Start and Run Capacitor Explained

By | November 12, 2014

start run hvac capacitors
What are Start RUN Capacitors?
Motors used in HVAC such as the condensing fan motors or blower fan motors sometimes need help to get 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.
Not all motors will have a start or run capacitor some can start and run by themselves.

Capacitors in HVAC can be separate or can be in a single package. When they are separate they are simply called “Single” and when they are combined into one package they are called “Dual Round”.
Here is a Dual Round Capacitor
dual round start run capacitor
cap 1
Here is a single Capacitor.
The Dual Round Capacitors are simply the way engineers are trying to save on space and cost. They could have put two capacitors in a unit but combined them in a single package.
A dual capacitor will most often have one side to start the compressor (Herm) and the other side to start the condensing fan motor. The third single leg on a dual capacitor is a shared common leg.
How Do They Work In a 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 then 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).
Buying a new HVAC Capacitor
A new Capacitor should always be installed with a new motor. A capacitor can be bought at a HVAC supply company there a usually at least a few even in a small town, also online Amazon is a good place to look.
Here is two common capacitors, the one on the left is a Dual Round Capacitor while the one on the right is Run Oval capacitor.
cap image
The Dual Capacitor is nothing more than two capacitors in the same housing; while the Run oval is a single capacitor and a HVAC system will usually have two.
Capacitors are measured by the Microfarad sometimes shown by the letters uf and Voltage. In any HVAC unit the capacitor must match the motor.
cap 2
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 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 Capcitor.

Testing a HVAC Capacitor
Testing a HVAC capacitor is done with a HVAC multi meter, the multi meter must be cable of reading the range that a HVAC capacitor can have. Many small electronic meters do not have this range. Here I am using the Fieldpeice HS36 multi meter with an Amp clamp
This test is being done on a Dual Run Capacitor 55+5 MFD (uf). The multi meter is on Farads and the leads are on C and FAN (positive and negative do not matter). The lower number is for the fan motor which is rated at 5 MFD (uf), and it reads 5.3 MFD (uf) so it is good. The leads C to Herm can be read also which would be for the compressor.
To test a Run Oval Capacitor simply touch the two leads. This one reads 4.5 MFD (uf) and is rated at 5 MFD (uf), so it is bad and needs replaced.

Replacing the 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.
1…..Turn off the power to the HVAC unit and verify it is off with a meter.
2…..Find the side panel were the electric is fed into the unit and remove the panel.
3…..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.
4….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 conations are correct.
(If you have two capacitors then one is for the compressor and one is for the fan motor.)


38 thoughts on “Start and Run Capacitor Explained

  1. Kathleen Lehman

    I have to replace both the start and run capacitors. Problem: The (Hard Start Capacitor) was removed and thrown away, so I can’t go wire to wire. I do not have any instructions, directions, or a schematic. I can’t identify the exact Ac/heat unit I have because there is no outside label with model#. I have a number from the inside panel where the wiring is located and I have some numbers off the motor below the fan (compressor)? The run cap was still there so I can hook that up, but not the Hard start. This is a Coleman Mobile Home AC/Heat unit. Compressor H29B18UABCA OC 25

    The run capacitor is a dual round (3 prong) the Hard Start Cap is 161-193/250

    Does some one have a diagram or know how to hook up?

    Where could I find a wire connection diagram and or model # of AC unit so I can get a schematic?

    1. Aaron Benetti Post author

      It is hard to say exactly what you need without seeing it but the capacitors should match the condensing fan motor and the compressor.

      Here is a Compressor H29B18UABCA listed on eBay that shows the inputs

      The condensing fan motor should have the UF listed on its name plate.

      A basic A/C unit should be wired as follows.
      One wire should go to the compressor and Herm on a capacitor.
      One wire will go to the capacitor and to a contactor for power.
      Depending on the unit one or two wires should go to the fan motor.

  2. Mike

    I have a 40/5 uf 370 volt capacitor currently installed in my heat pump. Is there any advantage to installing a 55/5 uf 370 volt. Or a 40/5 uf 440 volt capacitors or a 45/7.5.

    1. Aaron Benetti Post author

      The capacitor micro farads and amps should match a new one put in. Mixing in a higher amp capacitor will cause problems and possibly damage components.

  3. Jay Marker

    My a/c unit’s fan capacitor has gone out (visually inspected it), and the capacitor on my compressor is a dual cap. I (mistakenly) ordered a new 40/5 dual cap, and I was wondering if it’s possible to move from having separate cops to using just the dual cap.

    1. Aaron Benetti Post author

      It is best to replace the bad single capacitor. But yes technically as long as its wired correctly and the correct micro farads. A dual run is two capacitors in one package.

