Learn HVAC Capacitor and
What It Does to Our HVAC Unit

What is a HVAC capacitor?

Air conditioner capacitor is a device that storage electrical charge. It uses in HVAC unit to help run a motor or a compressor.

An ac capacitor will have capacitance. Capacitances are rated in microfarads or MFD. The symbol for micro is a Greek letter µ (mu) and the symbol for farad is the capital letter F.

In HVAC Units

There are two types of capacitor:

  • Run capacitor
  • Start capacitor

Run capacitor is always connected to PSC motor or compressor on all time to help run it. If a run capacitor for condenser unit is bad, you will hear sound from the compressor or the condenser fan will not start.

Run capacitors are use in:

  • Compressor
  • Condenser fan motors
  • Indoor blower motors
  • Induced draft or forced draft motor

Start capacitor stays in the circuit for few second and go off. It stays in the circuit longer enough to allow the motor to increase in torque. There a resistor on the start capacitor to bleed off the charge during off phase.

This HVAC capacitor will required relay to disengage it after startup. This relay is referring as potential or current relay.

Start capacitors are use in:

  • Increase compressor starting torque

There are two shape of an ac capacitor:

  • Oval
  • Round

In my area round capacitor is few dollars more expensive than oval capacitor. No – I’m not making this up.

HVAC capacitors are rated by:

  • Microfarad (MFD)
  • Voltage (Volt)
  • Manufacturing tolerance
  • Ambient temperature

Air conditioner capacitor rating

Two electrical rating, we pay close attention to. The first rating is capacitance; which is in microfarads (µF or MFD) and the second rating is the voltage for that capacitor.

Thing to remember:

We can always go up in voltage or equal to, but NEVER down in voltage

Let’s say a 5 µF and 370 volt capacitor is bad and we will replace it with 5 µF and 308 voltage capacitor. We can’t do that. We can have 440 volt to replace 370 volt capacitor, but we can never have a 308 volt replace 370 volt capacitor. Once of the main cause of the capacitor fail is incorrect voltage.

What is the best HVAC capacitor?

Capacitor label with EIA-456

Air conditioner capacitor has quality standards issues by Electronic Industries Association. This standard is EIA-456. When a capacitor have EIA-456 label on it, This mean the capacitor have been tested for 2,000 hours on a 125 percent above it rated voltage and 10°C above it actual rated temperature under Highly Accelerated Life Test (HALT) method.

For example, ac capacitor that is rated 30/5 MFD 370 VAC with 50°C, will be test at 463 VAC and 60°C for 2, 000 hours under Highly Accelerated Life Test (HALT) method. A HVAC capacitor that is test under Highly Accelerated Life Test (HALT) method have failing rated less than .50 percent. Choosing capacitor that meet EIA-456 standards will last longer.


The capacitor should meet the correct MFD requirement for the specific motor or compressor. What happen if we don’t know the correct MFD? We will discuss that later.

Higher MFD Rated

When we use a capacitor with higher MFD rating, this will happen.

  • Lower the motor amp draw
  • It will increase motor winding temp
  • Increase RPM of motor – it also increase motor torque

Lower rated HVAC capacitor will cause:

  • Decrease RPM
  • Increase motor amp draw, - this will also increase motor winding temp
  • Decrease efficiency
  • If it’s starting capacitor, it will decrease motor starting torque

Incorrect MFD rated for specific motor will put stress on it, this will all result in shorten the life of the motor, compressor, and the capacitor.

Manufacturing tolerance

Do you see a ± % in HVAC capacitor label? This is manufacturing tolerance for that specific capacitor.

It should said 5± %, 6± %, or +10% -5%.

Each ac capacitor will have different range. The ± % tells us the maximum and the minimum range of capacitor microfarads.

Ambient temperature

Air conditioner capacitor temp are rated in Celsius. Once of the main cause of the capacitor fail is due to higher temp. The higher the capacitor temp labels the better the capacitor life span.

As the temperature go up, the capacitance and it life span go down.

Single and Dual Capacitor

  • Single capacitor will have two terminal: fan or herm and common
  • Dual capacitor will have three terminal: Fan, Herm, and common (C)

Dual and single AC capacitor

As you can see on the bottom, there is important label:

  • HERM – for the compressor
  • FAN – for fan
  • C – common

Dual Ac capacitor head

Combined HVAC capacitor

Combine air conditioner capacitor to get the require MFD rating, in resident HVAC, the combine method work best if it is single capacitor and for commercial HVAC, combine dual capacitor is best.

Why can we combine dual capacitor for resident HVAC?

For example, we have two 30/5 MFD 370 VAC dual capacitor for the condenser unit. If we were to combine 30+30=60 and 5+5=10, we could get 60/10 MFD dual capacitor. In my area, I haven’t see condenser fan motor that require 10 MFD capacitors.

We could combine ac capacitor in series or in parallel. There is no different, we will still get the sum amount of the two capacitors MFD.

Lets said we needed a 20 MFD capacitor and we only have two 10 MFD with 370 volt capacitor. You add the 10+10=20 MFD and 370+370= 740 volt.

Here is the side effect, as the voltage increasing, the capacitor life spans decreasing. 740 volt is too much for resident HVAC units.

Side notes:

I understand the voltage on the combine capacitor need to be the SAME. Here is the thing, some HVAC manufacture combine volt and other company does not. For example, we have two 10 MFD 440 volt and other HVAC manufacture would add 440+440 = 880 volt and other would just leave 440 volt. What is the deal? Sorry, I digress, let’s continue.

