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How to measure the capacitance with standard run and dual run capacitors.

How to determine the difference between a run and a dual run capacitor.

The capacitor on the left is a "dual run round" and the capacitor on the right is a "run oval".

How to determine the terminals on a dual run capacitor.

The is the top of a dual run capacitor and if you look real close you can see the "c", "herm", and "fan" terminals.  Another way to know the terminal types is to count the number of connections that are located on each terminal.  This is common on most dual capacitors.   The fan has 2 connection points, herm or compressor has 3, and the common or c has 4.

How to measure the capacitance for a run capacitor.

Here is how to verify a run capacitor using a stanadard multimeter with a MFD option.  Just place the probes on both terminals.  In this case the reading is 7.7uf/mfd.  The rating is 7.5uf/mfd.  This is normal and validates the capacitor.

How to measure the capacitance for a dual run capacitor.

Here is how to verify a dual run capacitor using a stanadard multimeter with a MFD option.  Just place one of the probes on either the herm or fan terminal (depending on what you are checking) and the other probe on the common terminal.  In this case the reading is 43.8uf/mfd.  The rating is 45uf/mfd for the herm terminal.  You can then check the fan by moving the probe from the herm to the fan terminal and leave the second probe on the common terminal.

Capacitors are simple devices used in most HVAC equipment to assist compressors and motors with starting and running effectively. During the hot summer months the capacitor in the outdoor units failed at a high frequency rate. Changing them is a fairly simple process, but first you need to understand what is a capacitor.

A capacitor is a device that stores electricity. The amount of storage capacity is measured in Farads.

Capacitors come in all shape and sizes, but have the same function. The shape of the capacitor in HVAC equipment is usually dictacted on how it is mounted in the equipment. Can you use a oval run capacitor with the same rating as a round run capacitor? The answer is yes to this example and to many others. Another question is "What is a dual run capacitor?" The dual run capacitor is nothing more than 2 run capacitors in one housing. It will have two MFD/uf ratings such as... 5uf/40uf and can be found in a heat pump or air conditioning unit. The 5uf is for condenser fan motor and the 40uf is for the compressor.

  • Farad - the amount of storage potential
  • uf - stands for micro farad with "u" being the greek letter "mu" and mathematically is equal to 10 to the -6
  • mfd - is the same as uf and is used interchangeably

A failed or failing capacitor can cause motors and compressors not to run at all or they can try to start and make a humming sound repeatably. Motors can run slow and sometimes overheat and shut off only to cool down and start again.

Verify the capacitors prior to each heating and cooling season with a multimeter that can measure capacitance. The rule of thumb is to change the capacitor if the capacitance drops below 10% of the original value.

  • Example - the original value for a run capacitor for an AC condenser is 10MFD and the measurement is 8.5MFD or below, then it is time to change it.
  • Wiring example #1- A run capacitor has 2 terminals, which wire goes to which terminal? It doesn't matter.
  • Wiring example #2 - A dual run capacitor has 3 terminals, which wires go to which terminals? In this case the condenser motor will have a wire on the fan terminal (usually the lower uf/mfd value and is labeled on the capacitor as "fan") and the compressor "start" wire is connected on the "herm" terminal (usually on the higher uf/mfd value herm stands for hermatic compressor). The third terminal is labeled as "common" or "c".

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This guide's intention is to provide an understanding of the operation of a furnace. It is not intended as a hands-on teaching tool, or aide. All HVAC equipment should be serviced by a licensed HVAC technician. Consult your local, town, city, and state laws, ordinances, and regulations prior to accessing, engaging, troubleshooting, repairing, or servicing any HVAC equipment.

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