The Ultimate AC Capacitor Guide
As an HVAC technician, you’ve likely spent a great deal of time installing and replacing air conditioner capacitors for your customers. When a customer asks you for more information about these critical components of their HVAC systems, they’re offering you an opportunity to demonstrate your air conditioner expertise—and let them know they shouldn’t try to handle these high-voltage devices on their own.
What is an AC capacitor?
Capacitors are used in air conditioning systems, heat pumps, and furnaces. Replacing run capacitors was likely a large part of your apprenticeship tasks and probably makes up most of your AC service calls today.
If you’ve ever had customers ask exactly what a capacitor is, you might have described it as the air conditioner’s temporary battery for the sake of simplicity, while knowing it’s much more complex than that.
HVAC capacitors do store and then deliver energy to start and run the compressor and fan blower motors, but current doesn’t actually go through the capacitor. Instead, two thin metallic plates cover an insulating or dielectric material that electrons can’t flow through.
When voltage is applied to the capacitor, the current moves from one of those metallic plates to the other with every cycle of alternating current, until one plate holds as much charge as possible (its capacitance) and causes the current to stop.
In air conditioners, the direction of energy needs to change back and forth with alternating current, so AC capacitors are non-polarized. Other types of capacitors that are used in direct current applications are polarized.
Before diving into the different types of AC capacitors and which are the best to keep stocked in your truck for replacement, explore their key characteristics, including the importance of microfarads and voltage.
Microfarads (mfd or μF)
How much electrical charge a given AC capacitor can store, or capacitance, is expressed in microfarads. On HVAC parts, including capacitors, microfarads is written as μF or mfd/MFD.
HVAC capacitors can have an mfd as low as 3 or as high as 1,200, but most are in the range of 5 to 80 mfd. A capacitor that connects to the air conditioning compressor requires higher capacitance than a capacitor that connects to the fan motor.
If you use a capacitor with an mfd rating that’s too high for the air conditioner—particularly more than 10% higher—it will still run but can wear out the compressor faster with too much strain, and efficiency could decrease.
If you use a capacitor with an mfd rating that’s too low, the air conditioner may not start at all, or at minimum have long starting times, and you’ll need to replace the capacitor soon after.
Voltage
A capacitor’s voltage rating is the maximum electricity that can flow into and out of it. The higher the number, the more voltage and speed the capacitor can handle.
Keep in mind that the rating indicates the peak voltage a capacitor can tolerate. Because the capacitor isn’t discharging that voltage, you can replace a capacitor with one that has a higher rating—but never a lower rating.
For example, replacing a 370V capacitor with a 440V capacitor is fine, but the reverse isn’t true. Many manufacturers mark voltage on the part as 370/440V or 440/370V so HVAC technicians can see at a glance that the 440V capacitor can be used to replace a 370V capacitor.
Run capacitors are available in 250, 370, and 440 volts. Start capacitors use a lower voltage; their four major volt classifications are 125, 165, 250, and 330 volts.
The correct capacitor voltage is usually listed on the AC unit itself.
Frequency (Hz)
Frequency refers to the cycle of alternating current moving energy over those metallic plates in the capacitor. Measured in hertz (Hz), frequency is how many cycles, or completed events, occur per second.
Most HVAC capacitors are marked 50/60Hz, and you can use these to replace capacitors requiring either of those frequencies. But check your new capacitor carefully before swapping it out—some capacitors are marked as only 50Hz or 60Hz, and in that case, you’ll have to make sure you’re choosing the right replacement for an older one.
Terminal types, connections, and wire colors
Connecting the capacitor correctly is relatively straightforward but depends entirely on the unit you’re working on.
You connect to the terminal with the ¼" push-on tabs. Capacitor terminals usually have three or four tabs, but can also have just one, so make sure your capacitor has enough tab connections for the wiring you’ll need to complete.
Single-run and start capacitors for air conditioners have two terminals, while dual capacitors have three. You’ll use specific AC capacitor wiring colors for your connections, and terminals are identified with these standard markings:
C or COMMON: All HVAC capacitors have a common terminal for the power source, and these wire colors are usually black or blue.
F or FAN: This terminal connects to the fan motor, usually with a brown wire but sometimes an orange one. If the terminal is for the air handler connection, you’ll typically use a gray wire.
H or HERM: This terminal connects to start and run the (hermetically sealed) compressor, usually with yellow wire.
You may also encounter red, black, or blue wires in special cases or non-standard unit configurations.
Pro tip: Because a previous tech might have needed to use non-standard wire colors, make sure to clearly identify the correct wire before disconnecting and reconnecting.
Case shapes
AC capacitors come in a variety of case shapes—most commonly oval or round, but you will also encounter square or rectangular capacitors in some cases.
With all these different shapes, keep in mind that the capacitor case style doesn’t matter as much as other characteristics, such as mfd and voltage ratings, so long as you can fit the part into the mounting space.
Case sizes
AC capacitor sizes also vary, but much like the shape, the size should not matter as long as it fits in the provided space and meets the unit’s other requirements.