In Series for Single Capacitor

HVAC capacitor series

In Parallel for Dual Capacitor

HVAC capacitor paralled

Measuring the Microfarad of a Capacitor under Load

Yes, the title is correct; we can determine the microfarad of capacitor while it is in the line voltage. We will need to get the combine amperage of C & R between the compressor and the compressor contactor.

Air Conditioner capacitor load

The combine amperage draw of C & R should equal the amperage draw of S. If it does not equal, the HVAC capacitor may be bad and we will try something different.

As the compressor running normal, we will check the voltage between C and HERM on the run capacitor. We got the voltage and the amperage, and we will use this equation to determine the microfarad.

Microfarad Rating = (2652 X START AMPS) / RUN CAPACITOR VOLTS

For example, if the current draw on C & R is .40 amps, the start wire is 7 amps, and the volt between C and HERM is 440.

(2652 x 7)/440 = 42 microfarads

42 are within 6± % manufacture tolerance rating for a 40 MFD capacitor, so if we cannot find MFD on old HVAC capacitor; this is one of the methods. 2652 is a fix number and some people will use 2650 or 2654. It is still within the range.

How to Test a HVAC Capacitor

We will use Ohmmeter for Ohm and Digital Multimeter (DMM) for MFD to check and test ac capacitor. First, we will use the DMM to determine the correct amount of MFD for that specific air conditioner capacitor and then we use Ohm to check if the capacitor is bad. Let’s discharge the capacitor first.

How to discharge a capacitor?

You get a screwdriver that have insulated hander and put it on top of the capacitor terminal to discharge the capacitor. If it single capacitor, you put it on top of the two terminals and it should discharge it. If it dual capacitor, put the screwdriver on top of the three terminal.

how to discharge ac capacitor

According to book, it said we need capacitor discharge tool. This tool is made from two jumper wire with 20,000 OHM and 5-watt resistor.

How to check an air conditioner capacitor?

For safety reason, we are going to use the Single Hand method to Check

how to check a HVAC capacitor

We don’t need the two probes with two hand method. We need to get in the habit with two probes one head method.


To check the ac capacitor, first we need to know what the ACTUAL capacitance supposed to be for that specific capacitor. It should be something like: 5.0 MFD ± 5%, 45/5 ± 6% on the capacitor label.

Now that we know what to expect from our capacitor, here how we check it.

We are on µF or MFD section of the Digital Multimeter (DMM)

Check 5.0 MFD ± 6% single capacitor

Video - Will be here

Hold the two probes with one hand and check it. If we get 4.7 or 5.3 µF, it is within the ± 6% capacitor, it should be good.

Check 45/5 MFD ± 5% dual capacitor

Video - Will be here

The dual capacitors have three terminals: Check C to FAN and C to HERM. If we were to check FAN to HERM terminals, it provides the lower value. This does not tell us anything.

45/5 MFD ± 5% dual capacitor supposed to be within the ± % of the label and if the microfarad reading is below the ± % of the capacitor. The HVAC capacitor is bad.


A start capacitor does not have manufacturing tolerance. Ex, 5± %, 6± %, or +10% -5%.

air conditioner start capacitor head

On the top of the start capacitor, there is vent. This vent can indicate if a capacitor is bad. If the vent is push upward (bulging or hole ruptured), this is a sign the capacitor is bad or if there is discoloration or leak around the vents, the capacitor is defective.

Start capacitor have the same purpose as the run capacitor, except it is rated higher MFD than run capacitor. It danger then run capacitor.

Some start capacitor has bleeding resistor across two terminals. This bleeding resistor is not for the safety of the technician or it users. Actually, the bleeding resistor is designed to prolong the life of the relay. The bleeding resistor is to prevent arcing across the relay contacts and overheating it.

HVAC start capacitor resistor


If we check the MFD of capacitor with bleeding resistor, it will show the combine MFD. For example, my start capacitor is rated 88-106 MFD. If I check the microfarad of the capacitor with bleed resistor, it will show the start capacitor have 172 MFD.

If I were to check without the bleed resistor in the terminals, it will show the capacitor have 104 to 106 microfarads. So, check with bleed resistor will not show the actual microfarads.

Does run capacitor have bleeding resistor?

I have not seen them so far; I don’t see the need to have bleeding resistor for run a capacitor that stays in the circuit continuously. We done using MFD to check the capacitance; we are going to use the Ohm to test the air conditioner capacitor.

How to test a HVAC capacitor with Ohmmeter

Ohmmeter allows us to check if the HVAC capacitor has charge, shorted or open.

Ac capacitors charge

If we put ohmmeter across the capacitor terminal, it will rise and then fall back toward zero. This is good capacitor.


If air conditioner capacitor has a bleed resistor between the terminals, the value we read will not fall toward zero; instead, it will fall toward the value of the bleed resistor (17,000 – 20,000 Ohm). The best solution to test capacitor with bleed resistor is to disconnect one end of the resistor.

Shorted HVAC capacitor

A shorted capacitor will not rise, it will read 0. If a shorted capacitor with bleed resistor, it will rise to the value of the resistor.

Open capacitor

If capacitor is open, it will not rise or fall. It just read OL.

Ground Check

If an air conditioner capacitor is normal, we should read OL from terminal RUN and HERM to the capacitor metal body.

What kind of symptom does bad HVAC capacitor cause?

The motor will not run and we have to manually rotating the fan to make it work. It will also produce buzzing sound.

Basic Rule to replacing an air conditioner capacitor

When we replace a motor, we should always replace the capacitor and the capacitance of the new capacitor must be within ± 10% of the original capacitor. If we are replacing the start capacitor, the microfarad of the new capacitor must not exceed 20 percent of the original start capacitor.

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