HVAC Start and Run Capacitor Explained and Replacement – HVAC How To (2024)

HVAC Start and Run Capacitor Explained and Replacement – HVAC How To

Capacitors are used in HVAC to help electrical motors to start and keep running smoothly.

They are low-cost components that come in multiple packages, shapes, and sizes.

What are Start Run Capacitors?

A Start Capacitor holds a charge to start a motor moving.

A Run Capacitor keeps a motor running smoothly with no big up and down spikes.

A Start capacitor helps a motor start to turn, while a Run capacitor helps it to operate more smoothly.

Motors used in HVAC, such as the condensing fan motors or blower fan motors, sometimes need help to start moving and to stay running at a steady pace, with no heavy up and down spikes.

To do this, HVAC units use what are called Start and Run capacitors which can add more amps or smooth out voltage spikes.

Start Run capacitors are usually a single capacitor package but can be separated into individual units.

Not all motors will have a start or run capacitor; some can start and run by themselves.

HVAC Single Capacitor vs. Dual Round Capacitor

Capacitors in HVAC can be separated with two capacitors or can be in a single package.

They could have put two separate capacitors in an HVAC unit but combined them into one package.

Example Dual Round HVAC Capacitor Wiring Diagram

How Do Capacitors Work In an HVAC System?

A Start or Run Capacitor can be combined into one capacitor called a Dual Capacitor with three leads but can be split between two separate capacitors.

The Start Capacitor gives a fan motor the torque it needs to start spinning then stops, while the Run capacitor stays on, giving the motor extra torque when needed.

If the Start Capacitor fails, the motor will most likely not turn on. If a Run capacitor goes bad, then a motor can turn on, but the running amperage will be higher than normal, causing the motor to run hot and have a short life expectancy.

After replacing a bad Condensing Fan Motor, a new Start Run Capacitor should always be installed.

A Dual Capacitor has three connections HERM, FAN, and COM.

HERM, connects to the Hermetically Sealed Compressor.

FAN, connects to the Condenser Fan Motor.

COM, connects to the Contactor and provides power to the Capacitor.

If the unit has two capacitors, one is the Run Capacitor, and the other is the Start Capacitor. Keep in mind the compressor also often needs a capacitor which will be HERM (compressor).

How to Buy a New HVAC Start Run Capacitor

A new Capacitor should always be installed with a new motor. A capacitor can be bought at an HVAC supply company; there are usually at least a few even in a small town; also, online, Amazon or eBay is a good place to look.

Capacitors are measured by the Microfarad, sometimes shown by the letters “uf” and Voltage. In any HVAC unit, the capacitor must match the motor.

The voltage can go higher if necessary but never lower, while the MFD (uf) should always be the same. In the picture, this is a Dual Run Capacitor and reads 55+5 MFD (uf) 440 VAC. The larger number 55 MFD is for the compressor, while the lower number 5 MFD (uf) is for the fan motor.

The lower number is always going to be for the fan motor. Then the voltage is 440 Volts AC.

(The + -5 after the MFD is how much it the capacitor tolerance is rated to go up or down.)

To order a replacement for this capacitor, it would be 55+5 MFD (uf) and 440 volts AC Dual Run Capacitor.

Testing an HVAC Start Run Capacitor

How To Replace a Start Run Capacitor

A new fan capacitor should always be installed when a new motor is installed. It is always a good idea to take a picture or write down wire coloring and connections.

(If you have two capacitors one is for the compressor and one is for the fan motor.)

Turn off the power to the HVAC unit and verify it is off with a meter.

Find the side panel where the electric is fed into the unit and remove the panel.

Locate the Stat Run Capacitor, if it is a Dual Run capacitor there will be only one. If there is two then only the fan motor capacitor will need to be replaced.

Verify the MFD and voltages, then connect the new connections from the old capacitor to the new capacitor one leg at a time to be sure the connections are correct.

Start Capacitor Versus Run Capacitor – What’s the Difference?

One of the most common reasons for a broken AC or furnace is a failed capacitor.

In fact, the capacitor is one of the first things that a technician will check when your HVAC system goes down.

But what is the capacitor used for anyway? And what’s the difference between a start capacitor and a run capacitor?

