Why AC Capacitors Fail & How to Prevent It

Air conditioners need capacitors to start and run the motors. Capacitors give the AC motors the extra torque needed to start and run optimally and efficiently.

Capacitors stores charge higher than the one supplied by the power supply lines (usually 370-440 volts). That allows the capacitors to release the powerful electricity jolt needed to start and maintain the AC motor as they run.

bad air conditioner capacitor

AC capacitors will however fail from time to time and your HVAC system will just not work. So, what causes air conditioner capacitors to fail?

AC capacitors fail mostly due to overheating, usually caused by exposure to excessive heat or when the system runs for a long time. It could also be caused by power surges, lightning strikes, use of wrong capacitors or simply due to old age.

Damaged parts in the AC system can also cause the capacitor to fail. If one of the AC components has failed or is failing, it will cause the capacitor to stay connected to the circuit for longer periods of time causing it to overheat and burnout.

When an AC capacitor fails, its ability to store charge is considerably reduced. As such, the capacitor can’t provide the motor with the needed torque to start, which means that the AC will simply not start or takes too long to start and will most likely short-cycle.

A failed capacitor forces the whole system to work harder than it needs to which causes overheating of other AC components like the motors. If not replaced in good time, the motors will definitely burn out, resulting in more expensive repairs.

In the past, AC capacitors could last for as long as 30 years. However, in modern AC systems capacitors last for between 5 and 10 years. Some cheap capacitors will only last for as little as 2 years.

Some air conditioners may still run with a bad capacitor but you shouldn’t do it or even think of bypassing the capacitor. Doing so will cause the motors to work hard/strain, which results in the overheating of the motor windings and ultimately burn out.

Installing HVAC surge protector, using a capacitor with the correct rating, changing AC filters regularly, cleaning the coils, checking refrigerant levels and annual HVAC servicing are great ways of preventing the capacitor from failing.

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.