      1. Jay Marker

        Yeah, I know what the dual run is.. The smaller 5 uf cap has (obviously) with wires coming from it.. Would I just have to run those wires down to the dual run? Or would there be other wiring I would have to adjust?

        1. Jay Marker

          Meant to say it has 2 wires coming from it.. They both go back up into the fan motor

  4. Scott

    Thanks for the Ebay link. From what I’ve seen, offers a nice sizing chart for HVAC capacitors and furnace capacitors. Most of the capacitors I’ve looked at are $5 to $15 dollars, so a little less than Home Depot and Lowes.

    Replacement is really easy, but be sure to watch a youtube video on how to replace it like this one:

    Stay safe and hire a professional if you feel unsure about doing it yourself!

  5. Denise

    my husband recently changed out the disconnect because the breaker kept tripping but now the ac runs slow to cool the house. Could that be the capacitor needing to be replaced?

    1. Sid

      C – Common
      F – Fan
      H – Herm (Hermetically Sealed Compressor)

  6. Armando

    Well my condenser fan motor stopped working and just would hum and get hot. I was told it went out so i bought a replacement a aftermarket with the 4 wire ,and the direction wires and a ground wire.and it came with a run capacitor . I installed the fan motor and connected the brwn and brwn/wht wires to the capacitor that it came with. When I turn on the a/c system now it dont get cold, but the fan works now. Before the fan went out it would blow cold. So now im thinking the original 2 stage capacitor is my problem from the start.
    What do you think some feed back would be appreciated. Thank you.

    1. Aaron Benetti Post author

      If the condensing fan motor is working than so are its capacitors. Old capacitors should be replaced with a new motor or at least tested but if the fan is working than there is another problem.

  7. Gary Adams

    My fan went out on a Goodman 5 ton all in one I went out and heard the compressor running and before I cut the power to it it shut it self off. The squirrel fan was still running till I killed the power. I fixed fan turned breakers back on and nothing it’s like it is dead could this be the run capacitor any help would be great. Thanks

  8. John

    I’m going to sound like an idiot here, I’m aware when you replace anything you go by the previous specs to replace it with, but I was kind of thrown a random “test” so to speak where I had to match the right capacitor to the right motor, how exactly would you go about doing that?

  9. Fred

    I want to replace a dual 55/15 uf with a 55/5 uf and a 15 uf. On what terminal do I connect the jumper wires and to what terminal and which capacitors do I connect the three wires that were connected to the Common terminal on the original 55/15 uf capacitor.

    1. Terry Witzel

      Im sure you figured it out by now but the common wire will need to be jumped from your dual to the single and the fan wire will need to connect to the 15 and the compressor wire will connect to herm on the 55/5

  10. Jamie ODonnell

    Hello, I have an Armstrong unit and I believe I need a new capacitor. The past two days I’ve been jump starting it but now it doesn’t spin at all. The unit works otherwise but just no cold air. So the existing capacitor specs are 55+7.5 MDF and 370 VAC 50/60 HZ. I looked up my unit and the specs are 45+5 MDF 440 VAC. This is a new home to me and first summer. I’d prefer to use what the manufacturer states than to match what is currently there now. I’m going to buy both but unfortunately the proper one will not arrive until late next week and other tomorrow or next day. Hopefully I will hear back from someone prior to installation and appreciate any and all advice. Thanks!

    1. Aaron Benetti Post author

      The capacitor must match the motor. Sometimes motors are swapped out from the original since there is not one on hand. The amps and the voltage will be labeled on the motor itself. The larger MFD number is for the compressor and should also be matched.

  11. pradeep

    Can i use two capacitors in place of one capacitor in a cooler motor to increase the speed of cooler ?

  12. joyce

    My AC fan would not come on , so r eplaced a dual run capacitor today., for The fan started right away, but after15 to 20 min I noticed the thermostat didn’t lower. The fan stopped again. Also, I ordered the part from Amazon by the numbers on the capacitor, and the new one is is 2 inches shorter than original.
    any suggestions, in the middle of a heat wave .

  13. Eric

    What do you think about suggestions I’ve heard to replace the run capacitor as part of preventive maintenance, on a two-year basis? If someone doesn’t have the necessary multimeter to check capacitance and if deterioration but not failure of the capacitor is predictable and will still permit the equipment to run but with slower fan and compressor speed, less efficiency and greater wear, this would seem a good idea. I have two systems and when one run capacitor failed five years ago I replaced them in both units with no problems since. However, during a friendly chat with an a/c tech who was installing a new system for a neighbor, he suggested they be replaced routinely and on a bi-annual basis, and I’ve read that elsewhere as well. Please comment; thanks.