A start capacitor provides an initial jolt of electricity to get a motor running, whereas a run capacitor helps keep the motor running smoothly.

In this article, I’ll go over the differences between a start capacitor and a run capacitor.

I’ll also explain how these two types of capacitors work, and why some AC compressors have both of them.

How do you tell the difference between a start and run capacitor?

A start capacitor has a black plastic case, whereas a run capacitor has a metallic exterior.

Additionally, start and run capacitors have different functions:

Start capacitors need to deliver a high amount of charge over a short (<1 second) period of time. For this reason, start capacitors are designed for momentary use.

Run capacitors are designed to deliver a moderate amount of charge and will stay in the motor’s circuit while the motor is running. For this reason, run capacitors are designed for continuous use.

When working with HVAC systems, run capacitors are much more common.

Run capacitors are found hooked up to blowers, compressors, condenser fans, and sometimes even inducers.

Start capacitors in HVAC systems are rare.

In fact, the only place that you’re likely to find a start capacitor is in an AC condensing unit—connected to the compressor.

Even then, not all AC condensing units have start capacitors, since it’s cheaper to manufacture a condensing unit without one and they’re not always needed.

How a start capacitor works

A start capacitor works by creating a leading current in the motor’s start winding. The leading current creates a rotating magnetic field that the motor uses to start with an increased amount of torque.

After the motor starts up, the start capacitor and start winding are disconnected from the circuit, leaving the main winding to handle the load on its own.

So why is a start capacitor needed, anyway?

A start capacitor is required because a split-phase electric motor can not produce a rotating magnetic field on its own. A rotating magnetic field is required to start an electric motor.

Start capacitors are used in motors that require a high amount of starting torque. For example, you might find a start capacitor hooked up to your AC compressor in the form of a hard start kit (more on this later).

How a run capacitor works

A run capacitor works by creating a leading current in the motor’s auxiliary winding. The leading current creates a rotating magnetic field that helps the motor start. Once the motor is running, the run capacitor helps the motor run more efficiently.

Run capacitors are used in permanent split capacitor (PSC) motors—like those found in your home’s AC or furnace.

So why is a run capacitor needed for a PSC motor to work?

A run capacitor is needed to produce a rotating magnetic field in a PSC motor. The rotating magnetic field produces the torque required to start the motor. The run capacitor also helps the motor operate more efficiently.

One advantage of using a PSC motor with a run capacitor is its efficiency. PSC motors run more efficiently than capacitor start motors since the auxiliary winding stays energized while the motor is in operation.

The main drawback of a PSC motor is that it doesn’t have a high amount of starting torque.

But that’s not a big deal.

Most fans and compressors in HVAC systems use PSC motors—these devices generally don’t require a lot of starting torque.

Compressors and fans are designed to run continuously, so the increased efficiency from a run capacitor is highly beneficial.

In this case, your HVAC equipment will operate more efficiently and save you on energy costs in the long run.

Run capacitor farad rating

In order for your PSC motor to operate at maximum efficiency, its run capacitor must have the correct farad rating.

If a capacitor with the wrong farad rating is installed, then the motor’s windings won’t have an even magnetic field. This makes the electric motor “hesitiate” at certain points in its rotation—causing inefficiencies.

In the worst case, an incorrect farad rating on your motor’s run capacitor will cause your motor to overheat and burn out.

What about AC compressors that have both start AND run capacitors?

Some AC compressors have both start AND run capacitors.

The start capacitor for an AC compressor comes in a hard start kit.

The hard start kit consists of a start capacitor and an additional device that removes the start capacitor from the circuit after the compressor starts up.

If the start capacitor stays in the circuit after the motor starts, the compressor’s motor windings will overheat and burn out.

After the motor starts, the run capacitor stays in the circuit and helps the compressor motor run more efficiently.

Motor Capacitor FAQ

Motor Capacitor FAQ

Overview

Voltage
Capacitance
Frequency (Hz)
Connection Terminal Style
Case Shape
Case Size
Start vs. Run Capacitors

Start Capacitors

Applications
Specifications
How do I know if my start capacitor is bad?
My motor is slow to start. Is my start capacitor bad?
My start capacitor has a resistor on it. Do I need a replacement capacitor with one?
Can I use a higher voltage rated capacitor than the original?