Types of AC capacitors
An HVAC system can be manufactured to use two capacitors separately—a start capacitor and a run capacitor—or to use a dual run capacitor.
Start capacitors
Start capacitors deliver a higher amount of initial energy needed to get AC motors moving. After starting the compressor, the capacitor drops back and allows the run capacitor to maintain motor speed at a lower rate of power.
Start capacitors are rated 70 mfd and above, with most AC start capacitors exceeding 100 mfd in order to provide enough power to get the motor up to approximately 75% speed before the run capacitor takes over. Not all HVAC systems have one, but if a unit does have a start capacitor, it will also have a run capacitor.
Start capacitors can usually be identified at a glance by their black plastic exterior.
For single-phase AC systems in areas with low voltage or an unusually long line set, a hard start kit can “boost” the compressor with a larger start capacitor and a potential relay to divert excess energy once the motor is running. For budget-conscious customers, you might offer to install a hard start kit to extend an older compressor’s lifespan, but introducing more parts also creates more potential points of failure.
Run capacitors
In your day-to-day job, you’ll most frequently replace run capacitors. Sometimes referred to as single-run capacitors, these capacitors provide the energy necessary to keep the unit running consistently, without any spikes in voltage.
These capacitors usually feature a metal or gray plastic case and two terminals.
Start capacitors and run capacitors are not interchangeable. Single-run capacitors are designed for continuous duty, which means that they can provide consistent energy over a long period of time. A start capacitor cannot handle current for more than a few seconds at a time, so they will not be able to sustain the motor’s operation for long.
Dual capacitors
Also called dual-run capacitors, dual capacitors include two mfd ratings: one for the compressor and another for the fan motor. These capacitors are specially designed to provide both that initial burst of energy to get the motor moving and the consistent energy for continuous duty.
HVAC manufacturers tend toward dual capacitors in their units to save space and minimize maintenance complexity by reducing the number of components that might need to be replaced.
Dual capacitors have three terminals: C, F, and H.
Signs of a bad AC capacitor
A good-quality, correctly sized air conditioning capacitor can last anywhere from five to 20 years. A few factors—that aren’t all in your control—could cause your customers’ capacitors to wear out more quickly:
Operating in extreme cold or excessive heat and/or direct sunlight
Power surges or unstable voltage
Dust and debris buildup on capacitor or connections
Improper sizing with mfd, voltage, and frequency
Incorrect or failing wiring
Many HVAC systems offer warranties that extend to parts for a set amount of time. You can also order from brands like AmRad and MARS® that offer warranties on select capacitors.
If you’re inspecting the capacitor as part of your HVAC service contract instead of responding to an emergency call, check for the following bad AC capacitor symptoms:
Leaks, bulges, scorch marks, or discoloration on the capacitor
Condenser fan clicks off for a second before restarting
Little airflow or no cooling inside the home
Lights dim when air conditioner turns on or circuit breaker trips
Vibrations or loud sounds, such as buzzing, humming, or knocking
Air conditioner doesn’t turn on at all
If you don’t notice any of those issues or aren’t sure if a capacitor is starting to fail, you can test the capacitor to see if you need to replace it.
How to test an AC capacitor
AC capacitors are electrical components and must be handled carefully.
1. Disconnect the unit
Before doing anything, use the circuit breaker to disconnect power to the air conditioning unit and, if applicable, switch the unit’s service disconnect box to the OFF position.
2. Inspect the capacitor
You may be able to save yourself some testing time with a quick visual inspection. If the capacitor is leaking, burnt, or bulging, it will need to be replaced.
3. Discharge the capacitor
The capacitor must be discharged before proceeding. Failure to do so will result in harmful electric shock.
The easiest way to discharge a capacitor is using a screwdriver with a well-insulated handle. Place the metal shaft of the screwdriver across the terminals, ensuring the metal contacts both terminals at the same time.
For capacitors with more than two terminals, repeat this process with the remaining terminal as a precaution.
4. Remove and test the capacitor
Once the capacitor has been discharged, use pliers to detach the wiring. Remove any fasteners holding the capacitor in place.
Place the leads of your multimeter on the capacitor terminals. The reading should be close to the mfds listed on the capacitor itself. If the readings are significantly lower (roughly 6% or more), the capacitor should be replaced.
Stock the most common AC capacitors in your truck
You can use universal capacitors to replace multiple sizes and types of AC capacitors while saving valuable space in your truck, but because universal capacitors cost a bit more, that convenience can be expensive.
A good mix of universal capacitors along with the sizes you’ll most frequently need to have on hand can help you strike the right balance. So you don’t need to run back to the shop in the middle of the day, we recommend residential HVAC pros keep their work truck stocked with capacitors in a variety of configurations, mfds, and voltage ratings.
Pro tip: When organizing your work van, think vertically, using stackable bins and tool storage containers to store items along the interior walls—putting the largest items at the bottom and smaller items up top.
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