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What Is A Capacitor?


For HVAC technicians, a capacitor is simply a device that stores energy. It charges and discharges 60 times a second (in the US). On Run-Start-Common devices, it is connected between the Run and Start windings.

There are several reasons capacitors are used in the HVAC industry:
  1. Efficiency - A capacitor attempts to correct a poor Power Factor by adding Capacitance Reactance. Since a compressor or fan motor is an inductive device with Inductive Reactance, a capacitor will largely zero the reactances, resulting in a compressor or fan motor that runs cooler and uses less electricity.
  2. Cost - Older compressors and fan motors did not use capacitors at all. Instead, they were built much better than today's motors, which cost more in the manufacturing process.
  3. Start-up Boost - Run and Start capacitors will give an initial start boost. The run cap provides higher efficiency and is in-circuit all the time, while the start cap is typically switched out after startup with a potential relay or centrifugal switch.
There are two "plates" separated by a material called a dielectric. The dielectric can be made from many materials, but is generally an insulator. As a current is applied, separate charges build up on the plates. The resultant energy of the charges is then stored in the field between the two plates. Essentially, what is happening is that there are two separate charges that are being controlled in an electric field. Depending on what you want to use the capacitor for, there are various materials that can be used as a dielectric. If a very high efficiency is desired, a vacuum is the most often specified dielectric. Electrodes serve as the "plates." As the dielectric is a vacuum, there is very little resistance in the system, giving high efficiency. Radio systems are a good example of a use for this type of dielectric because efficiency and heat characteristics are of paramount importance. If tuning of the radio is an application of consideration, then an air dielectric may be a good choice. If high temperatures and voltages are a consideration, mica and glass make for excellent dielectrics. Control of heat is relatively simple, and the materials are abundant. One of the better characteristics of dielectrics is that they can be controlled to provide the amount of insulation between the "plates" that is desired. If a wire or series of wires are used as the dielectric, they can be twisted or untwisted to the degree of insulation desired.

HVAC Capacitors in General

CapacitorCapacitorsStart Capacitor

Run Capacitor Uses

Start Capacitor Uses

Dual Run Capacitor

Dual Run Capacitor

Symptoms of a Bad Run Capacitor

PCB Issues

Capacitor Check with Ohmmeter

Measure Rating of High-uf Capacitors

While system is running, take Voltage and Amperage readings.
Then use the following formula:
uf Rating Chart

Rules for Capacitor Replacement

  1. Voltage must be EQUAL TO or GREATER than the cap being changed
  2. Start Cap must be +- 20% of mfd
  3. Run Cap must be +- 10% of mfd

Quality Capacitors = Quality Service

by Jim Parisi, RE Air, Glendora, CA. Phone 626-914-7069.

Even the smallest of HVAC system components can have a profound effect on service and profitability.

A string of bad luck last summer, followed by some helpful information, opened my eyes to capacitor quality issues, and what everyone needs to know about them. I'm a Southern California HVAC residential and commercial service contractor. As a routine part of my service, I take readings on capacitors and replace any that are failed or weak.

Over the years, I've experienced occasional problems with capacitors failing within a year or so of installation. I thought that by switching suppliers and brands, I would eliminate the problem. My business did its best to provide a quality part to my customers, but I never knew of a way to measure the quality of a particular brand of capacitor. It was kind of hit and miss.

In the summer of 2010, I had a large preventive maintenance project that entailed servicing three or four buildings that had about 80 units each. The service included taking readings on all the capacitors and replacing the faulty ones, which amounted to about 180 compressor-sized capacitors and 125 smaller ones.

Within a couple months, I began to receive reports that the capacitors that had just been replaced were failing. When I pulled the capacitors out from the units, every one of them was bad or weak. They were brands I had used before and had purchased from a reputable supply house, so it really frustrated me. I contacted the supplier and the manufacturer about the problem right away. In the meantime, while waiting for their response, I proceeded to replace the capacitors again, using a different brand of capacitor, so much of the materials and all the labor costs were on me and my business.

In an effort to protect my customer's equipment, I had installed parts that I believed were of high quality. Instead, those parts had jeopardized their equipment, along with my reputation. As you can imagine, it doesn't look good to complete a preventive maintenance assignment, only to be called back because of a system failure. This leaves the customer to rethink their conclusions about your work.

Right about that time, I read an article written by Anthony Colone of Genteq, which talked about investing in quality capacitors. This article opened my eyes. Here was someone was speaking to the industry about special standards and ratings for capacitor quality set by the Electronic Industries Association (EIA). According to Colone, the reliability test outlined in EIA-456 is a Highly Accelerated Life Test (HALT) in which a set of capacitors is subjected to 125% of their rated voltage, and 10C above their rated temperature for 2,000 hours. EIA-456 calls for a first-year failure rate of no more than 0.50%.

Until I read the article, all I could do was test the new capacitors before they were installed, which I always did. But, as I learned from Colone, every capacitor tests the same out of the box, regardless of whether they're made in the U.S. or overseas. Only the manufacturers' HALT test data will prove the longevity and reliability of a capacitor. This was the first time I had been shown a way to determine which caps were good quality and which were not. It's very important for contractors to know the standards set for capacitors, so they can ask for test data from their suppliers.

I don't advertise, and my business has been built solely on the confidence and trust my customers have in me. I need to make money, and continue to look for ways to cut costs, if possible. But, if cost cutting means having a product that'll fail prematurely, then I don't want to use it. I can't afford to install bad parts — they make me look bad. Learning about HALT test ratings has given me peace of mind. Now, I don't have to put my trust in a supply house, because I know what information to look for. I have another project on a big building coming up, and expect to replace a lot of compressor and motor capacitors. Thanks to Genteq's research, I've done my homework and am confident that I'm using capacitors with a long field life.