Run Capacitors

How do I replace a run cap in my air conditioner?
Applications
Specifications
When to Replace
Why did my run capacitor fail?
How long should my run capacitor last?
Dual Run Capacitors
If I can't find a replacement for my dual run capacitor, can I use two separate run caps?

Overview

Voltage

A capacitor will have a marked voltage indicating its acceptable peak voltage, not its operational voltage. Therefore, you can select a capacitor with a voltage rating at or above the original capacitor. If you're using a 370 volt capacitor, a 370 or 440 volt one will work, though the 440 volt unit will actually last longer. However, you cannot replace a 440 volt capacitor with a 370 volt one without dramatically reducing its lifespan.

Capacitance

Select a capacitor with a capacitance value (given in MFD, uf or microfarad) that is equal to the original capacitor. Do not deviate from the original value, as it sets the operational characteristics of the motor.

Frequency (Hz)

Select a capacitor with the Hz rating of the original. Nearly all capacitors will be labeled 50/60.

Connection Terminal Style

Nearly every capacitor will use a ¼" flag style push-on connector. When replacing a capacitor, you will need to know how many terminals per terminal post are needed for your motor. Most start capacitors have two terminals per post, and most run capacitors will have either 3 or 4 terminals per post. Verify that the replacement has at least the number of connection terminals per connection post as the original motor capacitor.

Case Shape (Round vs. Oval)

Nearly all start capacitors have a round case. Round cases are by far the most common, but many motors still use oval designs. Electrically speaking, there is no difference. If space in the mounting box is not limited, the case style does not matter.

Case Size

Just like case shape, overall size makes no difference electrically. Select a capacitor that will fit within the space provided.

Start vs. Run Capacitors

Start capacitors give a large capacitance value necessary for motor starting for a very short period of time (usually seconds long). They are only intermittent duty and will fail catastrophically if energized too long. Run capacitors are used for continuous voltage and current control to a motor's windings and are therefore continuous duty. They are generally of a much lower capacitance value.

In unusual circ*mstances, a run capacitor could be used as a start capacitor, but the values available are much lower than the values usually available for dedicated start capacitors. The capacitance and voltage ratings would have to match the original start capacitor specification. A start capacitor can never be used as a run capacitor, because it cannot not handle current continuously.

View our video tutorial below to learn more about the differences between start and run capacitors.

Start Capacitors

Applications

Start capacitors are used to briefly shift phase start windings in single phase electric motors to create an increase in torque. They possess very large capacitance values for their size and voltage rating. As a result, they are only intended for intermittent duty. For this reason, start capacitors will fail after being left energized for too long due to a faulty starting circuit on a motor.

Specifications

Most start capacitors are rated for 50-1200 uf and 110/125, 165, 220/250, or 330 VAC. They are usually 50/60 Hz rated. Case designs are typically round and cast in black phenolic or Bakelite materials. Terminations are usually ¼" push on terminals with two terminals per connection post.

How do I know if my start capacitor is bad?

Most start capacitor failures are one of two types. Catastrophic failure is usually caused by an electric motor's starting circuit being engaged too long for the intermittent duty rating of a start cap. The top of the start cap has literally been blown off, and the insides have been partially or fully ejected. Similarly, a start cap may just exhibit a ruptured pressure relief blister. In either case, it's easy to tell that the start cap is in need of replacement.

My motor is slow to start. Is my start capacitor bad?

Your start capacitor may have lost its capacitance rating due to wear and age, or you may have other non-capacitor related issues that have to do with other motor components. You will want to measure the capacitance of your start capacitor to find out.

My start capacitor has a resistor on it. Do I need a replacement capacitor with one?

Most replacement start caps will not include a resistor. You can check the condition of the old one by checking the resistance value, or just replace it with a new one. This should read somewhere around 10-20k Ohms and around 2 watts. The resistors are usually either soldered or crimped to the terminals. The purpose of the resistor is to bleed off residual voltage in the capacitor after it has been disconnected from the circuit after motor start up. Not all start capacitors will use one, as there are other ways to accomplish this. The important part is if your original capacitor had one, you'll need to replace it on the new capacitor.

Can I use a higher voltage rated capacitor than the original?

Yes. Click here for more detail.

