Replacing a capacitor: When to replace and what are the indicators that it is bad?

Capacitors are often the reason for the downfall of your heating or air conditioning equipment. They are also one of the least expensive parts to have to replace, so it’s always a good idea to check it out. Here, I will describe a capacitor and where to locate it in your unit, determine if it is bad using no diagnostic equipment, as well as how to locate the correct replacement for your unit.

***CAUTION*** Capacitors are charged with electricity. When attempting to remove a capacitor, always make sure to kill the power to the equipment you are working on and discharge the contacts on the capacitor (herein referred to as legs) by using a screwdriver with a non conductive handle (like plastic or wood) and laying the metal portion across the contacts (legs). This will spark but what you have now done is discharged the capacitor and made it safe to touch and remove. When removing a single run capacitor, take note of which wires were connected and label them. When removing a dual capacitor after completing the steps above, make sure to take note of what labeled terminal each wire is going to and label them accordingly to ensure proper installation of the replacement. Failure to follow these steps could result in injury or death.

Step 1: Locating the capacitor
A capacitor will always be tethered to the wires of your motor, whether that be a condenser motor (outside unit), or your blower motor (inside unit if a furnace or air handler, outside if a package unit). The wire color to follow in most all cases will be the brown wire. If you trace that wire it should lead you to something that either looks like a can of soup (round) with 2 or 3 legs coming off of the top, or an oval can with the same type of configuration, 2 or 3 legs. The difference here is that a capacitor with 2 legs is what is known as a “RUN” capacitor and it’s only purpose is to help that motor run and nothing else. The capacitor with 3 legs on top is what is known as a “DUAL” capacitor which operates 2 parts, a condenser motor and a compressor, hence the “DUAL”. FYI… A compressor will only be located on an outdoor unit, so you will never find a DUAL capacitor on an inside furnace or air handler.

Step 2: Diagnosis
When inspecting the capacitor, there are a few easy tell tale signs to let you know if it is bad or not. If it is leaking, is puffed out so that it does not have a perfectly flat bottom or is giving off a bad smell, the capacitor is bad. Another way is when trying to start the outdoor unit, only one of the 2 items connected (the compressor or condenser motor) runs, that is another good indicator that the capacitor is bad. When dealing with the indoor furnace or blower unit and attempting to start that, if the wheel is not spinning, you can take a stick and try to spin it manually. If it kicks on when doing this, the capacitor is bad. This method can also be used on the outdoor unit if the compressor is running but the fan blade is not spinning. Simply by taking a stick and trying to spin the blade through the fan guard, if the fan thn kicks on your capacitor is bad.

Step 3: Determining which capacitor you have
After you have located and discharged the capacitor (as instructed above), you can in most cases find the value of the capacitor by simply reading the side of it. There are 3 key bits to locating the correct replacement. What you will be looking for are the MFD (microfarrad) of UF (same thing) ratings first. If a single run capacitor, it would look something like 5uf or 5mfd for example (The number can be higher or lower depending on what it is being used with). If on a dual capacitor, the numbers may look something like 40/5uf or 40/5mfd. What you now have is the microfarrad value. Next to look for is the voltage or VAC rating. Usually this only comes in 2 values, either a 370vac or a 440vac. (Note: If your unit uses a 370vac typically but you can onlyfind a 440vac, a 440vac will work in the place of the 370vac. This is not something that can be reversed. So you CANNOT replace a 440vac with a 370vac. This will cause damage to your unit and possibly start a fire.) The 3rd key bit which is not functionally important to the running of the system, but may be an issue with mounting and room restrictions is the shape, either oval or round. The shape effects the functionality in no way.

Step 4: Installing your new capacitor
After you have obtained your new capacitor, installation is a breeze. As always, remember to kill all power to the equipment it is going back into.

For a single run capacitor: These only have 2 legs, and hence will only use 2 wires. The polarity cannot be reversed on this type so once you have relocated (and hopefully labeled as capacitor wires at the time of removing the faulty capacitor) the capacitor wires, simply take one of them and plug it on to one of the legs of the new capacitor. Then take the other wire and plug it on to the other leg. It does not matter which one each wire goes to, as long as each wire has it’s own leg. Now you’re done!

For a dual capacitor: These are a bit more tricky than a single run capacitor because they use more wires. The dual capacitor will have 3 legs and each one having an embossed reference or initial next to it indicating what that leg is for. The “F” or “FAN” leg is for the brown wire from the fan motor. The “H” or “HERM” leg is for the compressor. And the “C” or “COM” terminal is for the common wires. Plug them in the same way as the one they were attached to before and that’s it, you’re done!

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  1. I also made a post on ac capacitor replacement, but I think your post is a lot more detailed. Thanks for sharing.

    ac unit capacitor

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