If the air conditioner isn’t working (or the heat pump), it may be because the capacitor is defective. An AC capacitor is an essential part of an HVAC system that often goes bad over time, and our best guess is that if you are reading this guide, something has gone wrong with your AC capacitor.
This guide covers everything you need to know about an AC capacitor, including what it is and how it works, how to identify the signs of a faulty AC capacitor, what happens if a capacitor fails, how much it costs to replace one, determining what capacitor to buy, how long they last, what causes them to fail, and how they are checked.
We explain all of this in an easy-to-understand way, answering all your questions about AC capacitors.
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What is an AC capacitor?
An AC capacitor is an electrical component within an HVAC system. It gives any motors an extra electricity boost to start and operate correctly. Depending on the HVAC motors, it may have a few capacitors.
Most HVAC systems will have one on the inducer motor, another on the blower fan motor, and one combined capacitor or two separate ones for the compressor motor and condenser fan motor.
Higher-end HVAC systems with variable speed motors usually have capacitors built into the control board and cannot be replaced. In these cases, the control board will need to be replaced.
What does an AC capacitor do
To describe how an AC unit capacitor works and what it does, we will compare it to a battery. A capacitor and a battery both store electricity. A capacitor can release all the electricity in seconds, whereas a battery will release its energy over time.
This fast release of energy helps the motors meet the energy demand it needs when starting up.
In HVAC and AC units, the motors require that extra electricity when starting up. A capacitor gives that extra electricity and then recharges and is ready to provide that boost of electricity when it is needed again.
Depending on the type of HVAC system, they may have an AC start capacitor, AC run capacitor, or an AC dual capacitor. This guide covers capacitors as a whole.
Because of how a capacitor works and what it is designed to do, they often become defective and must be replaced. Below we will cover some of the signs of a faulty capacitor.
Signs of a bad AC Capacitor
There are various signs to look for that may indicate a faulty capacitor. These signs include noise indications, visual indications, and sensory indications.
The first is a sensory indication. If when turning on the AC, it blows warm ambient air. It can indicate that the compressor's AC capacitor is not working. The compressor is a critical component of the refrigeration cycle. If the compressor isn’t working because the AC capacitor is defective, the HVAC system will not be able to cool the air.
Another sensory indication to look out for is when the HVAC system is turned on, and no air is flowing through the registers (vents), indicating that the blower wheel capacitor is defective. When the blower wheel capacitor is defective, the blower motor will not be able to start correctly, preventing air from flowing through the system and into the home.
One visual indication is that the outdoor condenser fan is not working when the AC is turned on. When the outdoor condenser fan isn’t working when the AC is on, there is a good chance the capacitor is defective. However, in higher-end HVAC systems that can operate at variable capacities, the outdoor fan motor may not work by design due to how they function.
Another way to visually identify a bad capacitor is because of an oily-like fluid built into the capacitor (similar to a battery). Often, if the oily-like fluid leaks, it allows dust and debris to stick and build up on or around the capacitor and electrical housing of the HVAC system. However, it may be hard to see this as it often requires opening up the electrical housing of the system.
Do not attempt to open the electrical housing yourself. HVAC systems use high voltage electricity.
In some cases, the capacitor may still work but is on the edge of failing. This can be identified if, when turning on the HVAC system, there is a brief, loud winding up or humming noise when the HVAC system starts.
We always recommend connecting with an HVAC pro before jumping to conclusions. Often, underlying conditions can cause the AC capacitor to go bad sooner than it should. And in other cases, you may have a functioning capacitor, but your HVAC system may not work for different reasons.
An HVAC professional will test the electrical capacitance of the capacitor and determine whether it is still good.
If you are concerned that the HVAC system will stop working because of a faulty capacitor, the best way to ensure the HVAC system operates all year round is to have yearly maintenance on your heating and cooling systems.
What happens when an AC capacitor fails
When an HVAC capacitor fails, a few things can happen to the HVAC system. If the outdoor unit (condensor) capacitor fails, either the compressor won’t start at all, or the compressor will start but the outdoor fan will not work. In both cases the capacitor should be replaced. Although the system will provide cool air when the outdoor fan is not working, the refrigeration cycle will not happen properly or efficiently which can cause more damage to the HVAC system and compressor over time.
If the indoor blower fan motors capacitor fails, the blower motor will not turn on, preventing air from flowing through the HVAC system and throughout the home.
