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Air Conditioner Capacitors

Air Conditioner Capacitors

Air Conditioner Capacitors
The capacitor in this photo is the round cylindrical device just below and to the right of the air conditioner contactor.

Capacitors can be found in air conditioners to assist the motors in your air conditioner. The assistance the air conditioner capacitors gives to an air conditioner can be either while it needs to start or when it is running. There is a big difference between a run capacitor and a start capacitor and I will discuss these differences in this article along with basic information to help you better understand your air conditioner capacitor. We will also offer basic information for troubleshooting air conditioner capacitors. After all, this site uses capacitor in its name so it is very apt to have some information on capacitors especially since this site relates to heating and cooling.

Air Conditioner Capacitors Basics

There are many different capacitors used for various things but here we are discussing capacitors used in HVAC for air conditioners, heat pumps, and even some you will find in hydronics for boilers. This is mostly applicable to single phase systems because three phase systems do not utilize capacitors. So this will be mainly applicable to residential air conditioners.

Discharging an Air Conditioner Capacitor (Important Safety Notice!!)

A word of warning before we proceed and you begin playing with a capacitor from your air conditioner system. Capacitors hold a charge of electrical energy and they can discharge into you meaning you can get shock by mishandling a capacitor.  To properly discharge the capacitor you should first make sure the power is off even before touching the capacitor. Then disconnect the wires to the capacitor. Then take a very large rated resistor which 10,000 ohms or higher. You do this by touching the resistor to the connections (where the wires connect) on the top of the resistor.  You have been warned!!

Air Conditioner Capacitor Ratings

Capacitors are rated in microfarads or MFD. This tells you the capacity of the capacitor by its rating. Another rating that can be found on a capacitor is voltage.  In most HVAC (includes air conditioning) systems you will find the capacitors have either a 370 volt rating or a 440 volt rating. If you have a bad capacitor and need to replace it and the old one is 370 volt rated then you can use the higher rated 440 volt capacitor as long as the new capacitor is the same microfarad rating. So if a blower motor needs new capacitor that requires 10 microfarads and you only have a 440 volt rated 10 microfarad capacitor when the old one is rated for 330 volts you are good if you use the 10 microfarad 440 volt capacitor.

You could also use an 11 microfarad capacitor. The rule on the microfarad rating is that you stay within plus or minus 10%. The best thing to do is to replace it with an exact replacement but when you are in a pinch as long as you stay within these rules you are okay.

Troubleshooting Air Conditioner Capacitors

When you have a problem with a motor in your air conditioner it is possible the problem is with the capacitor. Sometimes you may find the motor not turning at the right amount of RPM’s and other times the motor is not turning at all. This is in regards to blower motors found in the air handler and for condenser fan motors. Capacitors used for the compressor can go bad also but it is more difficult to determine if the compressor is the problem or the capacitor is the problem without checking everything out using the proper tools. Some capacitors I see that are bad usually look all swelled up beyond there normal size and some look they blew the top off. These will have an accompanying odor or smell like burnt oil. The oil may have coated the interior of the control panel. If you have a problem with a motor or compressor in the system is always a good thing to check out the capacitors.

Testing Air Conditioner Capacitors

Some capacitors when they fail will completely lose the circuit inside the capacitor that makes it work. It will appear to be an open circuit therefore you can use an ohm meter to test it to see that it is bad with an open circuit. Other capacitors will appear normal and show some resistance when an Ohm meter is used to test it but when tested with a capacitor tester it will show less than rated microfarads. Less than 10% of the microfarad rating and the capacitor needs to be replaced. Not all multi-meters have the ability to test capacitors. Those that do will have the microfarad symbol (μF) on them. The microfarad symbol or μF will be the setting you use to test the capacitor. Simply touch one lead on one side and one lead on the other side where the wires connect. Wait about 5 to 10 seconds and the microfarad reading will display on the meter.

Replacing Air Conditioner Capacitors

In replacing and electrical device or component the best rule to follow is to use an exact replacement. If an exact replacement is not available but you have several capacitors then you can wire them in parallel provided they meet the following formula:

Capacitor1 plus capacitor2 = capacitor total

So if you have a fan motor that needs a 15 microfarad capacitor and you have a 15 microfarad capacitor but you a 10 microfarad and 5 microfarad as long as you wire these in parallel you will be okay. I have done this before in a pinch but went to the supply house and bought the proper rated capacitor and replaced my pinch fix.

Sources:

Motor Capacitors on Wikipedia

Here is good article on How Start Capacitors Work from a site where I learned a lot about HVAC.

And a video reference for capacitors:

 

Air Conditioner Circuit Board Problem

Air Conditioner Circuit Board Problem

2 thoughts on “Air Conditioner Capacitors”

  1. Can I hook the capacitors up in series rather than in parallel as you suggest? If I do so how is the math done to calculate the MFD rating?

    1. Yes, it is possible however for most HVAC applications it is not applicable especially in the situation described above which is, in many cases, what happens. If you have two capacitors rated for the same capacitance or MFD rating and you wire them in series you will only get half of the rating for capacitance. The formula goes like this:

      Capacitors wired in series

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