Run Capacitors

Applications

Run capacitors are used to continuously adjust current or phase shift to a motor's windings in an effort to optimise the motor's torque and efficiency performance. They are designed for continuous duty, and as a result, have a much lower failure rate than start capacitors. They are commonly used in HVAC units.

Specifications

Most run caps are rated for 2.5-100 uf (microfarads) with voltage ratings of 370 or 440 VAC. They are usually 50/60 Hz rated. Case designs are round or oval, most commonly using either a steel or aluminum shell and cap. Terminations are usually ¼" push on terminals with 2-4 terminals per connection post.

When to Replace

As a general rule of thumb, a run capacitor will far outlast the same motor's start capacitor. A run cap will also fail or wear differently than a start cap, making troubleshooting slightly more involved.

When a run capacitor begins to perform outside the allowable range, it is most often indicated by a dropping of the rated capacitance value (the microfarad value has gone down). For most standard motors, a run capacitor will have a "tolerance" specified describing how close to the rated capacitance value that the actual value may be. This will be usually +/- 5 to 10%. For most motors, as long as the actual value is is within the 10% mark of the rated value, you're in good shape. If it drops outside of this range, you'll need to replace it.

In some cases, due to a defect in a capacitor's construction or sometimes caused by a non-capacitor related motor issue, a run capacitor will bulge from internal pressure. For most modern run capacitor designs, this will open the circuit, disconnecting the internal spiral membrane as a protective measure to prevent the capacitor from popping open.

If its bulging, time to replace. If you measure no continuity across the terminals, it is also time to replace.

Why did my run capacitor fail?

Below are some common reasons that run capacitors fail, but depending on how close the run capacitor is to its design life, it may be difficult to pinpoint the reason on a single factor.

Time - All capacitors have a design life. Several factors may be interchanged or combined to increase or reduce the life of a run capacitor, but once the design life is exceeded, the internals may begin to more rapidly decay and drop in performance. Simply put, a failure may be attributed to it being "just old."

Heat - Exceeding the design limit of operating temperature can have a big effect on run capacitor life expectancy. In general, motors that are operated in hot environments or with little ventilation will experience a dramatically reduced lifespan on their capacitors. The same can be caused by radiated heat from a generally hot running motor that causes the capacitor to run hot. In general, if you can keep your run capacitor cool, it will last a lot longer.

Current - When a motor is overloaded or has a failure in windings, it causes the current to climb, which can overload capacitors. This scenario is less commonly noticed, as it would usually be accompanied by a partial or complete failure of the motor.

Voltage - Voltage can have an exponential effect in shortening the design life of a capacitor. A run capacitor will have a marked voltage rating that should not be exceeded. For example, a capacitor is rated for 440 volts. At 450 volts, the life may be reduced by 20%. At 460 volts, the life may be reduced by 50%. At 470 volts, there is a 75% life reduction. The same can be applied in reverse to help increase the design life by using a capacitor with a voltage rating significantly higher than needed, though the effect will be less dramatic.

How long should my run capacitor last?

The life of a good quality aftermarket run capacitor (one that didn't come with your motor) would be 30,000 to 60,000 running hours. Factory-installed run capacitors sometimes have a much lower designed lifespan. In highly competitive industries where every part can have a significant impact on cost, or where a motor's intended use would likely be intermittent and infrequent, a lower grade of run capacitor may be selected with a design life of as little as 1000 hours. Additionally, all the factors from the section above ("Why did my run capacitor fail?") may dramatically modify the reasonable expected life of a run capacitor.

Dual Run Capacitors

Dual run capacitors are two run capacitors in one case. They have nothing else that makes them electrically special. They generally have connections marked "C" for "common", "H" or "Herm" for "Hermetic Compressor" and "F" for "Fan." They will also have two different capacitor ratings for the two different parts. You might see 40/5 MFD, meaning that one side is 40 microfarads (measurement of capacitance) and the other side is 5 microfarads. The smaller value will always be connected to the fan. The larger connection will always be connected to the compressor.

If I can't find a replacement for my dual run capacitor, can I use two separate run caps?

The dual run capacitor design's only advantage is that it comes in a small package with only 3 connections. There is no other difference. If there is enough space for mounting, using two separate run capacitors in place of your original dual run capacitor is an acceptable practice.