacitors have a dielectric fluid which vaporizes (especially due to overheating) which causes the case to mushroom/swell.

Is the dielectric fluid leaking from the capacitor? That is also a sign of a bad capacitor. The fluid prevents capacitors from overheating but when temperatures inside the capacitor are just so high the case will give in and the fluid will leak out.

A capacitor can however look good visually but still be bad and the only way to find out is by testing. I have written an article on how to test an AC capacitor. Read it here.

Why AC Capacitors Fail

Let us now look at why AC capacitors fail in more details. The following are the main causes of failure in air conditioner capacitors:

1. Overheating

Overheating is the main cause of failed capacitors. When a capacitor keeps overheating (especially when its temperatures hits 150 degrees Fahrenheit and above), its ability to hold charge (capacitance) is reduced.

A capacitor can overheat for several reasons:

  • Direct exposure to the sun – during the very hot months of summer, temperatures outside can get to crazy levels. Don’t also forget that the capacitor is located in the condenser unit which cools the house meaning that it is subjected to some of that heat too. Extra hot days also require the AC to worker harder than usual and all this factors can contribute to the failure of a capacitor
  • Dirty AC filter – When you fail to change your AC filter, the flow of air from the return ducts to the evaporator coil will be reduced, forcing the unit to work harder than it should. The capacitor will overheat and fail if that keeps on happening or happens for long.
  • Dirty AC coils – AC coils need to be cleaned regularly to allow heat exchange between the refrigerant and the surrounding air. If the coils are dirty, the system overworks and overheats in a bid to keep the house cool.
  • Low refrigerant levels – If your AC system is leaking out the refrigerant, it will run for longer periods of time that normal. The extra straining may cause the capacitor to overheat and fail.

2. Power Surges and Lightning Strikes

A power surge can also fry you capacitor. This happens when too much electricity is suddenly supplied to the unit either due to problems with the power supply or as a result of a lightning during thunderstorms.

3. Capacitor with the Wrong Rating

If you look at a capacitor, you will see 2 ratings:

  • Capacitance (in microfarads)
  • Voltage

You need to use a capacitor with the correct voltage and capacitance rating. Often when homeowners replace capacitors themselves, they may use an undersized capacitor which means it will fail out sooner than later as it will struggle to keep up.

You can replace an AC capacitor with another one of a higher voltage rating (some HVAC techs even recommend it) without any problems but a capacitor with a lower voltage rating will always fail prematurely.

4. Capacitor’s Age

Capacitors, just like other electrical components will fail as a result of old age. They definitely don’t last forever. Wear and tear as a result of internal heat will ultimately cause them to fail.

In the old days, capacitors could last for as long as 30 years or even the lifetime of the air conditioner unit. Most HVAC technicians have come across old capacitors which only failed because their terminals were just too rusty (Actually your capacitor can fail due to loose or rusty connections).

Unfortunately capacitors today don’t last that long. You will be lucky if yours lasts for more than 10 years. Some will only last for a year or two.

How to Prevent You AC Capacitor from Failing

The following are some of the ways to prevent AC capacitor failure:

1. Use an HVAC Surge Protector

The HVAC surge protector is installed in the electric circuit before the AC unit. It diffuses the excess voltage before it gets to the unit, in the event of a power surge.

2. Change Air Filter Regularly

Air filters are installed on the AC’s return air vents. They clean the air before it gets to the inside unit so that dust, pollen, lint and other particles are removed and not circulated back to the house.

It is recommended that you change your AC filter at least after 3 months. Luckily, replacing air filters is easy as I have outline in this post.

Clean filters keep the air flowing to the evaporator coil and the system doesn’t need to overwork, and capacitors will therefore not overheat.

2. Clean AC Coils

Dirty evaporator coils have the same effect as clogged air filters. They restrict airflow from return ducts forcing the whole system to worker harder and overheat. Cleaning the coils can prevent that from happening.

Also, cleaning the condenser coil outside prevents buildup of heat inside the unit which can also cause the capacitor to overheat.

3. Place the Outside Unit in a Shaded Area

To prevent the capacitor from heating due to direct exposure to sunlight, having the outside unit in a shaded area definitely helps, especially if you live in some of the hottest states.

4. Increase your Thermostat Setting

This may not be the most comfortable thing to do but when it is hottest outside is the time your AC works the hardest. It becomes a bigger challenge when you have set your thermostat too low.

Increasing your thermostat setting and making sure that your house is not losing heat due to bad insulation will help to take some of the load from the AC unit.

5. Use the Correct Capacitor

To prevent your capacitor from failing, replace it with a high-quality one and importantly with same rating (microfarads and voltage) as the old one.

6. Plan Annual AC Maintenance

Annual AC maintenance especially just before summer by a professional HVAC technician will help your air conditioner run smoothly during the entire summer months. They will inspect the ductwork, refrigerant levels, filters, coils etc. meaning that your capacitor will have a lower chance of failing.

And basically that is why air conditioners fail as well as steps to take to prevent them from failing. Just to warn you that capacitors carry a high charge and can shock you when mishandled and if you need to replace yours it is best to have it done by a licensed technician.