The start capacitor is one of the most crucial parts of any HVAC system. This AC capacitor is what supplies the initial surge of power to start the motor that powers the whole system, as that surge is about 300 to 500 percent of the normal draw of amperage for the system. The capacitor for AC systems is designed to last the entire life cycle of the unit…but not always.
A failed AC capacitor can have a number of causes:
- Overheated circuits elsewhere in the system
- Short circuits elsewhere in the system
- Extremely high outdoor temperatures for long stretches
- Normal wear and tear
- Power surges or lightning strikes
What the capacitor does
Every motor has a capacitor of some sort. Capacitors are a storage device for electricity, not unlike a battery. Along with supplying the big jolt of amperage needed to get the motor going, the capacitor regulates and limits excess power through the system, ensuring there’s a steady supply of power through the cooling cycle.
Once the capacitor starts to fail, the system won’t run properly and will have to work harder to get the job done and reach and maintain a desired temperature. It affects the outdoor unit in particular, hindering the entire cooling process.
These are all symptoms of a failing AC capacitor:
- A humming sound or loud click from the system, that can be heard inside the home
- Overheating or swollen size, bulging like a frozen soda can
- Smoke or a burning smell around the outdoor unit
- HVAC system performs poorly and doesn’t cool well
- Spike in utility bills
- System takes longer to start cooling, and shuts off at random times
- System doesn’t turn on at all
- AC capacitor is swollen similar to a frozen soda can
To be clear, some of these symptoms can point to other problems in the system, but if any of these problems present themselves, it’s logical to start looking at replacing the air conditioner capacitor as part of the diagnostic process.
Steps for replacing the capacitor
SAFETY NOTE: The first thing that should be done before anything else is to de-energize the system by throwing the circuit breaker. Also, the capacitor holds a great deal of energy and should be discharged before going any farther. There are other safety issues to keep in mind while handling the capacitor, which we’ll touch on in a moment.
A capacitor that’s on its way to total failure will show a substandard reading of microfarads when tested with a multimeter. Capacitors are usually cylindrical or oblong in shape and will begin to bulge at the top (near the terminals) or even leak oil from a rupture when they’re at the end of their service life.
When handling the capacitor, remember:
- Never, ever touch the terminals
- Never short the terminals with a metal object across them; this can cause arcing or even start a fire
- Discharging should be done through a resistance load, by a specialist
- Only use insulated pliers and tools
- A safe way to discharge the capacitor is to set your multimeter to DC and attach clips to the capacitor’s terminals; leave them connected until the DC reading on the multimeter slowly falls to zero
After removing the necessary access panels and locating the capacitor, you should make note of the voltage and capacitance specs of the old capacitor. Replacing it with one that has lower voltage and capacitance specs will cause unnecessary stress on the new one, shortening its service life. Dismount the capacitor, being sure to label the wires so that you can reconnect them properly on the new capacitor.
Install the new capacitor (access, mounting and other details can vary a lot from one make/model to another, you may need to consult a service manual), reconnect it to the proper terminals, re-energize the system and test. If it doesn’t restart properly, turn the power back off, discharge the new capacitor and double check that the wire leads were all connected properly.
AC capacitor cost
As of 2021, the cost of an AC capacitor can typically run between $175 and $250. Costs can vary depending on make and model as well as whether the capacitor is single-run or dual-run, and in some outlier cases the cost can be as much as $400. Still, the $175-250 range is a good ballpark number.
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