  14. Rob

    Can a capacitor emit an odor when it goes bad? This past weekend we had a terrible odor coming through the HVAC system all of a sudden in the middle of the night. The system continued to run fine, cool the house, and had no unusual noises. I had two different HVAC technicians check the system. The first wanted to change the blower motor and the second tested and replaced the capacitor for the blower motor. We have been running the system since then with no apparent odors now. I am holding off on replacing the blower motor at this time. But can a bad capacitor emit an odor like that? We have ruled out any dead animals, etc. And the smell is not really a burnt smell or burned plastic or wiring. Thanks!

  15. Vicente Chaco

    Can I go from a dual cap to a single cap? If so, how do I wire it in?

  16. Robert

    Okay, so I’ve got a Weatherking unit and the compressor would buzz but the fan wouldn’t turn. Called a professional, he spun the fan manually to “kick start” it and the fan worked, so he told me it was likely the ‘start’ capacitor in the round dual run unit. He called into his office for something and came back to tell me that he would not be able to do a single thing more without my authorization because it turned out I only had a 5 year warranty and not a 10 year as I thought I had so I’d actually have to pay for the capacitor replacement… At $300 labor. He pulled me aside and told me “Look, this part is less than $50 and takes only a few minutes to install” when I explained to him that I can’t afford $300.

    Well here’s where I’m an idiot: I forgot to take pictures of how the old one was wired when I discharged it and removed it to have something to show the folks at Home Depot when I went to buy a new one. The panel I remove to get to the capacitor is ZERO help because it shows an oval single run, and there’s only a space for a round dual run like the one I took out so when I wired it by the diagram on the panel of 2xOrange to fan, 1xBrown to Herm, and 1xPurple to common, the compressor no longer buzzes and the thermostat doesn’t “click” when switched to cool.

    What wiring configuration should I be using for a Weatherking 13JPL and does it matter which of the posts on the Herm, Fan, and Com terminals I connect the wires to?

  17. Robert

    Correction to my comment, I was going from memory.

    It’s 1xBrown to fan from the Outdoor Fan Motor, 1xOrange from OFM to common, 1xOrange from compressor contactor to common, 1xPurple from Compressor to Herm. It turns out, while I was outside looking over the diagram again, the compressor sprung to life. I guess it just took a few minutes.

    Fan still won’t spin… Well, it’ll spin freely when it’s off and tries to spin when it’s on but it’s as if it’s stuck. Guess it’s time to order a new fan motor. *sigh*

  18. Mr Fluffy

    Need direction! Old Carrier AC has dual round 40+12. Does no one make them anymore? And would a 40+10 have a snowball’s chance of getting the fan spinning if rated for 12?

    1. John

      You can split them up and make them singles if you cant find the dual round.

  19. jon

    I have a single capacitor 40 MFD on my Rheem. Both condenser and fan motor are connected to it. Fan motor is shot. Replacing fan motor and getting new dual capacitor. Is capacitance halved as it is currently wired? 20 MFD to compressor, 20 MFD to fan? New fan is 3 MFD. How do I know compressor capacitance?

  20. eric brewer

    Can i use this -Dual Run- RoToM 80/5 MFD 440/370v, 3 prong capacitor, to start a lawn tractor? Or are they only for AC application’s? Its all jibberish to me!!! I can’t even figure out which wire would go to what! C-Fan-Herm?

  21. Carl Harris

    How do I jump from a dual start capacitor to a just run only a capacitor for a blower motor

  22. Bill

    I have Tempstar 12 -model NAC260AKC3. I lost Capacitor and want to buy new one. I can find fan label is 5.0 mfd/370 vac. Compressor only shows 1 PH, RLA 25.0, LRA 148. Anybody knows what is Capacitor I should buy? such as 80/5 370, 50/5 370 or any combination?


  23. Bam

    I had a power failure at home and the breaker tripped blowing the dual round capacitor. I replaced and the 5 ton central ac is cooling great again, but the system is turning off and on every 2-3 minutes. What can cause that? I turned the unit off to avoid high electricity cost and I’m not sure it will damage anything.

  24. Andrew

    Hi there, how do I identify which terminal is which on a new capacitor?? I have purchased a new capacitor for my air conditioning unit… the problem I have is that the new capacitor has no markings and no way of telling which terminal does what.
    It is a moulded plastic type one with four terminals individually sticking up from the top.
    The old one is metal and the terminals although there are still four, there is two pairs.
    any help would be appreciated.

  25. angelo

    i have a condenser unit problem,the fan is not working but the compressor is,i changed the capacitor for the fan and started working for maybe a day but after that it stop again,,i check the new capacitor but it seems ok,,do you i need a replacement for the fan?how can i check if its already dead


Leave a Reply

Your email address will not be published. Required fields are marked *