Investing in Quality Capacitors Saves Money

by Anthony Colone (October 18, 2010)

We all know that capacitors are necessary to the proper functioning of any HVAC system. Yet, some in the HVAC industry have elected to make capacitor selection decisions based primarily on price. After all, they only cost a couple dollars, so if you can get them for a few cents less, then that amounts to some savings you can bring to your bottom line, right?

Here’s what you need to know: All capacitors are not created equal! Using low-quality capacitors can result in significant damage to your customers’ systems, and can also cost you hundreds of dollars per year in service callbacks — not to mention damage to customer relationships, wear and tear to on-call technicians, and reduced profitability.

Let’s take a look at what capacitors do and how they’re made, so you can better understand the importance of using high-quality capacitors.


Capacitors are a very important component of modern HVAC systems, as they are used in ac motors to provide starting torque and to improve the motor’s running efficiency. For both functions, the capacitors are connected in series with the start and main windings of the motor.

Depending on the function of the capacitor within the ac motor circuit, it can be called a start capacitor or a run capacitor. The start capacitor provides the motor with the torque needed to move the motor from standstill, and then automatically disconnects itself from the circuit. The run capacitor helps to maintain the rotational field under optimum conditions, helping the motor to run more efficiently at a higher power factor; it remains connected at all times. This is the reason it is called a run capacitor.

These capacitors generally are found in single-phase ac motors. This is because a single-phase ac source rises and falls from zero very rapidly, and the capacitor helps by producing a current-to-voltage lag on the windings. Since this current builds up more slowly, it gives time to the motor armature to react to the rotational field.

Capacitors are sort of like batteries — both store electrical energy, but they work differently. A battery uses chemicals to produce electrical energy and release it very slowly through a circuit, sometimes taking several years to disperse all the energy (in the case of a watch battery, for instance). A capacitor, which stores energy in the form of an electrostatic field, generally releases its energy much more rapidly — often in seconds or less. This can make a large, charged capacitor extremely dangerous if used or handled improperly.


The Electronic Industries Association (EIA) issues quality standards for capacitors. EIA-456 provides standards for metalized film dielectric capacitors intended for use with, among other things, ac motors. The reliability test outlined in EIA-456 is a Highly Accelerated Life Test (HALT) in which a set of capacitors is subjected to 125 percent of their rated voltage and 10o˚C above their rated temperature for 2,000 hours. For example, a capacitor that is rated at 5uf/440 vac, with an operating temperature of 70˚C, is tested at 550 vac and 80˚C for 2,000 hours. This HALT test simulates 60,000 hours of field life. If you estimate 5,000 hours of capacitor operational time per year, a 60,000-hour capacitor could last approximately 12 years in the field. EIA-456 calls for a first-year failure rate of no more than 0.50 percent.

High-end HVAC manufacturers/OEMs adhere to EIA standards, and thus require capacitors that meet EIA-456. Unfortunately, some capacitor manufacturers, while claiming that their products adhere to EIA-456, use poor-quality materials and/or flawed manufacturing processes.

If one compares any two capacitors in the industry, the initial readings for both capacitors will more than likely reflect the ratings specified on the capacitor label. Based upon the initial readings, it would seem that both are good capacitors. However, it’s in the rigorous HALT testing that capacitor quality can be determined more definitively.

In February 2010, capacitors from one domestic supplier, one offshore supplier, and Genteq were placed side by side and tested against EIA-456. In tests that ran upwards of 2,000 run-hours, the failure rates of lower-quality capacitors was 7-12 times greater than that of higher-quality capacitors — literally, only a savings of a few pennies for a much reduced run time.

When capacitors do not meet the performance ratings specified on their labels, and when used with motors in HVAC systems, they can severely damage the motors by:

• reducing the speed of the motor, which:

– increases the motor’s temperature;

– causes bearing wear and insulation breakdown; and

— increases the noise;

• lowering the motor’s efficiency, which causes additional energy consumption and costs for the customer;

• creating improper operation of the entire equipment, which:

– results in improper cycling;

– increases system noise; and

– provides unwanted stress on other system components.

Needless to say, a motor is expensive to replace.


Bargain brands may have the same labeling and claim to meet the same standards as more expensive capacitors. However, they may contain poor-quality materials and may be manufactured using processes that yield lower quality and reliability. This combination of poor quality materials and inferior manufacturing processes may significantly reduce the life of a capacitor. In short, cheap capacitors may fail faster.

There also is a safety perspective to consider. In the last few years, customers have asked Genteq to perform failure analysis of bargain capacitors that have exhibited safety-related conditions such as wiring overheating and melting, capacitor case bursting, and melting of the terminal assemblies of the capacitor, all of which may be attributable, depending on the circumstances, to the use of lower-quality materials.

In approximately the last three to four years, many contractors and OEMs have made capacitor selection decisions based primarily on price. Now, however, there is a growing realization that capacitor quality is very important and that lower-quality capacitors may damage both HVAC systems and customer relationships.

David Allen, owner of Allen’s Air Conditioning, Inc., Tuscumbia, Ala., began looking at his company’sprocesses and procedures to determine ways he could keep the company profitable when the business slowed last year. In reviewing the company’s callback logs, he was surprised to see the number of return visits his technicians were making associated with capacitor warranty claims. He did his own research, and discovered that he was losing hundreds of dollars in callbacks to replace faulty capacitors. Through intensive research, David has decided that installing high quality components for his customers provides the best overall return for him and his customers.