Cost to replace AC capacitor
An AC capacitor itself is not too expensive. The more costly part of replacing a capacitor is the labor due to the knowledge and danger involved in replacing one.
The time to diagnose and replace an AC capacitor can be 15-30 minutes of work. The cost for the AC capacitor can be between $6 to $30+ online or in-store, and the labor cost to replace an AC capacitor can be anywhere from $150 and upwards of $350.
The cost of replacing an AC capacitor is between $150 and $380, labor and materials included.
Replacing a capacitor the wrong way can damage the new capacitor and the motors and, in worse cases, cause injury or death.
We recommend connecting with an HVAC pro to replace an AC capacitor. An HVAC pro knows how to diagnose a faulty capacitor and has the experience and knowledge to replace one.
If you purchase an ac capacitor online or in-store, buy the correct microfarad rating required for the HVAC system's motors.
Who makes the best ac capacitors
Like any product, there are more expensive and less expensive variations. The same applies to AC capacitors. A less expensive option may work but will probably fail faster, whereas a more expensive one is better engineered and uses higher quality materials that help them last longer.
The choice is yours whether you want to invest in a better quality AC capacitor. However, it is essential that the capacitor meets the requirements of the motor. A capacitor is measured by its microfarad rating, which measures how much energy it can store. The higher the rating, the more energy the capacitor can store. Each motor is unique, and the capacitor needs to meet the energy requirements of the motor.
In most cases, it is possible to match the microfarad rating of the new capacitor with the old one. However, oftentimes, we are unable to be sure that the old capacitor is the correct one, either because the previous owners or an inexperienced HVAC technician may have replaced it with the wrong one.
To properly match the capacitor, the motor and specifications need to be checked. We always recommend connecting with an HVAC pro in case of a faulty AC capacitor.
How long does an ac capacitor last
Under ideal conditions, an AC capacitor should last upwards of 10-15 years. However, how long an AC capacitor lasts on an AC is depends on the operating conditions of the HVAC system. It is rare that an HVAC system operates in ideal conditions. Below we will discuss the various reasons an AC capacitor will fail sooner.
What causes an AC capacitor to fail
An AC capacitor will fail sooner the more strain it has on it. And since HVAC systems are designed to work under ideal conditions any variation from these ideal conditions can cause the capacitor to fail sooner.
In order for an HVAC system to work in ideal conditions, it needs to be properly designed and installed. When an HVAC system isn’t designed properly it can cause the system to work under high pressures, which will put more strain on the capacitor while running and during start up.
In addition to high pressures putting strain on the capacitor, poor insulation and high infiltration rates will cause the system to cycle on and off more frequently, causing it to wear and break down sooner.
How to check if an ac capacitor is bad
An AC capacitor is checked with the help of a multimeter and the terminals that the wires connect to. A multimeter is a device that is able to read electrical ratings. AC capacitors are measured in microfarads.
There are a few ways to check the if the capacitor is still good. One method is when the system is on and the other is when the system is off and the capacitor is isolated from the system. Which method is better is a topic of debate between HVAC pros.
Either way a capacitor should be checked by a professional because even when they are isolated they contain a high level of electrical charge.
When checking an isolated capacitor, the multimeter should be set to the microfarad reading. Next the capacitor should be checked between the C (common) terminal and the other terminals. Depending on the capacitor it may have a few terminals.
The reading between the C (common) terminal and the other terminal is the microfarad rating.
When checking a capacitor if the system is running, there is a formula that involves testing the amperage going to the motor, multiplying that by 2,652, and dividing it by the voltage reading of the C (common) and the other terminal that is being checked.
The formula is (AMPS / 2,652) x Voltage = Microfarad rating
Each capacitor has the microfarad rating stated on the side of it that indicates a properly functioning capacitor. Most capacitors can operate within 5-10%+- of the indicated microfarad rating.
If the AC capacitor is checked and tested and falls outside of that 5-10% range, it should be replaced.
An AC capacitor is a critical component of an HVAC system. It provides the additional electricity needed to start and run the motors of an HVAC system.
Now that you know what an ac capacitor is, what it does, the signs to look for to identify a bad ac capacitor, what happens if an AC capacitor is bad, what kind and how much it costs to replace one, what causes them to fail, and how long they last. You can confidently approach HVAC pros if you think you have a faulty capacitor.
If you are considering upgrading your existing or purchasing a new HVAC system, check out Air Guide or Air Design to learn about the various types of HVAC systems and the importance of a well designed one. Then we can connect you with a vetted Air Pro to complete your HVAC project.